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Corrosion mechanism of the as-cast and as-extruded biodegradable Mg-3.0Gd-2.7Zn-0.4Zr-0.1Mn alloys
Accepted Manuscript Corrosion mechanism of as-cast and as-extruded biodegradable Mg-3.0Gd-2.7Zn-0.4Zr-0.1Mn alloys
Zhenzhen Gui, Zhixin Kang, Yuanyuan Li PII: DOI: Reference:
S0928-4931(17)34030-4 https://doi.org/10.1016/j.msec.2018.11.037 MSC 9057
To appear in:
Materials Science & Engineering C
Received date: Revised date: Accepted date:
7 October 2017 19 November 2018 24 November 2018
Please cite this article as: Zhenzhen Gui, Zhixin Kang, Yuanyuan Li , Corrosion mechanism of as-cast and as-extruded biodegradable Mg-3.0Gd-2.7Zn-0.4Zr-0.1Mn alloys. Msc (2018), https://doi.org/10.1016/j.msec.2018.11.037
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ACCEPTED MANUSCRIPT Corrosion mechanism of as-cast and as-extruded biodegradable Mg-3.0Gd-2.7Zn-0.4Zr-0.1Mn alloys Zhenzhen Gui, Zhixin Kang*, Yuanyuan Li
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Guangdong Key Laboratory for Advanced Metallic Materials Processing, National Engineering Research Center of Near-Net-Shape Forming for Metallic
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Materials, School of Mechanical and Automotive Engineering, South Clhina
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University of Technology, Guangzhou, China
*Corresponding author, E-mail address: [email protected]
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Tel.:+86 20-87111116 Fax: +86 20-87112111
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Abstract:
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Electrochemical measurements and immersion tests were adopted to investigate
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corrosion mechanism of as-cast and as-extruded Mg-3.0Gd-2.7Zn-0.4Zr-0.1Mn (GZKM-1) alloys. Results of immersion tests indicate that corrosion resistance of the
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as-extruded GZKM-1 alloy is better. Galvanic corrosion between α-Mg and
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β-(MgZn)3Gd compound is the main corrosion mechanism of the two GZKM-1 alloys. Larger α-Mg peeled off from the as-cast samples, then expanded corrosion reaction along discontinuous β-(MgZn)3Gd compound making a pit corrosion phenomenon in macro view. Granular β-(MgZn)3Gd or precipitations fell off from the as-extruded samples, and then expanded corrosion reaction along the concentration of the particle β-(MgZn)3Gd (precipitations). Most areas of the as-extruded alloy are protected well 1
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by denser protective film.
Keywords: Magnesium; Corrosion evolution; Electrochemical properties; Immersion process; Mg-3.0Gd-2.7Zn-0.4Zr-0.1Mn; Hanks’ solution
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1. Introduction
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Magnesium and its alloys show great potential as biodegradable implant
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materials resulting from their biocompatibility [[1], [2], [3]]. Commercial magnesium
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alloys with proper mechanical properties usually contain some damage elements such as Al, which will be harmful to human being as implant materials [[4], [5]]. Then new
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designed magnesium-based (Mg-based) alloys are expected to meet the demands of mechanical and corrosion combination properties for biodegradable application [[6],
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[7], [8], [9], [10]]. With low standard electrode potential, Mg-based alloys exhibit poor corrosion resistance in vivo, which will lead to destroy mechanical integrity
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before healing [[11], [12], [13], [14]]. Poor corrosion resistance limits development of these new designed Mg-based alloys to some extent [[4], [5]]. Extrusion was widely
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adopted to modify the microstructure and the mechanical properties of as-cast biodegradable magnesium alloys [[4], [15], [16], [17]]. However, the effect of extrusion on corrosion behavior of biodegradable Mg-based alloys is controversial. Researchers have different conclusions when they studied different Mg-based biodegradable alloys. Liao [[17]] stated that the corrosion properties of the as-extruded Mg-Al-Mn-Ca 2
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(AMX602) alloy were better than that of the as-cast AMX602 alloy in the aspect of immersion test and the electrochemical analysis. Peng [[18]] found that backward extrusion was one of the effective methods to enhance corrosion properties of the as-cast Mg-Zn alloys. Zhou [[19]] reported that the extruded Mg-2Zn-0.2Mn alloy
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exhibited better corrosion resistance compared with that of the as-cast alloy. However,
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Zhang [[20]] observed that the corrosion resistance of the AZ91 alloy deteriorated
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after extrusion, for the reason of existing dislocation density, twins and grain
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boundaries in the extruded AZ91 alloy. Chen [[21]] confirmed that although the as-cast ZK60 alloy corroded by pitting corrosion, it showed good corrosion resistance
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compared with the as-extruded and T6 treated ZK60 alloys. Jeong [[22]] confirmed that the improvement of corrosion resistance after extrusion in the Mg-Ca alloys with
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Ca ≥ 1%, but it was not obvious in the pure Mg and Mg-0.4 Ca alloy. According to these researches, the effect of extrusion on the corrosion resistance of Mg-based
Our
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alloys was different from the alloy elements and compositions. previous
work
[[23]]
designed
a
new
magnesium
alloy
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Mg-3.0Gd-2.7Zn-0.4Zr-0.1Mn (GZKM-1), and the as-extruded GZKM-1 alloy possessed excellent mechanical and good corrosion properties in Hanks’ solution, showing sound potential for biodegradable implant application. Hence, further detailed studies on corrosion behavior of GZKM-1 alloys are crucial to clarify the mechanism of corrosion process. This is also significant to develop GZKM-1 alloys as biodegradable implant materials. In this paper, corrosion mechanisms of the as-cast 3
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and as-extruded GZKM-1 alloys are studied by electrochemical analysis, immersion test and characteristics of the corrosion process.
2. Materials and Methods
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2.1 Materials and samples preparation
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It is well known that small amounts of Fe can have significant influence on Mg
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corrosion properties [[24]], so the amount of Fe in the metal and master alloys should
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be controlled strictly. Commercially high purity Mg (99.95%, wt %), high purity Zn (99.95%), Mg-30%Gd, Mg-30%Zr and Mg-10%Mn master alloys were used to
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prepare the experimental alloy. Semi-continuous casting was applied to obtain as-cast GZKM-1 ingot with a diameter of 100 mm. After casting, a cylindrical with 50 mm in
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height was cut from the ingot and was solid solution treated at 520 °C for 10 hours. After heat treatment, two cylindrical with diameter of 49.5 mm and height of 50 mm
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were cut from the treated cylindrical. Then the treated cylindrical was extruded at 350 °C with an extrusion ratio of 8.4:1 and a ram speed of 4 mm/s. The detailed
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information of GZKM-1 alloys please refers to our previous work [[23] ]. Samples for the electrochemical measurements cut to 15 mm in diameter and 3 mm in thickness, and then ground with serials of grand papers till 2000-grit. Samples for the immersion experiments were prepared with 35 mm in length, 8 mm in width and 5 mm in thickness and then drilled a hole with 2 mm in diameter. Then samples were ground to 2000-grit sand papers. Surface area of each sample obtained from its 4
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initial length, width and thickness.
2.2 Electrochemical measurements and immersion tests Hanks’ solution was used to carry out the electrochemical measurements and
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immersion tests [[23]]. A three-electrode electrochemical cell was utilized to taking
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the open circuit potential (OCP), potentiodynamic polarization (PDP) and
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electrochemical impedance spectroscopy (EIS) experiments on an electrochemical workstation (IM6ex, Zahner) to evaluate the corrosion properties of the alloys. The
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three-electrode electrochemical cell contains a silver/silver chloride (Ag/AgCl,
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saturated KCl) electrode as the reference electrode, a platinum (Pt) mesh as the counter electrode and the specimen under investigation as the working electrode with
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an exposed area of 1 cm2. The OCP reflected the initiation and propagation of
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corrosion process. The OCP tests started immediately after immersing the samples in the Hanks’ solution at 37 ± 0.2 °C, lasting for 1800 seconds to achieve constant
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potential. The polarization curves were acquired at a scanning rate of 1 mV/s. The
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polarization tests of the as-cast and as-extrude GZKM-1 alloys were recorded after immersion for 1800 seconds in Hanks’ solution at 37 ± 0.2 °C. Data were recorded from 100 kHz to 100 mHz with a 5 mV sinusoidal perturbing signal at the open-circuit potential for EIS tests and presented as Nyquist and Bode plots. The EIS tests were carried out without agitation or circulation and without disturbing the corrosion system [[25]]. To ensure the reproducibility and reliability, all 5
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electrochemical measurements were repeated three times and typical curves were given in the results. 1. Immersion tests for calculating weight loss were taken in Hanks’ solution at 37 ± 0.2 °C according to the stand of ASTM G31-72 [[26]]. The prepared samples
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hung at a beaker within 200 ml Hanks’ solution, with a ratio of media (ml)
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volume to specimen surface (cm2) was 20 : 1, utilizing a fine nylon rope through
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the drilled hole. After weight loss experiment, each sample was carried out and
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cleaned in a solution (consisting of 200 g CrO3, 10 g AgNO3 and 1000 ml distilled water) at 80 oC water bath in an ultrasonic cleaner for 5-10 mins,
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followed by washing with distilled water and drying in a stream of warm air. The weight of samples before immersion and after removal of corrosion products
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weighted to an accuracy of 0.1 mg. To investigate the corrosion process of the alloys, real-time immersion tests were carried out in Hanks’ solution at 37 ±
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0.2 °C for 1 min, 2 min, 10 min, 30 min, 1 h and 5 h. The samples were prepared as stated in the former immersion tests. Preparation for hydrogen evolution
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experiment is similar to weight loss test. The detail information of hydrogen evolution experiment carried out as shown in Fig. 1 [[27]]. Corrosion products were determined using X-ray diffraction (XRD, D8 ADVANCE, Bruker, Germany). The XRD experiments were performed with Cu Kα radiation (λ =1.5418 Å), and the step size was 0.02o with a scan rate of 0.1 s/step.
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2.3 Surface morphology of the immersed GZKM-1 alloys The corroded surface morphology of the samples was observed by Leica DVM6 (Germany) and scanning electron microscopy (SEM) (NOVA NANOSEM 430, the
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Netherlands) equipped with energy dispersive X-ray spectrometry (EDS).
