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PEO coating with Ce-sealing for corrosion protection of LPSO Mg–Y–Zn alloy
Surface & Coatings Technology 383 (2020) 125253
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PEO coating with Ce-sealing for corrosion protection of LPSO Mg–Y–Zn alloy a,⁎
b
a
a
b
M. Mohedano , P. Pérez , E. Matykina , B. Pillado , G. Garcés , R. Arrabal a b
a
T
Departamento de Ingeniería Química y de Materiales, Facultad de Ciencias Químicas, Universidad Complutense, 28040 Madrid, Spain Departamento de Metalurgia Física, CENIM-CSIC, Avenida Gregorio del Amo 8, E-28040 Madrid, Spain
ARTICLE INFO
ABSTRACT
Keywords: LPSO Mg-Y-Zn magnesium alloy Plasma electrolytic oxidation Protective coating Corrosion Ce-based sealing
Plasma Electrolytic Oxidation coatings (PEO), without and with Ce-based sealings, were developed on a Mg–Y–Zn alloy containing long period stacking order (LPSO) phases in order to improve its corrosion behaviour. Specimens were investigated in terms of morphology, composition and corrosion behaviour (polarization and hydrogen evolution measurements). PEO improved the corrosion resistance compared to the bulk material. Sealing for 5 min plugged the coating pores and cracks with Ce-rich compounds, resulting in improved long-term corrosion behaviour. Sealing for 1 h resulted in excessive dissolution of the coating and deterioration of the inner barrier layer.
1. Introduction Magnesium alloys with long period stacking ordered (LPSO) phases have been recently developed to improve the low strength of conventional Mg-based materials at room and elevated temperatures [1]. An LPSO phase can be described as a long range stacking of basal planes with periodic enrichment of transition metals and Y or RE elements. Several variants of the Mg hexagonal close-packed structure (rhombohedral and hexagonal Bravais lattices) have been reported for LPSO phases depending on the stacking sequence of the close-packed atomic layers [2–4]. It is believed that LPSO phases behave as alloy-strengthening components due to their high Young's modulus and promote the grain refinement effect during dynamic recrystallization [5]. Among LPSO Mg alloys, particular attention has been drawn to the Mg–Y–Zn system due to its superior mechanical properties, which can be tailored through alloy design [6–8] and thermomechanical treatments [1,6,9–13]. Accordingly, most of the studies have focused on microstructure development and deformation mechanisms with the aim of improving the mechanical properties of these alloys (e.g. tensile strength, creep resistance, fracture toughness and fatigue strength) [14]. However, the corrosion behaviour and suitability for surface treatment of LPSO Mg alloys are much less studied fields, with only a few works recently published. Previous studies have shown that the distribution and volume fraction of LPSO phases play an important role in corrosion related phenomena. Zhang [15] reported that Mg–Zn–Y cast alloys with a continuous distribution of LPSO phase show higher corrosion resistance than those with a discontinuous network. Li [16] reported that the
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LPSO phase could serve as a cathode, significantly accelerating the degradation of the α-Mg matrix for high volume fractions of LPSO phases. Results obtained by Perez [17] revealed that the orientation of the LPSO phase also influences the corrosion performance with the longitudinal section revealing worse corrosion resistance than the transversal one. Up to date, there is no simple alloying or casting/forging strategy capable of reducing the corrosion rate of LPSO Mg alloys down to the values usually reported for conventional AZ alloys (0.1–15 mm/year) [18]. This brings up the necessity for surface treatment in order to exploit the outstanding mechanical properties of these alloys. Surface modification is an effective way for improving the corrosion resistance of Mg-based materials. However, this strategy has not been explored so far for LPSO Mg alloys. Among the different techniques that have been applied to Mg and its alloys (e.g., conversion treatments, solgel, gas-phase deposition processes, laser surface melting and organic/ polymer coatings), plasma electrolytic oxidation (PEO) has shown to improve both corrosion and wear resistances [19–24]. PEO involves anodic polarization up to voltages above dielectric breakdown, resulting in short-lived microdischarges and formation of ceramic-like coatings. PEO coatings are typically hard and enriched in substrate- and electrolyte-derived constituents that form crystalline and amorphous phases [25]. Coatings normally exhibit a two or three-layered microstructure comprising a nanometer barrier layer close to the substrate, an outer layer with variable porosity and, in some cases, a dense intermediate layer. Sealing of the outer porous part is required when PEO coatings are exposed to highly corrosive media. Various post-treatments have been
Corresponding author. E-mail address: [email protected] (M. Mohedano).