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2.4 Corrosion rate calculation of Mg-based alloys in Hanks’ solution
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Corrosion phenomenon requires generation of electrons (anodic reaction), consumption of electrons (cathodic reaction), and a medium for electron transport
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(electrolyte) [[28]]. For magnesium in aqueous medium, electron generation occurs
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predominantly with metal dissolution reaction, which is given by equation (1). Hydrogen evolution is the predominant cathodic reaction, given by equation (2) for
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consumption of electrons. The main chemical reactions of biomedical Mg-based
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alloys in Hanks’ solution are shown as following equations (1-4) [[3], [4]]: (1)
2H2O 2e H 2 2OH (cathodic)
(2)
2H 2O O2 4e 4OH (cathodic)
(3)
2Mg 2 4OH 2Mg(OH) 2 (productio n)
(4)
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Mg Mg 2 2e (anodic)
The hydrogen evolution volume reflects the Mg-ion dissolved in the surface. Hydrogen evolution rate represents corrosion rate of the biomaterial alloys in certain degree. The corrosion rate of the weight loss (CRWL) calculated in mm per year by the weight loss as shown in formula (5) [[29]]: 7
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CRWL
ΔM m 365 24 10 87600 ΔM m (mm/yr) Sm Tm ρ m Sm Tm ρ m
(5)
According to the corrosion reaction depicted as equations (1-4), one molecule of hydrogen evolved for each atom of corroded Mg. One mol (i.e. 24.31 g) of Mg metal
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corrodes for each mol (i.e. 22.4 L) of hydrogen gas produced [[29]]. The corrosion
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rate of the hydrogen evolution (CRHE) calculated according to the evolution volume of hydrogen as shown in formula (6) [[29][29]]:
VH 2
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22.4 S m Tm ρ m
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S m Tm ρ m
(mm/yr )
(6)
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CRHE
24.31 ΔVH 2 365 24 10 -2
where M m (g) is the weight loss of magnesium alloys, Sm (cm2) is the surface
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area of magnesium alloys exposed in Hanks’ solution, ρ m (g/cm3) is the density of the magnesium alloy, Tm (h) is the total immersion time of magnesium alloys in hand and
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VH 2 (ml) is the hydrogen evolution volume.
Extrapolation of Tafel lines is one of the most popular techniques for evaluating
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the corrosion of Mg alloys at present [[29], [30]]. The extrapolation of anodic and/or cathodic Tafel lines for charge transfer controlled reactions gives the corrosion current
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density, icorr, at the corrosion potential, Ecorr [[30]]. With Stern equation, polarization resistance (Rp) was calculated though the slope of anodic and/or cathodic Tafel lines (a, c). To evalute the corrosion rate of Mg-based alloys with the Tafel extrapolation method, the corrosion current density, icorr (mA/cm2) is estimated by Tafel extrapolation of the cathodic branch of the polarization curve, and icorr is related to the average penetration corrosion rate (Pi, mm/yr) using equation formula (7) [[29]]: 8
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Pi = 22.85 icorr
(7)
3. Results 3.1 Results of immersion test
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The corrosion rates calculated by weight loss and hydrogen evolution of as-cast
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and hot extruded GZKM-1 alloys immersed in Hanks’ solution at 37 ± 0.2 °C for 240
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h are shown in Fig. 2. The two calculation methods reveal similar tendency of
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corrosion rate. The corrosion rates calculated by hydrogen evolution of the samples are less than that calculated by weight loss. Because some gas maybe escape from the
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cup or dissolve in the water. The corrosion rate calculated by weight loss is adopted as the standard for evaluating the corrosion resistance of the alloys, while the corrosion
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rate calculated by hydrogen evolution is adopted as a reference and shows the variation of the corrosion rate. The corrosion rate of the hot extruded GZKM-1 alloy
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is less than the as-cast GZKM-1 alloy. The corrosion rate of the hot extruded GZKM-1 calculated by weight loss reaches 0.45 ± 0.14 mm/yr, showing up a well
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corrosion resistance. The corrosion rates (evaluated by weight loss method) of the GZKM-1 alloys and some typical Mg-base alloys prepared by various methods [[31], [32], [35][34], [35]] are listed in table 1. Among all these results, the corrosion rate of the as-extruded GZKM-1 alloy shows competitive advantage.
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3.2 Results of electrochemical properties Fig. 3 (a) shows the OCP curves of the as-cast and as-extruded GZKM-1 alloys immersed in Hanks’ at 37 ± 0.2 °C. The potential of as-extruded GZKM-1 alloy
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increases gradually in the first 400 seconds due to thickening of the protective surface film, then the OCP stays almost the same at about -1.535 V. The OCP of the as-cast
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GZKM-1 alloy increases in the first 150 seconds and then transient decreasing
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appears. After 250 seconds, the OCP of the as-cast GZKM-1 alloy is still increasing
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until reaching 1600 seconds, and then stays at about -1.575 V. The representative results of potentiodynamic polarization tests displayed in Fig. 3 (b). It can be seen that
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the corrosion potential (Ecorr) of the as-extruded GZKM-1 alloy is about 140 mV
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higher than that of the as-cast alloy. The cathodic branch provided an extensive linear
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Tafel region, and the icorr, Ecorr, and Pi values of GZKM-1 alloys evaluated from polarization curves are included in Table 2. The results display that the corrosion
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current density (icorr, μm/cm2), the corrosion potential (Ecorr, V) and the average
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penetration corrosion rate (Pi, mm/yr) of the as-cast and as-extruded alloys are almost at the same level, while the icorr and Pi of the as-cast alloy is a little bit higher than that of the as-extruded alloy. The Ecorr of the as-cast alloy is about 140 mV less than that of the as-extruded alloy. These indicate that the as-extruded GZKM-1 alloy shows a better corrosion resistance in Hanks’ solution compared to the as-cast alloy. Besides, the error bands of icorr, Ecorr and Pi of as-cast GZKM-1 alloy are higher than that of the 10
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as-extruded alloy, which indicates that the corrosion properties of the as-cast alloy show higher dispersion. Fig. 4 shows the EIS results for both the as-cast and as-extruded GZKM-1 alloys in Hanks’ solution. Two capacitive loops exist in the Nyquist plots of GZKM-1 alloys
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as shown in Fig. 4. The higher and medium frequency capacitive loop related to the
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charge transfer resistance and the lower frequency capacitive loop attributed to the
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mass transport in the solid phase arising from diffusion of ions through the corrosion
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product layer [[36], [37]]. Equivalent circuit was used to analysis the EIS results. Parameters of the components in the equivalent circuit were also listed in Fig. 5. The
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Rct was a parameter for evaluating the ability of charge transporting between metal surface and solution. When the surface oxidation film fails to protect the inner metal,
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Rct could be a parameter for evaluating the corrosion process. From the fitting results, the Rct of the extruded alloy is higher than that of the as-cast alloy, obviously.
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3.3 Surface morphology of immersed alloys
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Fig. 5 illustrates the SEM images of the as-cast and as-extruded GZKM-1 alloys: (a) and (e) show the microstructures of the as-cast and extruded alloy respectively. Microstructure of GZKM-1 alloy mainly contained α-Mg matrix and eutectic compounds β-(MgZn)3Gd. The differences of microstructure between the as-cast and as-extrude GZKM-1 alloy are the grain size and the distribution of β-(MgZn)3Gd. The -Mg and phases were pointed out in Fig. 5 (a) and (e). The grain size of the as-cast 11
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extrusion direction. Fig. 5 (b) and (f) shows surface morphologies of the as-cast and
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longitudinal section of the extruded alloy after immersing in Hanks’ solution for 240 h.
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Corrosion pits exist in both as-cast and as-extruded alloys. The magnified images (c-d)
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indicate that the probable corrosion process of as-cast alloy containing the protective oxidation film cracks, and the film peels off. Compare to the as-cast alloy, the
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as-extruded alloy exists denser oxidation film as shown in Fig. 5 (g-h). Fig. 6 shows map scanning of corrosion surface after immersing in Hanks’
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solution for 240 h: (a-h) as-cast alloy with complete protective oxidation film; (i-p) as-cast alloy with oxidation film peeled off and (q-x) as-extruded alloy with protective
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oxidation film. The complete protective oxidation film of the as-cast alloy consists of elements: Mg, O, Ca, P, C, Zn and Gd. The element Zn and Gd mainly distributes in
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the grain boundaries. Some big cracks of the oxidation film also start at the grain boundaries as shown in Fig. 6 (a, g-h). Once the oxidation film peeled off, the original microstructure exposure to the Hanks’ solution again as shown in Fig. 6 (i-p). The protective oxidation film of as-extruded alloy contains elements: Mg, O, Ca, P, C and a bit of Zn and Gd. Small amount of Zn and Gd is distribute along extrusion direction as shown in Fig. 6 (w-x), which is similar to the distribution of β-(MgZn)3Gd in the 12
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as-extruded microstructure as shown in refs [[23]].
3.4 Corrosion products of immersed alloys Fig. 7 shows the XRD results of the corrosion products of the as-cast and
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as-extruded GZKM-1 alloys immersing in Hanks’ solution for 240 h. The result
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indicate that the the as-cast alloy containes less corrosion production including MgO,
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Mg(OH)2 and a bit of CaCO3. The as-extruded alloy also containes a little
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hydroxyapatite (HA).
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4. Discussion
4.1 Corrosion evolution of the as-cast and as-extruded alloys
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The two different microstructure characteristics result in two kinds of corrosion
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evolutions. The results of the as-cast alloy and the as-extruded alloy immersed in Hanks’ solution both show pitting and uniform corrosion. Fig. 8 appears corrosion
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morphology with removal of corrosion products of the as-cast (a-e) and as-extruded
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(f-j) GZKM-1 alloys as further evidence. Fig. 8 (a-c) displays the surface of the as-cast alloy, which appears large area of the corrosion trace and most of the surface appears the netlike compounds β-(MgZn)3Gd. The magnified area exhibits two morphologies of the network eutectic compounds in Fig. 8 (d) continuous network (marked as ellipse) and Fig. 8 (e) discontinuous network (marked as rectangle), respectively. The pitting corrosion of the as-extruded alloy seems more severe than 13
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that of the as-cast alloy. The pits mainly exist in edges, and the other areas protected well by the denser oxidation film as shown in Fig. 8 (f). Those serious pits form at the area where was enriched in the particle β-(MgZn)3Gd phase, and the pits expand along the distribution of the β-(MgZn)3Gd phase as shown in Fig. 8 (i-j).
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Fig. 9 displays the macro-morphologies of the GZKM-1 alloys after removal of
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corrosion products and 3D measurement results: (a-b) as-cast alloy and (c-d)
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as-extruded alloy. Fig. 9 (a) appears a large-area corrosion model and Fig. 9 (c) shows
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a local pitting corrosion model. Severe pits of the as-cast and as-extruded alloys always occurred at the edge of the samples. However, 3 D imaging and measurement
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is hard to carry out at the edges of samples. Fig. 9 only presents the typical surface morphologies. The deepest pit in the as-cast alloy and as-extruded alloy is about 420
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μm and 400 μm respectively according to the 3D measurement in Fig. 9 (b) and (d), indicating that a similar corrosion level between the as-cast and as-extruded alloys in
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some severe areas.
Real time corrosion experiments were carried out to observe the initial corrosion
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behavior of the as-cast and as-extruded GZKM-1 alloys. Two kinds of GZKM-1 alloys samples are immersed in Hanks’ solution for 1 min, 2 min, 10 min, 30 min, 1 h and 5 h. Then the surface morphologies are observed by SEM with EDS. Fig. 10 displays the corrosion process of the as-cast alloy: Fig. 10 (a-b) immersed for 1 min; (c-d) immersed for 2 min; (e-f) immersed for 10 min; (g-h) immersed for 30 min; (i-j) immersed for 1 h and (k-l) immersed for 5 h. Once the sample contacts Hanks’ 14
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solution, the alloy undergoes on the equations (1-4) and forms partial protective Mg(OH)2 and evolutes hydrogen. In the first minute, corrosion reaction undergoes on some high activity area of α-Mg and forms very small pits. With the corrosion reaction going on, the pits become more and distribute more intensively. The
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boundaries of grains are high activity areas and easy to form galvanic corrosion.
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These pits expand and crack with Cl- existing in Hanks’ solution. After immersed for
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30 min, some regions form protective oxidation film as shown in Fig. 10 (g-h). The
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protective oxidation films are not stable in the solution with Cl- and hydrogen evolution. Parts of film crack gradually near the grain boundaries for existing galvanic
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couple as shown in Fig. 10 (j). Once the protective films broking and exposing the flesh alloy, the former corrosion reaction recycles with the longer immersion time. It
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means that the film formation and breaking are to take place simultaneously at the initial corrosion stage.
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The initial corrosion area of the as-extruded alloy also begins at high activity region. Some precipitations (such as ZnZr phase) become nucleation particles, and
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NaCl etc. adhere to the precipitations as shown in Fig. 11 (b). With the ions adsorption and accumulation, a protective film covers on the initial Mg(OH)2 film and isolates the alloy from the Hanks’ solution as shown in Fig. 11 (d). The protective film becomes porous and loose because of existing Cl- as shown in Fig. 11 (f). With the immersion time goes on, the porous and loose film breaks, and sever pit corrosion goes on near the particle β-(MgZn)3Gd phase and some precipitations as shown in Fig. 15
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11 (g-l). To indicate the effect of the oxidation film on corrosion process, potentiodynamic polarizations performed after samples immersed in Hanks’ for 15 h as shown in Fig. 12. The corrosion potential (Ecorr, V) of the as-cast and as-extruded
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alloys immersed for 15 h enhance about 110 mV and 35 mV respectively compare to
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the curves immersed for 0 h. This indicates that the partially protective oxidation
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films on the surface of as-cast and as-extruded alloys make the corrosion reaction
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more difficult in terms of thermodynamics. On the polarization curves, the value of pitting potential (Ept) usually indicates the tendency for localized corrosion, and a
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more positive Ept means a less likely localized corrosion [[38], [39]]. After immersing in Hanks’ solution for 15 h, the Ept of the as-cast alloy enhances about 240 mV,
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indicating that the oxidation film of the as-cast alloy shows a positive protective and reduces the tendency of pitting of the as-cast alloy. Note that the as-cast alloy shows a
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higher Ept than that of the as-extruded alloy after immersing for 15 h. This indicates that the as-extruded alloy appears a high tendency of pitting after the oxidation film
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formed. The corrosion current density (icorr) obtained by Tafel extrapolation method displays that icorr (as-extruded, 15 h)> icorr (as- extruded, 0 h) >icorr (as-cast, 15 h)> icorr (as-cast, 0 h). This indicates that the oxidation film of as-extruded alloy immersed for 15 h shows a positive protection, as the corrosion current density (icorr) is a kinetic parameter that signifies the reaction rate.
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4.2 Corrosion mechanism of the as-cast and as-extruded alloys In a summary, the corrosion models of the as-cast and as-extrude GZKM-1 alloys exhibit in Fig. 13: (a-e) as-cast alloy and (f-j) as-extruded alloy. The as-cast alloy forms partial protective Mg(OH)2 and evolutes hydrogen as shown in Fig. 13 (a).
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Corrosion reaction mainly undergoes on some high activity area of α-Mg and forms
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very small pits. The boundaries of grains are high activity areas. With corrosion
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process, the corrosion product forms and accumulates. Then the formation and
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breaking of protective film is to achieve dynamic equilibrium as shown in Fig. 13 (b). α-Mg and β-(MgZn)3Gd form galvanic corrosion near boundaries as shown in Fig. 13
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(c). Then α-Mg peeled off form network β-(MgZn)3Gd with corrosion aggregating.
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The β-(MgZn)3Gd contains two kinds: the continuous network compounds (like the
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ellipse area in Fig. 8(e)) and discontinuous network compounds (like the rectangle in Fig. 8(e)). The continuous network compounds block the expanding of corrosion
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reaction. The discontinuous network compounds block the expanding of corrosion
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reaction to some extent. However, the broken network part will expose the flesh α-Mg, which will form galvanic corrosion and aggregate the tendency of pitting as shown in Fig. 13 (d). Once α-Mg peeled off form network β-(MgZn)3Gd, corrosion expands along the discontinuous network β-(MgZn)3Gd to form pits as shown in Fig. 13 (e). The as-extruded alloy processes refined grains and flowing texture structure, and detailed microstructure please refers our previous work [[23]]. Corrosion reaction 17
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β-(MgZn)3Gd and α-Mg forms micro-galvanic couple to accelerate corrosion reaction
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as shown in Fig. 13 (g). The surround α-Mg of β-(MgZn)3Gd is eroded to form hole,
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resulting in β-(MgZn)3Gd and other precipitations fall off as shown in Fig. 13 (h). The
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corrosion reaction tends to expand along the distribution and concentration of β-(MgZn)3Gd and precipitations and forms pits as shown in Fig. 13 (i-j). The most
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area of as-extruded alloy were protected well for the following reasons. Increased reactivity coupled with more sites for nucleation of an oxide film on the surface of
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grain-refined materials is posited to result in a more rapid formation of a protective layer [[23], [40]]. For Mg-based alloys, the oxide film may be MgO and RE2O3 [[25],
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[41], [42]]. The attack rate of the protective layer is controlled by the diffusion of the reactants through the surface film, and it follows that process-induced microstructure
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changes may influence this diffusion-controlled process [[23]]. The corrosion current density (icorr) obtained from potentiodynamic polarizations (0 h, 15 h) indicates that the protective film of the as-extruded alloy has more advantage than that of the as-cast alloy. Micro-galvanic interaction between the -Mg and second phases causes the corrosion of Mg alloys [[43]]. Galvanic corrosion (α-Mg, β/precipitations) is the main 18
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corrosion mechanism of the as-cast and as-extruded GZKM-1 alloys. The biggest difference is that the initial coarse grains (α-Mg) peel from the samples of the as-cast alloy, while the particle β-(MgZn)3Gd and precipitations fall off from the samples of the as-extruded alloy when galvanic corrosion occurs in the grain boundary. Once the
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large size α-Mg peeled off, fresh α-Mg exposes to solution and form new galvanic
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couples. The peeled α-Mg surrounded by Hanks’ solution, which aggravate the
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corrosion process. Meanwhile, peeled β-(MgZn)3Gd and precipitations are much
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smaller than the coarse grains, which helps to reduce the corrosion rate. Although the pits formation tendency of the as-extruded alloy is higher than that of the as-cast alloy,
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large α-Mg peels off making a more negative effect on corrosion resistance compared to that much smaller size β-(MgZn)3Gd and precipitations fall off making. The
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protective oxidation film of the as-extruded alloy has advantage of the as-cast alloy. Most areas of the as-extruded alloy are protected well in Hanks’ solution. Making
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continuous network compounds β-(MgZn)3Gd may enhance the corrosion resistance of the as-cast GZKM-1 alloy, and reducing particle β-(MgZn)3Gd and precipitations
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may increase the corrosion resistance of the as-extruded GZKM-1 alloy. This may be a further study of the GZKM-1 alloys for biodegradable application.
5. Conclusions The corrosion behaviors of as-cast and as-extruded GZKM-1 alloys in Hanks’ solution were studied in detail from electrochemical analysis to immersion corrosion 19
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test. The conclusions summarized as follows: (1) The corrosion resistance of the as-extruded GZKM-1 alloy is higher than that of the as-cast alloy in terms of the thermodynamics and kinetics and the immersion tests.
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(2) The main corrosion mechanism of the as-cast and as-extruded GZKM-1
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alloys is galvanic corrosion, forming between α-Mg and β-(MgZn)3Gd and/or
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precipitations.