https://doi.org/10.1016/j.surfcoat.2019.125253 Received 8 October 2019; Received in revised form 2 December 2019; Accepted 11 December 2019 Available online 12 December 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 383 (2020) 125253
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phosphate-cerium based solution led to a decrease in the corrosion resistance due to the dissolution of the PEO film during the sealing treatment. In the present study, PEO coatings sealed in cerium nitrate solution are developed and characterized for the first time on an LPSO Mg alloy. The corrosion behaviour was evaluated in a NaCl solution by electrochemical and hydrogen measurements. The influence of the Ce-based sealing duration on the corrosion resistance was also investigated.
Table 1 Composition in weight percent of the Mg-Y-Zn alloy determined by X-ray fluorescence analysis. Element
Y
Zn
Fe
Mn
Mg
wt%
6.8 ± 0.1
2.2 ± 0.1
0.02 ± 0.002
< 0.01
Bal.
evaluated in the past, most of them adapted from sealing processes used in conventional anodizing [21,26–28]. Among them, sealings in Cecontaining solutions are quite promising due to the efficient blockage of the coating pores and defects by Ce-rich hydroxides and oxides. For instance, Lim [29] and Laleh [30] reported an improvement in the corrosion resistance of PEO coated AZ31 alloy after post-treatment in cerium chloride solutions. Similarly, Mingo [21] and Sun [27] demonstrated the superior corrosion performance of PEO coatings sealed in cerium nitrate solutions when compared to other sealing procedures. Author's previous work [31] revealed that the corrosion performance of PEO coated specimens increases with increasing Ce concentration and sealing time due to a higher accumulation of Ce products within coating pores and cracks. However, this is not always the case as reported by Phuong [32], who found that long post-treatments in a
2. Material and methods 2.1. Material The LPSO Mg–Y–Zn alloy was cast in a resistance melting furnace and extruded at 400 °C employing an extrusion ratio of 18:1 and a rectangular profile of 4 × 20 mm2. Specimens of size 30 × 20 × 3 mm3 were ground successively with silicon carbide papers to P1200 grit size for a final roughness of Sa 0.42 ± 0.02 μm and cleaned with iso-propanol prior to corrosion measurements and PEO treatment. The composition of the alloy is given in Table 1.
Fig. 1. (a) 3D SEM reconstruction and (b, c) higher magnification micrographs along the extrusion direction of the Mg-Y-Zn alloy. 2
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Fig. 2. Surface potential maps and potential profiles of the (a,b) LPSO phase and (c,d) Y-rich inclusion.
2.2. Plasma electrolytic oxidation and Ce-sealing
were prepared by grinding through successive grades of silicon carbide paper, with final polishing to 1 μm diamond finish. Image analysis of coatings plan views was performed with ImageJ software in three different SEM micrographs. Volta potential maps were obtained with a Nanoscope IIIA MultiMode scanning kelvin force microscope (SKPFM) operating with a 100 nm tip-to-sample distance. Temperature and relative humidity were kept at 20–23 °C and 40–65%, respectively. Phase composition was examined by X-ray diffraction (XRD) using a Philips X'Pert diffractometer (Cu Kα = 1.54056 Å). The XRD patterns were recorded using Bragg-Brentano geometry (0.01° per second, 2θ range from 10° to 90°) and grazing angle of 0.5° (2θ range from 20° and 45°).
Specimens were masked with Lacquer 45 resin (McDermid plc.) to isolate a working area of ~3 cm2. PEO treatments were conducted parallel to the extrusion direction in an alkaline silicate-based electrolyte (0.086 mol/L sodium silicate, 0.15 mol/L sodium hydroxide and 0.046 mol/L sodium fluoride) at (20 ± 1) °C using a voltage-controlled 2 kW power supply (EAC-S2000, ET Systems electronic) and a 2 L double jacketed cell with a stainless steel (AISI 316) counter-electrode. The electrical parameters were as follows: 60 s ramp, 400 V/−30 V square signal, 500 Hz and 400 mA cm−2 rms current density limit. Two sealing post-treatments were conducted for 5 and 60 min at (30 ± 1) °C by immersion in 10 g/L cerium(III) nitrate hexahydrate, 0.3 g/L hydrogen peroxide and 1 g/L boric acid aqueous solution. Hydrogen peroxide and boric acid were used as accelerators [33,34]. Previous experiments by the authors revealed that short sealing times (< 10 min) yielded better results for this alloy system than longer ones (> 40 min). The selected times, 5 and 60 min, were chosen as representative extremes. The sealing solution was prepared using high purity chemicals and magnetic stirring for at least 30 min. A total volume of 250 mL of sealing solution, with a very low stirring rate, was used for treating batches of 3 specimens. Fresh solutions were prepared after treatment of 15 specimens. After sealing the specimens were dried in warm air.