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(3) Large α-Mg peels off during corrosion process and corrosion reaction expands along discontinuous β-(MgZn)3Gd making the peeled α-Mg and flesh α-Mg
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between discontinuous β-(MgZn)3Gd expose in Hanks’ solution, which aggravating corrosion process and making a negative effect on corrosion resistance of the as-cast
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GZKM-1 alloy. Compared to the as-cast alloy, much smaller size β-(MgZn)3Gd and precipitations fall off during corrosion reaction and expand along the distribution of
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particle β-(MgZn)3Gd of the as-extruded alloy. (4) The protective film of as-extruded alloy has advantage than that of the as-cast
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alloy for the reason easily and rapid formation protective layer to some extent. (5) Make more continuous network compounds β-(MgZn)3Gd and reduce particle β-(MgZn)3Gd and precipitations are feasible suggestions for increasing the corrosion resistance of the as-cast and as-extruded GZKM-1 alloys for further studying.
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ACCEPTED MANUSCRIPT Acknowledgement This work was financial supported by the ‘Fabrication of a new generation of medical implant materials and its engineering research’, No.2013440002091065.
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Mg-Nd-Zn-Zr alloys by double extrusion Mater. Sci. Eng. B, 177 (2012), pp. 1113-1119 [41] D. Orlov, K.D. Ralston, N. Birbilis, Y. Estrin Enhanced corrosion resistance of Mg alloy ZK60 after processing by integrated extrusion and equal channel angular pressing Acta Mater., 59 (2011), pp. 6176-6186 [42] W.W. He, E.L. Zhang, K. Yang 26
ACCEPTED MANUSCRIPT Effect of Y on the bio-corrosion behavior of extruded Mg-Zn-Mn alloy in Hank’s solution Mater. Sci. Eng. C, 30 (2010), pp. 167-174 [43] A. Atrens, G.L. Song, M. Liu, Z. Shi, F. Cao, M.S. Dargusch Review of recent developments in the field of magnesium corrosion
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Figure captions Fig. 1 Schematic diagram of hydrogen evolution experiment Fig. 2 Corrosion rate calculated by weight loss and hydrogen evolution of as-cast and hot extruded GZKM-1 alloys immersed in Hanks’ solution for 240 h
time and (b) typical potentiodynamic polarization curves in Hanks’ solution
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Fig. 3 Electronic properties of the as-cast and as-extrude GZKM-1 alloys: (a) changes of OCP with immersion
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Fig. 4 Equivalent circuit as well as EIS and fitting results for the as-cast and as-extruded GZKM-1 alloys in
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Hanks’ solution
Fig. 5 SEM images: (a) and (e) microstructures of the as-cast and extruded alloy respectively; (b) and (f) surface
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morphologies of the as-cast and longitudinal section of hot extruded alloy after immersing in Hanks’ for 240 h; (c-d) amplification of (b) at different positions; (g-h) amplification of (f) at different positions
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Fig. 6 Map scanning of corrosion surface after immersing in Hanks’ solution for 240 h: (a-h) as-cast alloy with complete protective oxidation film; (i-p) as-cast alloy with oxidation film peeled off and (q-x) as-extruded alloy
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Fig. 7 XRD results of corrosion products obtained after immersing in Hanks’ solution for 240 h Fig. 8 Surface morphology with removal of corrosion products after immersion in Hanks’ solution for 240 h: (a-e) the as-cast and (f-j) the extruded GZKM-1 alloy
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Fig. 9 Typical surface morphology of the as-cast and as-extruded alloy immersing in Hanks’ solution for 240 h and 3D measurement of the deepest pits
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Fig. 10 SEM images of surface morphologies of the as-cast alloy in Hanks’ solution: (a-b) immersed for 1 min; (c-d) immersed for 2 min; (e-f) immersed for 10 min; (g-h) immersed for 30 min; (i-j) immersed for 1 h and (k-l) immersed for 5 h Fig. 11 SEM images of surface morphologies of the as-extruded alloy in Hanks’ solution: (a-b) immersed for 1 min; (c-d) immersed for 2 min; (e-f) immersed for 10 min; (g-h) immersed for 30 min; (i-j) immersed for 1 h and (k-l) immersed for 5 h Fig. 12 Potentiodynamic polarization curves in Hanks’ solution (0 h and 15 h)
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Table captions Table 1 Corrosion rate of different Mg-base alloys prepared by various methods calculated by weight loss
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Table 2 Parameters evaluated from polarization curves of the GZKM-1 alloys
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Fig. 1 Schematic diagram of hydrogen evolution experiments
Fig. 2 Corrosion rate calculated by weight loss and hydrogen evolution of as-cast and hot extruded
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GZKM-1 alloys immersed in Hanks’ solution for 240 h
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Fig. 3 Electronic properties of the as-cast and as-extrude GZKM-1 alloys: (a) changes of OCP with
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Fig. 5 SEM images: (a) and (e) microstructures of the as-cast and extruded alloy respectively; (b) and (f)
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Fig. 6 Map scanning of corrosion surface after immersing in Hanks’ solution for 240 h: (a-h) as-cast
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Fig. 7 XRD results of corrosion products obtained after immersing in Hanks’ solution for 240 h
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Fig. 9 Typical surface morphology of the as-cast and as-extruded alloy immersing in Hanks’
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Fig. 11 SEM images of surface morphologies of the as-extruded alloy in Hanks’ solution: (a-b)
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Fig. 13 Schematic of the corrosion mechanism of as-cast (a-e) and as-extruded GZKM-1 alloy (f-j)
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Table 1 Corrosion rate of different Mg-base alloys prepared by various methods calculated by weight loss Alloys
Treatment
Solution
Time
Corrosion rate
(h)
(mm/yr)
Refs
As-cast
Hanks’
240
2.18 ± 0.18
[23]
Mg-3.0Gd-2.7Zn-0.4Zr-0.1Mn
As-extruded
Hanks’
240
0.35 ± 0.19
[23]
AZC611
As-extruded
0.6 M NaCl
72
AZCW6110
As-extruded
0.6 M NaCl
72
AZ61
As-extruded
0.6 M NaCl
AZC610
As-extruded
0.6 M NaCl
AZC611
As-extruded
0.6 M NaCl
Mg-2.33Zn-0.84Gd
As-extruded
Mg-2.33Zn-0.84Gd
As-extruded
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Mg-3.0Gd-2.7Zn-0.4Zr-0.1Mn
[28]
0.31 ± 0.06
[28]
0.71 ±0.04
[29]
72
1.66 ± 0.10
[29]
72
1.84 ± 0.23
[29]
Hanks’
240
0.27
[30]
Hanks’
240
0.20
[30]
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1.84 ± 0.23
T4
0.1 M NaCl
144
1.343
[31]
Mg–6.7Zn–1.3Y–0.6Zr
As-forged
0.1 M NaCl
144
2.182
[31]
Mg-5Li-1Al
As-rolled
3.5 wt.% NaCl
336
~9.44
[32]
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Mg–6.7Zn–1.3Y–0.6Zr
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Note: The results in refs [31-32] displayed in mg/cm2/day. Here the results re-calculated according to formula (5)
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to display in mm/yr. The density of the Mg-based alloys all selected as 1.74 g/cm3.
Table 2 Parameters evaluated from polarization curves of the GZKM-1 alloys
Ecorr
Rp
Pi
Materials
(μA/cm2)
(V)
()
(mm/yr)
As-cast
16.45 ± 10.10
-1.6433 ± 0.0529
2115 ± 407
0. 38 ± 0.23
As-extruded
14.97 ± 3.87
-1.5043 ± 0.0081
2460 ± 326
0.34 ± 0.09
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Graphical abstract
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ACCEPTED MANUSCRIPT Highlights:
Corrosion mechanism of GZKM-1 alloys is galvanic corrosion.
Corrosion expansions of as-cast and as-extruded GZKM-1 alloys are studied. Decrease galvanic corrosion is a goal to reduce corrosion rate in further
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studying.
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Figure 1
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Zhenzhen Gui, Zhixin Kang, Yuanyuan Li PII: DOI: Reference:
S0928-4931(17)34030-4 https://doi.org/10.1016/j.msec.2018.11.037 MSC 9057
To appear in:
Materials Science & Engineering C
Received date: Revised date: Accepted date:
7 October 2017 19 November 2018 24 November 2018
Please cite this article as: Zhenzhen Gui, Zhixin Kang, Yuanyuan Li , Corrosion mechanism of as-cast and as-extruded biodegradable Mg-3.0Gd-2.7Zn-0.4Zr-0.1Mn alloys. Msc (2018), https://doi.org/10.1016/j.msec.2018.11.037
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ACCEPTED MANUSCRIPT Corrosion mechanism of as-cast and as-extruded biodegradable Mg-3.0Gd-2.7Zn-0.4Zr-0.1Mn alloys Zhenzhen Gui, Zhixin Kang*, Yuanyuan Li
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Guangdong Key Laboratory for Advanced Metallic Materials Processing, National Engineering Research Center of Near-Net-Shape Forming for Metallic
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Materials, School of Mechanical and Automotive Engineering, South Clhina
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University of Technology, Guangzhou, China
*Corresponding author, E-mail address: [email protected]
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Tel.:+86 20-87111116 Fax: +86 20-87112111
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Abstract:
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Electrochemical measurements and immersion tests were adopted to investigate
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corrosion mechanism of as-cast and as-extruded Mg-3.0Gd-2.7Zn-0.4Zr-0.1Mn (GZKM-1) alloys. Results of immersion tests indicate that corrosion resistance of the
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as-extruded GZKM-1 alloy is better. Galvanic corrosion between α-Mg and
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β-(MgZn)3Gd compound is the main corrosion mechanism of the two GZKM-1 alloys. Larger α-Mg peeled off from the as-cast samples, then expanded corrosion reaction along discontinuous β-(MgZn)3Gd compound making a pit corrosion phenomenon in macro view. Granular β-(MgZn)3Gd or precipitations fell off from the as-extruded samples, and then expanded corrosion reaction along the concentration of the particle β-(MgZn)3Gd (precipitations). Most areas of the as-extruded alloy are protected well 1
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by denser protective film.