2.4. Corrosion tests Hydrogen evolution tests were carried out by triplicate in 0.5 wt% NaCl solution at room temperature, (22 ± 2) °C, during 15 days of immersion. Details of the procedure, design and the relationship between the volume of hydrogen and the mass loss of the specimen can be found elsewhere [35]. Potentiodynamic polarization measurements were performed in naturally aerated 0.5 wt% NaCl solution at room temperature by using a computer-controlled potentiostat (GillAC, ACM Instruments) and a three-electrode cell with a graphite counter-electrode and a silver-silver chloride 3 M KCl reference electrode. The cathodic and anodic branches were obtained separately after 1 h of immersion at 0.3 mVs−1 from the OCP (open circuit potential) to −100 mV and + 3000 mV, respectively, with a 5 mA cm−2 current limit.
2.3. Characterization Specimens were examined by scanning electron microscopy (SEM) using a JEOL JSM-6400 instrument equipped with Oxford Link energydispersive X-ray (EDS) microanalysis hardware. Coatings cross-sections
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Fig. 3. Plan view SEM micrographs of the coatings: (a,b) PEO, (c,d) PEO_SCe_5 min and (e,f) PEO_SCe_1h.
Table 2 Surface characteristics of the studied coatings.
PEO PEO_SCe_5min PEO_SCe_1h
Surface porosity (%)
Pore size (μm)
Number of pores, (Pores μm−2)
1.9 ± 1.0 0.8 ± 0.2 1.8 ± 0.9
0.38 ± 0.05 0.19 ± 0.02 0.28 ± 0.09
(4.29 ± 0.09) × 10−02 (0.38 ± 0.09) × 10−02 (4.1 ± 0.1) × 10−02
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Fig. 4. Cross-section examination of the unsealed PEO coating. (a,b) BSE micrographs and (c, d, e, f) X-ray elemental maps.
3. Results
shaped Y-rich intermetallics (46.5Al, 0.9Zn, 52.6Y, in at.%) are also observed (Fig. 1c). Recrystallized fine α-Mg grains (~1 μm in diameter and ~60% area fraction) are the result of partial dynamic recrystallization during high temperature processing [36]. 3D characterization of this alloy in a previous study revealed that the volume fraction of the LPSO phase is close to 25% and that strings of this phase are composed of independent fibers of 200–300 μm in length [17]. Previous studies have pointed out the cathodic behaviour of LPSO phases based on the preferential corrosion of the surrounding α-Mg matrix, particularly for lamellar LPSO phases [16,37], but only a few
3.1. Characterization 3.1.1. Mg–Y–Zn alloy Fig. 1 shows the 3D SEM reconstruction and higher magnification micrographs along the extrusion direction for the Mg–Y–Zn alloy. The material shows fully recrystallized α-Mg grains surrounded by coarse and elongated α-Mg grains (98.3Al, 0.6Zn, 1.1 Y, in at.%) and bright contrast strings of LPSO phases (95.3Al, 2.1Zn, 2.6Y, in at.%). Cuboid-
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Fig. 5. Cross section examination of the PEO_Ce_5min coating. (a,b) BSE micrographs and (c, d) X-ray elemental maps.
works have presented SKPFM results [38]. Our results show Volta potential differences of the order of ~60 mV, confirming the cathodic activity of the LPSO phase (Fig. 2 a,b). This value is much lower than that reported by Li [16] in a Mg–Zn–Y cast alloy (250 mV). The authors believe the latter result to be misleading as it was determined right at the LPSO/α-Mg interface, where SKPFM artefacts are quite common [39]. Y-rich cuboid intermetallics reveal higher potential values (~140 mV) than the LPSO phase (Fig. 2c,d). This might be due to the higher Y + Zn/Mg ratio (calculated based on at.%) of cuboids (~1.15) compared with that of LPSO phase (~0.05) which leads to a more noble reduction potential [40,41].