Keywords: Magnesium; Corrosion evolution; Electrochemical properties; Immersion process; Mg-3.0Gd-2.7Zn-0.4Zr-0.1Mn; Hanks’ solution
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1. Introduction
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Magnesium and its alloys show great potential as biodegradable implant
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materials resulting from their biocompatibility [[1], [2], [3]]. Commercial magnesium
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alloys with proper mechanical properties usually contain some damage elements such as Al, which will be harmful to human being as implant materials [[4], [5]]. Then new
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designed magnesium-based (Mg-based) alloys are expected to meet the demands of mechanical and corrosion combination properties for biodegradable application [[6],
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[7], [8], [9], [10]]. With low standard electrode potential, Mg-based alloys exhibit poor corrosion resistance in vivo, which will lead to destroy mechanical integrity
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before healing [[11], [12], [13], [14]]. Poor corrosion resistance limits development of these new designed Mg-based alloys to some extent [[4], [5]]. Extrusion was widely
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adopted to modify the microstructure and the mechanical properties of as-cast biodegradable magnesium alloys [[4], [15], [16], [17]]. However, the effect of extrusion on corrosion behavior of biodegradable Mg-based alloys is controversial. Researchers have different conclusions when they studied different Mg-based biodegradable alloys. Liao [[17]] stated that the corrosion properties of the as-extruded Mg-Al-Mn-Ca 2
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(AMX602) alloy were better than that of the as-cast AMX602 alloy in the aspect of immersion test and the electrochemical analysis. Peng [[18]] found that backward extrusion was one of the effective methods to enhance corrosion properties of the as-cast Mg-Zn alloys. Zhou [[19]] reported that the extruded Mg-2Zn-0.2Mn alloy
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exhibited better corrosion resistance compared with that of the as-cast alloy. However,
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Zhang [[20]] observed that the corrosion resistance of the AZ91 alloy deteriorated
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after extrusion, for the reason of existing dislocation density, twins and grain
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boundaries in the extruded AZ91 alloy. Chen [[21]] confirmed that although the as-cast ZK60 alloy corroded by pitting corrosion, it showed good corrosion resistance
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compared with the as-extruded and T6 treated ZK60 alloys. Jeong [[22]] confirmed that the improvement of corrosion resistance after extrusion in the Mg-Ca alloys with
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Ca ≥ 1%, but it was not obvious in the pure Mg and Mg-0.4 Ca alloy. According to these researches, the effect of extrusion on the corrosion resistance of Mg-based
Our
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alloys was different from the alloy elements and compositions. previous
work
[[23]]
designed
a
new
magnesium
alloy
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Mg-3.0Gd-2.7Zn-0.4Zr-0.1Mn (GZKM-1), and the as-extruded GZKM-1 alloy possessed excellent mechanical and good corrosion properties in Hanks’ solution, showing sound potential for biodegradable implant application. Hence, further detailed studies on corrosion behavior of GZKM-1 alloys are crucial to clarify the mechanism of corrosion process. This is also significant to develop GZKM-1 alloys as biodegradable implant materials. In this paper, corrosion mechanisms of the as-cast 3
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and as-extruded GZKM-1 alloys are studied by electrochemical analysis, immersion test and characteristics of the corrosion process.
2. Materials and Methods
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2.1 Materials and samples preparation
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It is well known that small amounts of Fe can have significant influence on Mg
SC
corrosion properties [[24]], so the amount of Fe in the metal and master alloys should
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be controlled strictly. Commercially high purity Mg (99.95%, wt %), high purity Zn (99.95%), Mg-30%Gd, Mg-30%Zr and Mg-10%Mn master alloys were used to
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prepare the experimental alloy. Semi-continuous casting was applied to obtain as-cast GZKM-1 ingot with a diameter of 100 mm. After casting, a cylindrical with 50 mm in
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height was cut from the ingot and was solid solution treated at 520 °C for 10 hours. After heat treatment, two cylindrical with diameter of 49.5 mm and height of 50 mm
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were cut from the treated cylindrical. Then the treated cylindrical was extruded at 350 °C with an extrusion ratio of 8.4:1 and a ram speed of 4 mm/s. The detailed
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information of GZKM-1 alloys please refers to our previous work [[23] ]. Samples for the electrochemical measurements cut to 15 mm in diameter and 3 mm in thickness, and then ground with serials of grand papers till 2000-grit. Samples for the immersion experiments were prepared with 35 mm in length, 8 mm in width and 5 mm in thickness and then drilled a hole with 2 mm in diameter. Then samples were ground to 2000-grit sand papers. Surface area of each sample obtained from its 4
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initial length, width and thickness.
2.2 Electrochemical measurements and immersion tests Hanks’ solution was used to carry out the electrochemical measurements and
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immersion tests [[23]]. A three-electrode electrochemical cell was utilized to taking
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the open circuit potential (OCP), potentiodynamic polarization (PDP) and
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electrochemical impedance spectroscopy (EIS) experiments on an electrochemical workstation (IM6ex, Zahner) to evaluate the corrosion properties of the alloys. The
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three-electrode electrochemical cell contains a silver/silver chloride (Ag/AgCl,
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saturated KCl) electrode as the reference electrode, a platinum (Pt) mesh as the counter electrode and the specimen under investigation as the working electrode with
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an exposed area of 1 cm2. The OCP reflected the initiation and propagation of
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corrosion process. The OCP tests started immediately after immersing the samples in the Hanks’ solution at 37 ± 0.2 °C, lasting for 1800 seconds to achieve constant
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potential. The polarization curves were acquired at a scanning rate of 1 mV/s. The
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polarization tests of the as-cast and as-extrude GZKM-1 alloys were recorded after immersion for 1800 seconds in Hanks’ solution at 37 ± 0.2 °C. Data were recorded from 100 kHz to 100 mHz with a 5 mV sinusoidal perturbing signal at the open-circuit potential for EIS tests and presented as Nyquist and Bode plots. The EIS tests were carried out without agitation or circulation and without disturbing the corrosion system [[25]]. To ensure the reproducibility and reliability, all 5
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electrochemical measurements were repeated three times and typical curves were given in the results. 1. Immersion tests for calculating weight loss were taken in Hanks’ solution at 37 ± 0.2 °C according to the stand of ASTM G31-72 [[26]]. The prepared samples
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hung at a beaker within 200 ml Hanks’ solution, with a ratio of media (ml)
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volume to specimen surface (cm2) was 20 : 1, utilizing a fine nylon rope through
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the drilled hole. After weight loss experiment, each sample was carried out and
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cleaned in a solution (consisting of 200 g CrO3, 10 g AgNO3 and 1000 ml distilled water) at 80 oC water bath in an ultrasonic cleaner for 5-10 mins,
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followed by washing with distilled water and drying in a stream of warm air. The weight of samples before immersion and after removal of corrosion products
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weighted to an accuracy of 0.1 mg. To investigate the corrosion process of the alloys, real-time immersion tests were carried out in Hanks’ solution at 37 ±
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0.2 °C for 1 min, 2 min, 10 min, 30 min, 1 h and 5 h. The samples were prepared as stated in the former immersion tests. Preparation for hydrogen evolution
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experiment is similar to weight loss test. The detail information of hydrogen evolution experiment carried out as shown in Fig. 1 [[27]]. Corrosion products were determined using X-ray diffraction (XRD, D8 ADVANCE, Bruker, Germany). The XRD experiments were performed with Cu Kα radiation (λ =1.5418 Å), and the step size was 0.02o with a scan rate of 0.1 s/step.
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2.3 Surface morphology of the immersed GZKM-1 alloys The corroded surface morphology of the samples was observed by Leica DVM6 (Germany) and scanning electron microscopy (SEM) (NOVA NANOSEM 430, the
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Netherlands) equipped with energy dispersive X-ray spectrometry (EDS).
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2.4 Corrosion rate calculation of Mg-based alloys in Hanks’ solution
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Corrosion phenomenon requires generation of electrons (anodic reaction), consumption of electrons (cathodic reaction), and a medium for electron transport
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(electrolyte) [[28]]. For magnesium in aqueous medium, electron generation occurs
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predominantly with metal dissolution reaction, which is given by equation (1). Hydrogen evolution is the predominant cathodic reaction, given by equation (2) for
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consumption of electrons. The main chemical reactions of biomedical Mg-based
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alloys in Hanks’ solution are shown as following equations (1-4) [[3], [4]]: (1)
2H2O 2e H 2 2OH (cathodic)
(2)
2H 2O O2 4e 4OH (cathodic)
(3)
2Mg 2 4OH 2Mg(OH) 2 (productio n)
(4)
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Mg Mg 2 2e (anodic)
The hydrogen evolution volume reflects the Mg-ion dissolved in the surface. Hydrogen evolution rate represents corrosion rate of the biomaterial alloys in certain degree. The corrosion rate of the weight loss (CRWL) calculated in mm per year by the weight loss as shown in formula (5) [[29]]: 7
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CRWL
ΔM m 365 24 10 87600 ΔM m (mm/yr) Sm Tm ρ m Sm Tm ρ m
(5)
According to the corrosion reaction depicted as equations (1-4), one molecule of hydrogen evolved for each atom of corroded Mg. One mol (i.e. 24.31 g) of Mg metal
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corrodes for each mol (i.e. 22.4 L) of hydrogen gas produced [[29]]. The corrosion
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rate of the hydrogen evolution (CRHE) calculated according to the evolution volume of hydrogen as shown in formula (6) [[29][29]]:
VH 2
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22.4 S m Tm ρ m
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S m Tm ρ m
(mm/yr )
(6)
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CRHE
24.31 ΔVH 2 365 24 10 -2
where M m (g) is the weight loss of magnesium alloys, Sm (cm2) is the surface
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area of magnesium alloys exposed in Hanks’ solution, ρ m (g/cm3) is the density of the magnesium alloy, Tm (h) is the total immersion time of magnesium alloys in hand and
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VH 2 (ml) is the hydrogen evolution volume.
Extrapolation of Tafel lines is one of the most popular techniques for evaluating
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the corrosion of Mg alloys at present [[29], [30]]. The extrapolation of anodic and/or cathodic Tafel lines for charge transfer controlled reactions gives the corrosion current
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density, icorr, at the corrosion potential, Ecorr [[30]]. With Stern equation, polarization resistance (Rp) was calculated though the slope of anodic and/or cathodic Tafel lines (a, c). To evalute the corrosion rate of Mg-based alloys with the Tafel extrapolation method, the corrosion current density, icorr (mA/cm2) is estimated by Tafel extrapolation of the cathodic branch of the polarization curve, and icorr is related to the average penetration corrosion rate (Pi, mm/yr) using equation formula (7) [[29]]: 8
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Pi = 22.85 icorr
(7)
3. Results 3.1 Results of immersion test
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The corrosion rates calculated by weight loss and hydrogen evolution of as-cast
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and hot extruded GZKM-1 alloys immersed in Hanks’ solution at 37 ± 0.2 °C for 240
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h are shown in Fig. 2. The two calculation methods reveal similar tendency of
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corrosion rate. The corrosion rates calculated by hydrogen evolution of the samples are less than that calculated by weight loss. Because some gas maybe escape from the
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cup or dissolve in the water. The corrosion rate calculated by weight loss is adopted as the standard for evaluating the corrosion resistance of the alloys, while the corrosion
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rate calculated by hydrogen evolution is adopted as a reference and shows the variation of the corrosion rate. The corrosion rate of the hot extruded GZKM-1 alloy
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is less than the as-cast GZKM-1 alloy. The corrosion rate of the hot extruded GZKM-1 calculated by weight loss reaches 0.45 ± 0.14 mm/yr, showing up a well
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corrosion resistance. The corrosion rates (evaluated by weight loss method) of the GZKM-1 alloys and some typical Mg-base alloys prepared by various methods [[31], [32], [35][34], [35]] are listed in table 1. Among all these results, the corrosion rate of the as-extruded GZKM-1 alloy shows competitive advantage.
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3.2 Results of electrochemical properties Fig. 3 (a) shows the OCP curves of the as-cast and as-extruded GZKM-1 alloys immersed in Hanks’ at 37 ± 0.2 °C. The potential of as-extruded GZKM-1 alloy
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increases gradually in the first 400 seconds due to thickening of the protective surface film, then the OCP stays almost the same at about -1.535 V. The OCP of the as-cast
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GZKM-1 alloy increases in the first 150 seconds and then transient decreasing
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appears. After 250 seconds, the OCP of the as-cast GZKM-1 alloy is still increasing
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until reaching 1600 seconds, and then stays at about -1.575 V. The representative results of potentiodynamic polarization tests displayed in Fig. 3 (b). It can be seen that
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the corrosion potential (Ecorr) of the as-extruded GZKM-1 alloy is about 140 mV
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higher than that of the as-cast alloy. The cathodic branch provided an extensive linear
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Tafel region, and the icorr, Ecorr, and Pi values of GZKM-1 alloys evaluated from polarization curves are included in Table 2. The results display that the corrosion
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current density (icorr, μm/cm2), the corrosion potential (Ecorr, V) and the average
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penetration corrosion rate (Pi, mm/yr) of the as-cast and as-extruded alloys are almost at the same level, while the icorr and Pi of the as-cast alloy is a little bit higher than that of the as-extruded alloy. The Ecorr of the as-cast alloy is about 140 mV less than that of the as-extruded alloy. These indicate that the as-extruded GZKM-1 alloy shows a better corrosion resistance in Hanks’ solution compared to the as-cast alloy. Besides, the error bands of icorr, Ecorr and Pi of as-cast GZKM-1 alloy are higher than that of the 10
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as-extruded alloy, which indicates that the corrosion properties of the as-cast alloy show higher dispersion. Fig. 4 shows the EIS results for both the as-cast and as-extruded GZKM-1 alloys in Hanks’ solution. Two capacitive loops exist in the Nyquist plots of GZKM-1 alloys
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as shown in Fig. 4. The higher and medium frequency capacitive loop related to the
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charge transfer resistance and the lower frequency capacitive loop attributed to the
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mass transport in the solid phase arising from diffusion of ions through the corrosion
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product layer [[36], [37]]. Equivalent circuit was used to analysis the EIS results. Parameters of the components in the equivalent circuit were also listed in Fig. 5. The
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Rct was a parameter for evaluating the ability of charge transporting between metal surface and solution. When the surface oxidation film fails to protect the inner metal,
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Rct could be a parameter for evaluating the corrosion process. From the fitting results, the Rct of the extruded alloy is higher than that of the as-cast alloy, obviously.
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3.3 Surface morphology of immersed alloys
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Fig. 5 illustrates the SEM images of the as-cast and as-extruded GZKM-1 alloys: (a) and (e) show the microstructures of the as-cast and extruded alloy respectively. Microstructure of GZKM-1 alloy mainly contained α-Mg matrix and eutectic compounds β-(MgZn)3Gd. The differences of microstructure between the as-cast and as-extrude GZKM-1 alloy are the grain size and the distribution of β-(MgZn)3Gd. The -Mg and phases were pointed out in Fig. 5 (a) and (e). The grain size of the as-cast 11
ACCEPTED MANUSCRIPT alloy is about 200 μm (except some grains are smaller), while the grains are sharply refined to 10 μm (or smaller) after hot extrusion. The presentation of the β-(MgZn)3Gd in the as-cast alloy is netlike at the grain boundaries. While the β-(MgZn)3Gd in the as-extruded alloy is small particles (~ 5 μm) distributed along the
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extrusion direction. Fig. 5 (b) and (f) shows surface morphologies of the as-cast and
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longitudinal section of the extruded alloy after immersing in Hanks’ solution for 240 h.
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Corrosion pits exist in both as-cast and as-extruded alloys. The magnified images (c-d)
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indicate that the probable corrosion process of as-cast alloy containing the protective oxidation film cracks, and the film peels off. Compare to the as-cast alloy, the
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as-extruded alloy exists denser oxidation film as shown in Fig. 5 (g-h). Fig. 6 shows map scanning of corrosion surface after immersing in Hanks’
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solution for 240 h: (a-h) as-cast alloy with complete protective oxidation film; (i-p) as-cast alloy with oxidation film peeled off and (q-x) as-extruded alloy with protective
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oxidation film. The complete protective oxidation film of the as-cast alloy consists of elements: Mg, O, Ca, P, C, Zn and Gd. The element Zn and Gd mainly distributes in
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the grain boundaries. Some big cracks of the oxidation film also start at the grain boundaries as shown in Fig. 6 (a, g-h). Once the oxidation film peeled off, the original microstructure exposure to the Hanks’ solution again as shown in Fig. 6 (i-p). The protective oxidation film of as-extruded alloy contains elements: Mg, O, Ca, P, C and a bit of Zn and Gd. Small amount of Zn and Gd is distribute along extrusion direction as shown in Fig. 6 (w-x), which is similar to the distribution of β-(MgZn)3Gd in the 12
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as-extruded microstructure as shown in refs [[23]].
3.4 Corrosion products of immersed alloys Fig. 7 shows the XRD results of the corrosion products of the as-cast and
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as-extruded GZKM-1 alloys immersing in Hanks’ solution for 240 h. The result
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indicate that the the as-cast alloy containes less corrosion production including MgO,
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Mg(OH)2 and a bit of CaCO3. The as-extruded alloy also containes a little
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hydroxyapatite (HA).
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4. Discussion
4.1 Corrosion evolution of the as-cast and as-extruded alloys
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The two different microstructure characteristics result in two kinds of corrosion
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evolutions. The results of the as-cast alloy and the as-extruded alloy immersed in Hanks’ solution both show pitting and uniform corrosion. Fig. 8 appears corrosion
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morphology with removal of corrosion products of the as-cast (a-e) and as-extruded
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(f-j) GZKM-1 alloys as further evidence. Fig. 8 (a-c) displays the surface of the as-cast alloy, which appears large area of the corrosion trace and most of the surface appears the netlike compounds β-(MgZn)3Gd. The magnified area exhibits two morphologies of the network eutectic compounds in Fig. 8 (d) continuous network (marked as ellipse) and Fig. 8 (e) discontinuous network (marked as rectangle), respectively. The pitting corrosion of the as-extruded alloy seems more severe than 13
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that of the as-cast alloy. The pits mainly exist in edges, and the other areas protected well by the denser oxidation film as shown in Fig. 8 (f). Those serious pits form at the area where was enriched in the particle β-(MgZn)3Gd phase, and the pits expand along the distribution of the β-(MgZn)3Gd phase as shown in Fig. 8 (i-j).
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Fig. 9 displays the macro-morphologies of the GZKM-1 alloys after removal of
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corrosion products and 3D measurement results: (a-b) as-cast alloy and (c-d)
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as-extruded alloy. Fig. 9 (a) appears a large-area corrosion model and Fig. 9 (c) shows
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a local pitting corrosion model. Severe pits of the as-cast and as-extruded alloys always occurred at the edge of the samples. However, 3 D imaging and measurement
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is hard to carry out at the edges of samples. Fig. 9 only presents the typical surface morphologies. The deepest pit in the as-cast alloy and as-extruded alloy is about 420
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μm and 400 μm respectively according to the 3D measurement in Fig. 9 (b) and (d), indicating that a similar corrosion level between the as-cast and as-extruded alloys in
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some severe areas.
Real time corrosion experiments were carried out to observe the initial corrosion
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behavior of the as-cast and as-extruded GZKM-1 alloys. Two kinds of GZKM-1 alloys samples are immersed in Hanks’ solution for 1 min, 2 min, 10 min, 30 min, 1 h and 5 h. Then the surface morphologies are observed by SEM with EDS. Fig. 10 displays the corrosion process of the as-cast alloy: Fig. 10 (a-b) immersed for 1 min; (c-d) immersed for 2 min; (e-f) immersed for 10 min; (g-h) immersed for 30 min; (i-j) immersed for 1 h and (k-l) immersed for 5 h. Once the sample contacts Hanks’ 14
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solution, the alloy undergoes on the equations (1-4) and forms partial protective Mg(OH)2 and evolutes hydrogen. In the first minute, corrosion reaction undergoes on some high activity area of α-Mg and forms very small pits. With the corrosion reaction going on, the pits become more and distribute more intensively. The
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boundaries of grains are high activity areas and easy to form galvanic corrosion.
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These pits expand and crack with Cl- existing in Hanks’ solution. After immersed for
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30 min, some regions form protective oxidation film as shown in Fig. 10 (g-h). The
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protective oxidation films are not stable in the solution with Cl- and hydrogen evolution. Parts of film crack gradually near the grain boundaries for existing galvanic
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couple as shown in Fig. 10 (j). Once the protective films broking and exposing the flesh alloy, the former corrosion reaction recycles with the longer immersion time. It
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means that the film formation and breaking are to take place simultaneously at the initial corrosion stage.