The cross-sections of the unsealed PEO layer along with the Si, F, O and Y X-ray elemental maps are shown in Fig. 4. Backscattered scanning electron (BSE) micrographs (Fig. 4a and b) reveal a coating with a thickness of (5.4 ± 1.2) μm and a typical three-layered structure [25,44–46], consisting of an outer part with through-going discharge channels, an intermediate section with smaller pores and a compact inner barrier layer (< 500 nm). X-ray elemental maps show Si species homogeneously distributed throughout the coating thickness (Fig. 4c) and a F-rich inner region (Fig. 4d), indicating a rapid inward migration of fluoride ions during the oxide formation [47]. LPSO phases are oxidized and incorporated into the coating in a localized manner, resulting in Y-rich regions. These regions did not significantly affect neither the coating thickness nor the morphology. This behaviour is quite different from that recently reported by Martin et al. [48]. The differences might be ascribed to the different anodizing conditions and composition of secondary phases. Fig. 5 and Fig. 6 show significant differences between the 5 min and 1 h sealed PEO coatings. After 5 min, the PEO layer does not undergo significant changes and, as marked by the brighter areas of the BSE micrograph and X-ray elemental maps, Ce-rich compounds are preferentially deposited within the crater-like pores [31] (Fig. 5a). The BSE micrograph at higher magnification (Fig. 5b) and the distribution of F (Fig. 5d) suggest that the barrier inner layer remains intact and, therefore, the anticorrosion properties ascribed to this inner layer are not compromised. Sealing for 1 h (Fig. 6), on the other hand, leads to
3.1.2. PEO coatings Surface morphologies of the PEO coated specimens before and after sealing are shown in Fig. 3. The unsealed coating reveals a crater-like morphology with microcracks and micropores located at the sites of the discharge channels due to residual stresses and gas evolution during the coating formation [42] (Fig. 3a and b). After post-treatment, a Ce-rich superficial layer is formed, mimicking the morphology of the PEO coating and decreasing the number and size of open pores (Fig. 3c, d, e and f). Cracks observed in the Ce-rich layer may be associated with its dehydration (Fig. 3d and f) [43]. The best sealing properties, based on surface porosity, pore size and number of pores, are obtained for the 5 min post-treatment (Table 2).
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Fig. 6. Cross section examination of the PEO_Ce_1h coating. (a,b) BSE micrographs and (c, d) X-ray elemental maps.
important changes in the PEO coating. These are summarized as follows; i) decrease in the coating thickness up to 40% in some areas; ii) visible large pores through the film thickness; and iii) deterioration of the barrier layer which becomes hardly visible (Fig. 6b). Indeed, the Xray map shows a depletion in F in the inner region (Fig. 6d) indicating that the barrier layer has been partially dissolved. The XRD pattern of the unsealed PEO coating obtained at BraggBrentano geometry (Fig. 7 a) shows magnesium oxide (MgO) as the main constituent. Further analysis using grazing angle geometry (Fig. 7 b) reveals the formation of forsterite (Mg2SiO4), suggesting that Si element is incorporated as crystalline compound in the outer region of the coating. No crystalline F containing phases are identified. The presence of CeO2 compound is detected after both sealing post-treatments which can be more clearly observed in the grazing angle measurements (Fig. 7 b). It is worth to note that after Ce based post-treatments the Mg2SiO4 peaks disappeared, indicating the partial dissolution of the coating in contact with the acidic media. Other Ce-rich amorphous compounds may form, but they were not identified in the present study. In all the patterns, Mg peaks correspond to the substrate.