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The initial corrosion area of the as-extruded alloy also begins at high activity region. Some precipitations (such as ZnZr phase) become nucleation particles, and
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NaCl etc. adhere to the precipitations as shown in Fig. 11 (b). With the ions adsorption and accumulation, a protective film covers on the initial Mg(OH)2 film and isolates the alloy from the Hanks’ solution as shown in Fig. 11 (d). The protective film becomes porous and loose because of existing Cl- as shown in Fig. 11 (f). With the immersion time goes on, the porous and loose film breaks, and sever pit corrosion goes on near the particle β-(MgZn)3Gd phase and some precipitations as shown in Fig. 15
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11 (g-l). To indicate the effect of the oxidation film on corrosion process, potentiodynamic polarizations performed after samples immersed in Hanks’ for 15 h as shown in Fig. 12. The corrosion potential (Ecorr, V) of the as-cast and as-extruded
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alloys immersed for 15 h enhance about 110 mV and 35 mV respectively compare to
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the curves immersed for 0 h. This indicates that the partially protective oxidation
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films on the surface of as-cast and as-extruded alloys make the corrosion reaction
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more difficult in terms of thermodynamics. On the polarization curves, the value of pitting potential (Ept) usually indicates the tendency for localized corrosion, and a
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more positive Ept means a less likely localized corrosion [[38], [39]]. After immersing in Hanks’ solution for 15 h, the Ept of the as-cast alloy enhances about 240 mV,
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indicating that the oxidation film of the as-cast alloy shows a positive protective and reduces the tendency of pitting of the as-cast alloy. Note that the as-cast alloy shows a
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higher Ept than that of the as-extruded alloy after immersing for 15 h. This indicates that the as-extruded alloy appears a high tendency of pitting after the oxidation film
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formed. The corrosion current density (icorr) obtained by Tafel extrapolation method displays that icorr (as-extruded, 15 h)> icorr (as- extruded, 0 h) >icorr (as-cast, 15 h)> icorr (as-cast, 0 h). This indicates that the oxidation film of as-extruded alloy immersed for 15 h shows a positive protection, as the corrosion current density (icorr) is a kinetic parameter that signifies the reaction rate.
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4.2 Corrosion mechanism of the as-cast and as-extruded alloys In a summary, the corrosion models of the as-cast and as-extrude GZKM-1 alloys exhibit in Fig. 13: (a-e) as-cast alloy and (f-j) as-extruded alloy. The as-cast alloy forms partial protective Mg(OH)2 and evolutes hydrogen as shown in Fig. 13 (a).
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Corrosion reaction mainly undergoes on some high activity area of α-Mg and forms
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very small pits. The boundaries of grains are high activity areas. With corrosion
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process, the corrosion product forms and accumulates. Then the formation and
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breaking of protective film is to achieve dynamic equilibrium as shown in Fig. 13 (b). α-Mg and β-(MgZn)3Gd form galvanic corrosion near boundaries as shown in Fig. 13
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(c). Then α-Mg peeled off form network β-(MgZn)3Gd with corrosion aggregating.
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The β-(MgZn)3Gd contains two kinds: the continuous network compounds (like the
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ellipse area in Fig. 8(e)) and discontinuous network compounds (like the rectangle in Fig. 8(e)). The continuous network compounds block the expanding of corrosion
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reaction. The discontinuous network compounds block the expanding of corrosion
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reaction to some extent. However, the broken network part will expose the flesh α-Mg, which will form galvanic corrosion and aggregate the tendency of pitting as shown in Fig. 13 (d). Once α-Mg peeled off form network β-(MgZn)3Gd, corrosion expands along the discontinuous network β-(MgZn)3Gd to form pits as shown in Fig. 13 (e). The as-extruded alloy processes refined grains and flowing texture structure, and detailed microstructure please refers our previous work [[23]]. Corrosion reaction 17
ACCEPTED MANUSCRIPT mainly undergoes on some high activity area of α-Mg and the grain boundaries with form protective oxidation film and corrosion products as shown in Fig. 13 (f). Then the formation and damage of protective film is to achieve dynamic equilibrium. The particle β-(MgZn)3Gd failed to block the expanding of corrosion. On the contrary,
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β-(MgZn)3Gd and α-Mg forms micro-galvanic couple to accelerate corrosion reaction
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as shown in Fig. 13 (g). The surround α-Mg of β-(MgZn)3Gd is eroded to form hole,
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resulting in β-(MgZn)3Gd and other precipitations fall off as shown in Fig. 13 (h). The
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corrosion reaction tends to expand along the distribution and concentration of β-(MgZn)3Gd and precipitations and forms pits as shown in Fig. 13 (i-j). The most
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area of as-extruded alloy were protected well for the following reasons. Increased reactivity coupled with more sites for nucleation of an oxide film on the surface of
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grain-refined materials is posited to result in a more rapid formation of a protective layer [[23], [40]]. For Mg-based alloys, the oxide film may be MgO and RE2O3 [[25],
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[41], [42]]. The attack rate of the protective layer is controlled by the diffusion of the reactants through the surface film, and it follows that process-induced microstructure
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changes may influence this diffusion-controlled process [[23]]. The corrosion current density (icorr) obtained from potentiodynamic polarizations (0 h, 15 h) indicates that the protective film of the as-extruded alloy has more advantage than that of the as-cast alloy. Micro-galvanic interaction between the -Mg and second phases causes the corrosion of Mg alloys [[43]]. Galvanic corrosion (α-Mg, β/precipitations) is the main 18
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corrosion mechanism of the as-cast and as-extruded GZKM-1 alloys. The biggest difference is that the initial coarse grains (α-Mg) peel from the samples of the as-cast alloy, while the particle β-(MgZn)3Gd and precipitations fall off from the samples of the as-extruded alloy when galvanic corrosion occurs in the grain boundary. Once the
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large size α-Mg peeled off, fresh α-Mg exposes to solution and form new galvanic
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couples. The peeled α-Mg surrounded by Hanks’ solution, which aggravate the
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corrosion process. Meanwhile, peeled β-(MgZn)3Gd and precipitations are much
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smaller than the coarse grains, which helps to reduce the corrosion rate. Although the pits formation tendency of the as-extruded alloy is higher than that of the as-cast alloy,
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large α-Mg peels off making a more negative effect on corrosion resistance compared to that much smaller size β-(MgZn)3Gd and precipitations fall off making. The
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protective oxidation film of the as-extruded alloy has advantage of the as-cast alloy. Most areas of the as-extruded alloy are protected well in Hanks’ solution. Making
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continuous network compounds β-(MgZn)3Gd may enhance the corrosion resistance of the as-cast GZKM-1 alloy, and reducing particle β-(MgZn)3Gd and precipitations
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may increase the corrosion resistance of the as-extruded GZKM-1 alloy. This may be a further study of the GZKM-1 alloys for biodegradable application.
5. Conclusions The corrosion behaviors of as-cast and as-extruded GZKM-1 alloys in Hanks’ solution were studied in detail from electrochemical analysis to immersion corrosion 19
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test. The conclusions summarized as follows: (1) The corrosion resistance of the as-extruded GZKM-1 alloy is higher than that of the as-cast alloy in terms of the thermodynamics and kinetics and the immersion tests.
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(2) The main corrosion mechanism of the as-cast and as-extruded GZKM-1
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alloys is galvanic corrosion, forming between α-Mg and β-(MgZn)3Gd and/or
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precipitations.
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(3) Large α-Mg peels off during corrosion process and corrosion reaction expands along discontinuous β-(MgZn)3Gd making the peeled α-Mg and flesh α-Mg
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between discontinuous β-(MgZn)3Gd expose in Hanks’ solution, which aggravating corrosion process and making a negative effect on corrosion resistance of the as-cast
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GZKM-1 alloy. Compared to the as-cast alloy, much smaller size β-(MgZn)3Gd and precipitations fall off during corrosion reaction and expand along the distribution of
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particle β-(MgZn)3Gd of the as-extruded alloy. (4) The protective film of as-extruded alloy has advantage than that of the as-cast
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alloy for the reason easily and rapid formation protective layer to some extent. (5) Make more continuous network compounds β-(MgZn)3Gd and reduce particle β-(MgZn)3Gd and precipitations are feasible suggestions for increasing the corrosion resistance of the as-cast and as-extruded GZKM-1 alloys for further studying.
20
ACCEPTED MANUSCRIPT Acknowledgement This work was financial supported by the ‘Fabrication of a new generation of medical implant materials and its engineering research’, No.2013440002091065.
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References
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[1] F. Witte The history of biodegradable magnesium implants: A review
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Acta Biomater., 6 (2010), pp. 1680-1692
[2] X. Li, X.M. Liu, S.L. Wu, K.W.K. Yeung, Y.F. Zheng, P. K. Chu
From bulk to surface
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Acta Biomater., 45 (2016), pp. 2-30
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Design of magnesium alloys with controllable degradation for biomedical implants:
[3] Y.Q. Chen, W.T. Zhang, M.F. Maitz, M.Y. Chen, H. Zhang, J.L. Mao, Y.C. Zhao, N. Huang,
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G.J. Wan
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Comparative corrosion behavior of Zn with Fe and Mg in the course of immersion degradation in phosphate buffered saline
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Corros. Sci., 111 (2016), pp. 541-555 [4] Y.F. Zheng, X.N. Gu, F. Witte
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Biodegradable metals
Mater. Sci. Eng. R, 77 (2014), pp. 1-34 [5] Y.J. Chen, Z.G. Xu, C. Smith, J. Sankar Recent advances on the development of magnesium alloys for biodegradable implants Acta Biomater., 10 (2014), pp. 4561-4573 [6] X.B. Zhang, G.Y. Yuan, L. Mao, J.L. Niu, W.J. Ding Biocorrosion properties of as-extruded Mg-Nd-Zn-Zr alloy compared with commercial AZ31 and WE43 alloys 21
ACCEPTED MANUSCRIPT Mater. Lett., 66 (2012), pp. 209-211 [7] Z.J. Li, X.N. Gu, S.Q. Lou, Y.F. Zheng The development of binary Mg-Ca alloys for use as biodegradable materials within bone Biomaterials, 29 (2008), pp. 1329-1344
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[8] H. Li, Q.M. Peng, X.J. Li, K. Li, Z.S. Han, D.Q. Fang Microstructures, mechanical and cytocompatibility of degradable Mg-Zn based
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Corrosion of magnesium and its alloys
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ACCEPTED MANUSCRIPT Corrosion behavior of magnesium and magnesium-hydroxyapatite composite fabricated by friction stir processing in Dulbecco’ sphosphate buffered saline Corros. Sci., 104 (2016), pp. 319-329 [15] C.Y. Zhao, F.S. Pan, L. Zhang, H.C. Pan, K. Song, A.T. Tang Microstructure, mechanical properties, bio-corrosion properties and cytotoxicity of
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[16] H. Ha, H.J. Kim, S. Baek, B Kim, S Sohn, H Shin, H.Y. Jeong, S.H. Park, C.D. Yim, B.S.