obviously more corrosion resistant. This is evidenced by the lower current densities of the anodic/cathodic branches and the well-defined passive region of ~400 mV between Ecorr and the pitting potential, Epit (−1.19 VAg/AgCl). Ce-sealings yielded PEO coatings with higher Ecorr and Epit values (~ −1.2 VAg/AgCl and − 0.63 VAg/AgCl, respectively) and lower corrosion current densities, icorr, namely, up to three orders of magnitude lower than the uncoated alloy (10−9 vs 10−6 A cm−2). This was mainly associated with an increased surface passivity (~600 mV passive range) and further inhibition of the anodic reaction when compared to the unsealed coating. It needs to be noted that sealing for 1 h shifts the polarization curve towards higher current values for both the anodic and cathodic branches in comparison with the 5 min sealing. This is a clear indication of coating dissolution and insufficient blockage of the Ce-rich layer, which was already mentioned in Section 3.1.2. Fig. 9 shows the collected hydrogen versus time for the studied materials up to 15 days of immersion in 0.5 wt% NaCl. The kinetic laws calculated from the experimental data are gathered in Table 4 and were approximated by a linear equation y = bt + a, where “y” coordinate represents the collected hydrogen in mL cm−2 and “t” is the immersion time in days. As expected from the electrochemical measurements, the uncoated material reveals a high corrosion rate with significant amounts of hydrogen evolved from the surface. PEO coatings (without and with post-treatments) reduce considerably the hydrogen evolution when compared to the bulk material (up to ~24 times in the best case for PEO_SCe_5min). Also, two trends can be distinguished depending on
3.2. Corrosion behaviour The potentiodynamic polarization results obtained after immersion in 0.5 wt% NaCl for 1 h are shown in Fig. 8 and Table 3. The corrosion potential, Ecorr, of the bulk material is higher (−1.50 VAg/AgCl) than that of the PEO coated specimen (−1.61 VAg/AgCl), although the latter is
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Fig. 7. XRD patterns of studied materials. (a) Bragg-Brentano geometry (b) grazing angle.
corrosion experiments.
the immersion time: up to 3 days of immersion (short term) all the coatings show effective anticorrosion properties with values of accumulative hydrogen lower than 3 mL cm−2. For longer immersion times, all the coated materials show some level of degradation, which is typical for this type of porous oxide layers without top layers such as paints. According to the total hydrogen collected, an opposite effect of Ce-based sealing post-treatments on the corrosion behaviour of PEO coating is observed: post-treatment conducted for 5 min reveals a beneficial effect whereas 1 h sealing has a negative influence. This failure of the PEO_SCe_1h specimen was not observed in the electrochemical tests, which shows the importance of carrying out long-term
4. General discussion LPSO Mg alloys are known to be highly susceptible to corrosion in aqueous media due to galvanic coupling between the cathodic LPSO phases and the α-Mg matrix. Therefore, the amount, composition and distribution of these phases are all factors needing consideration. Table 5 summarizes the influence of some of these factors and shows examples of literature data on the corrosion of LPSO Mg alloys together with some of the experimental results obtained in the present study
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Fig. 8. DC polarization curves of studied materials after 1 h immersion in 0.5 wt% NaCl at 20 °C. Table 3 Parameters obtained from polarization curves in 0.5 wt% NaCl. Specimen
Ecorr (VAg/AgCl)
icorr (A cm−2)
Mg–Y–Zn PEO PEO_SCe_5min PEO_SCe_1h
−1.50 −1.61 −1.20 −1.25
2.4 7.1 2.0 1.3
× × × ×
10−06 10−08 10−09 10−08
Table 4 Kinetic laws calculated from hydrogen measurements. Epit (VAg/AgCl) − −1.19 −0.63 −0.63
[16,17,37,38,49,50]. Detailing the corrosion performance of the bulk material is not the final goal of the present work, although it is worth to note that the studied Mg–Y–Zn alloy shows slightly lower corrosion rate in comparison with the available data. This is likely to be associated with the low Volta potential differences obtained for the LPSO/α-Mg (~60 mV) that do not promote the micro galvanic corrosion process. Moreover, inclusions Y-rich with higher Volta potential differences are expected to play a minor role in corrosion processes due to their negligible contribution to the total cathode area. In this particular study, the combination of PEO processing and Ce-
Material
Kinetic law: y = b t + a; [y (mL cm−2), t (days)]
Time (days)
R2
Mg–Y–Zn PEO PEO_SCe_5min PEO_SCe_1h
y y y y y
0 0 0 0 8
0.99 0.98 0.95 0.83 0.99
= = = = =
11.61 t − 11.70 0.88 t − 0.46 0.48 t − 0.38 2.17 t 6.26 t − 30.50
≤ ≤ ≤ ≤ ≤
t ≤ 15 t ≤ 15 t ≤ 15 t≤8 t ≤ 15
sealing was evaluated for the first time as a way for improving the corrosion performance of an LPSO Mg alloy. SEM/EDS and XRD characterization revealed that PEO processing can successfully be applied to a Mg alloy containing LPSO phases. The typical three-layered structure of AC PEO coatings is obtained and the intermetallics are oxidized/ incorporated into the coating without disrupting its thickness or morphology (Fig. 10a). The F-rich inner barrier layer (< 500 nm) is the main responsible for the anticorrosion properties of the coating, whereas the outer layers with their interconnected porosity only serve as a short-term corrosion barrier [25,51,52]. Electrochemical (short-term) and hydrogen measurements (long-
Fig. 9. (a) Volume of hydrogen evolved from the cathodic reaction with immersion time of studied materials in 0.5 wt%NaCl solution. (b) Detail for better comparison of PEO coated specimens.