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[19] S.J. Zhou, L. Liu, N. Huang Effect of Extrusion Ratio on Microstructure, Mechanical and Degradation Properties of Mg-2Zn-0.2Mn Biomedical Alloy Integr. Ferroelectr., 154 (2014), pp. 166-174 [20] T. Zhang, Y.W. Shao, G.Z. Meng, Z.Y. Cui, F.H. Wang Corrosion of hot extrusion AZ91 magnesium alloy: I-relation between the microstructure and corrosion behavior 23
ACCEPTED MANUSCRIPT Corros. Sci., 53 (2011), pp. 1960-1968 [21] J.X. Chen, L.L. Tan, K. Yang Effect of heat treatment on mechanical and biodegradable properties of an extruded ZK60 alloy Bioact. Mater., 2 (2017), pp. 19-26
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[22] Y.S. Jeong, W.J. Kim Enhancement of mechanical properties and corrosion resistance of Mg-Ca alloys
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[26] American Society for Testing and Materials ASTM-G31-72: standard practice for laboratory immersion corrosion testing of metals In Annual Book of ASTM Standards, ASTM, Philadelphia, PA, USA, 2004 [27] G.L. Song, A. Atrens, D. St.John An hydrogen evolution method for the estimation of the corrosion rate of magnesium alloys Magnesium Technology, (2001), pp. 255-262 24
ACCEPTED MANUSCRIPT [28] G.R. Argade, K. Kandasamy, S.K. Panigrahi, R.S. Mishra Corrosion behavior of a friction stir processed rare-earth added magnesium alloy Corros. Sci., 58 (2012), pp. 321-326 [29] Z.M. Shi, M. Liu, A. Atrens Measurement of the corrosion rate of magnesium alloys using Tafel extrapolation
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Effect of alloyed Ca on the microstructure and corrosion properties of extruded AZ61 Mg alloy
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[33] H.W. Miao, H. Huang, Y.J. Shi, H. Zhang, J. Pei, G.Y. Yuan Effects of solution treatment before extrusion on the microstructure, mechanical
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ACCEPTED MANUSCRIPT Effect of rolling-induced microstructure on corrosion behaviour of an as-extruded Mg-5Li-1Al alloy sheet Corros. Sci., 119 (2017), pp. 14-22 [36] M.I. Jamesh, G. Wu, Y. Zhao, D.R. McKenzie, M.M.M. Bilek, and P.K. Chu Electrochemical corrosion behavior of biodegradable Mg-Y-RE and Mg-Zn-Zr alloys in
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conversion coating containing Ni2+ and Co2+
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[40] X.B. Zhang, Z.Z. Wang, G.Y. Yuan, Y.J. Xue Improvement of mechanical properties and corrosion resistance of biodegradable
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Mg-Nd-Zn-Zr alloys by double extrusion Mater. Sci. Eng. B, 177 (2012), pp. 1113-1119 [41] D. Orlov, K.D. Ralston, N. Birbilis, Y. Estrin Enhanced corrosion resistance of Mg alloy ZK60 after processing by integrated extrusion and equal channel angular pressing Acta Mater., 59 (2011), pp. 6176-6186 [42] W.W. He, E.L. Zhang, K. Yang 26
ACCEPTED MANUSCRIPT Effect of Y on the bio-corrosion behavior of extruded Mg-Zn-Mn alloy in Hank’s solution Mater. Sci. Eng. C, 30 (2010), pp. 167-174 [43] A. Atrens, G.L. Song, M. Liu, Z. Shi, F. Cao, M.S. Dargusch Review of recent developments in the field of magnesium corrosion
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Figure captions Fig. 1 Schematic diagram of hydrogen evolution experiment Fig. 2 Corrosion rate calculated by weight loss and hydrogen evolution of as-cast and hot extruded GZKM-1 alloys immersed in Hanks’ solution for 240 h
time and (b) typical potentiodynamic polarization curves in Hanks’ solution
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Fig. 3 Electronic properties of the as-cast and as-extrude GZKM-1 alloys: (a) changes of OCP with immersion
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morphologies of the as-cast and longitudinal section of hot extruded alloy after immersing in Hanks’ for 240 h; (c-d) amplification of (b) at different positions; (g-h) amplification of (f) at different positions
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Fig. 6 Map scanning of corrosion surface after immersing in Hanks’ solution for 240 h: (a-h) as-cast alloy with complete protective oxidation film; (i-p) as-cast alloy with oxidation film peeled off and (q-x) as-extruded alloy
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Fig. 7 XRD results of corrosion products obtained after immersing in Hanks’ solution for 240 h Fig. 8 Surface morphology with removal of corrosion products after immersion in Hanks’ solution for 240 h: (a-e) the as-cast and (f-j) the extruded GZKM-1 alloy
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Fig. 9 Typical surface morphology of the as-cast and as-extruded alloy immersing in Hanks’ solution for 240 h and 3D measurement of the deepest pits
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Fig. 10 SEM images of surface morphologies of the as-cast alloy in Hanks’ solution: (a-b) immersed for 1 min; (c-d) immersed for 2 min; (e-f) immersed for 10 min; (g-h) immersed for 30 min; (i-j) immersed for 1 h and (k-l) immersed for 5 h Fig. 11 SEM images of surface morphologies of the as-extruded alloy in Hanks’ solution: (a-b) immersed for 1 min; (c-d) immersed for 2 min; (e-f) immersed for 10 min; (g-h) immersed for 30 min; (i-j) immersed for 1 h and (k-l) immersed for 5 h Fig. 12 Potentiodynamic polarization curves in Hanks’ solution (0 h and 15 h)
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Table captions Table 1 Corrosion rate of different Mg-base alloys prepared by various methods calculated by weight loss
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Table 2 Parameters evaluated from polarization curves of the GZKM-1 alloys
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Fig. 1 Schematic diagram of hydrogen evolution experiments
Fig. 2 Corrosion rate calculated by weight loss and hydrogen evolution of as-cast and hot extruded
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Fig. 3 Electronic properties of the as-cast and as-extrude GZKM-1 alloys: (a) changes of OCP with
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Fig. 5 SEM images: (a) and (e) microstructures of the as-cast and extruded alloy respectively; (b) and (f)
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Fig. 6 Map scanning of corrosion surface after immersing in Hanks’ solution for 240 h: (a-h) as-cast
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Fig. 7 XRD results of corrosion products obtained after immersing in Hanks’ solution for 240 h
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Fig. 8 Surface morphology with removal of corrosion products after immersion in Hanks’ solution
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Fig. 10 SEM images of surface morphologies of the as-cast alloy in Hanks’ solution: (a-b) immersed for 1 min; (c-d) immersed for 2 min; (e-f) immersed for 10 min; (g-h) immersed for 30 min; (i-j) immersed for 1 h and (k-l) immersed for 5 h
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Fig. 11 SEM images of surface morphologies of the as-extruded alloy in Hanks’ solution: (a-b)
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Fig. 13 Schematic of the corrosion mechanism of as-cast (a-e) and as-extruded GZKM-1 alloy (f-j)
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Table 1 Corrosion rate of different Mg-base alloys prepared by various methods calculated by weight loss Alloys
Treatment
Solution
Time
Corrosion rate
(h)
(mm/yr)
Refs
As-cast
Hanks’
240
2.18 ± 0.18
[23]
Mg-3.0Gd-2.7Zn-0.4Zr-0.1Mn
As-extruded
Hanks’
240
0.35 ± 0.19
[23]
AZC611
As-extruded
0.6 M NaCl
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AZCW6110
As-extruded
0.6 M NaCl
72
AZ61
As-extruded
0.6 M NaCl
AZC610
As-extruded
0.6 M NaCl
AZC611
As-extruded
0.6 M NaCl
Mg-2.33Zn-0.84Gd
As-extruded
Mg-2.33Zn-0.84Gd
As-extruded
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Mg-3.0Gd-2.7Zn-0.4Zr-0.1Mn
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0.31 ± 0.06
[28]
0.71 ±0.04
[29]
72
1.66 ± 0.10
[29]
72
1.84 ± 0.23
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Hanks’
240
0.27
[30]
Hanks’
240
0.20
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1.84 ± 0.23
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0.1 M NaCl
144
1.343
[31]
Mg–6.7Zn–1.3Y–0.6Zr
As-forged
0.1 M NaCl
144
2.182
[31]
Mg-5Li-1Al
As-rolled
3.5 wt.% NaCl
336
~9.44
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Mg–6.7Zn–1.3Y–0.6Zr
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Table 2 Parameters evaluated from polarization curves of the GZKM-1 alloys
Ecorr
Rp
Pi
Materials
(μA/cm2)
(V)
()
(mm/yr)
As-cast
16.45 ± 10.10
-1.6433 ± 0.0529
2115 ± 407
0. 38 ± 0.23
As-extruded
14.97 ± 3.87
-1.5043 ± 0.0081
2460 ± 326
0.34 ± 0.09
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Graphical abstract
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ACCEPTED MANUSCRIPT Highlights:
Corrosion mechanism of GZKM-1 alloys is galvanic corrosion.
Corrosion expansions of as-cast and as-extruded GZKM-1 alloys are studied. Decrease galvanic corrosion is a goal to reduce corrosion rate in further
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Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
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Figure 10
Figure 11
Figure 12
Figure 13