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Table 5 Examples of corrosion data for LPSO Mg alloys. [Reference] Alloy
Corrosion data/Comments
[38] Mg15Gd2Zn0.39Zr -As-cast -T4
3.5 wt% NaCl As-cast Ecorra −1.57 VAg/AgCl icorr 4.33 × 10−2 mA cm−2 HER200h 1.02 mL cm−2 days−1 Pa 2.32 mm year−1
[50] Mg0.75Zn2Y -Gravity-cast -Injection-cast -Ribbon consolidated alloy (Mg0.75Zn2Y, Mg0.75Zn2Y0.5Al,Mg0.75Zn2Y1.5Al)
[49] Mg15.24Gd4.75Zn eAs cast -T4 -T6 -T6F
[16] Mg0.9Zn1.6Y Mg2.1Zn5.2Y Mg3.1Zn7.6Y eAs cast
[17] Mg97Y2Zn1
[37] Mg4.59Zn8.92Y0.47Ti -As-cast −400 °C × 10 h −520 °C × 6 h −520 °C × 6 h + 400 °C × 10 h
T4 fLPSO 27% Ecorra −1.44 VAg/AgCl icorr 1.16 × 10−1 mA cm−2 HER200h 0.12 mL cm−2 days−1 Pa 0.27 mm year−1 The improved long term corrosion behavior of T4 treated alloy with LPSO structure was ascribed to: the lower VPD of LPSO vs α-Mg (in comparison with the eutectic phase) and the lower volume fraction of cathodic constituents. 0.17 M NaCl (Hydrogen evolution)/ 0.1 M NaCl (Electroquemical measuerements) Mg0.75Zn2Y Gravity-cast Mg0.75Zn2Y Injection-cast Ecorrb −1.63 VAg/AgCl Ecorrb −1.72 VAg/AgCl −1 P 2.8 mm year P 1.4 mm year−1 Mg0.75Zn2Y Ribbon-consolidated alloy Mg0.75Zn2Y0.5Al Ribbon-consolidated alloy Ecorrb −1.64 VAg/AgCl Ecorrb −1.71 VAg/AgCl P 1.0 mm year−1 P 0.8 mm year−1 Mg0.75Zn2Y1.5Al Ribbon-consolidated alloy Ecorrb −1.72 VAg/AgCl P 0.6 mm year−1 - The cooling rate strongly influence the corrosion performance. Rapidly solidified alloys showed lower corrosion rate. - The best corrosion behavior was achieved for the alloy combining aluminum addition and a rapid solidification 3.5 wt% NaCl As-cast T4 fLPSO 19.68% fLPSO 18.74% Ecorra −1.53 VAg/AgCl Ecorra −1.57 VAg/AgCl −2 −2 icorr 8.942 × 10 mA cm icorr 4.507 × 10−2 mA cm−2 P 323.39 mm year−1 P 17.30 mm year−1 T6 T6F Ecorra −1.58 VAg/AgCl Ecorra −1.63 VAg/AgCl icorr 1.604 × 10−2 mA cm−2 icorr 2.630 × 10−2 mA cm−2 P 4.98 mm year−1 P 9.75 mm year−1 LPSO phase was formed in T6 and T6F alloys and acted as corrosion barrier improving the degradation behavior. 0.1 M NaCl Mg-0.9Zn-1.6Y Mg-2.1Zn-5.2Y fLPSO 3.6% fLPSO 20.3% Ecorra −1.64 VAg/AgCl Ecorra −1.60 VAg/AgCl icorr 1.2 × 10−2 mA cm−2 icorr 6.7 × 10−2 mA cm−2 Pa 0.23 mm year−1 Pa 3.42 mm year−1 Weight lossb 0.5 mg cm−2 days−1 Weight lossb4.5 mg cm−2 days−1 Mg-3.1Zn-7.6Y ΔVLPSO-matrix 250 mV fLPSO 36.2% Ecorra −1.61 VAg/AgCl icorr 2.6 × 10−2 mA cm−2 Pa 0.91 mm year−1 Weight lossb 2.0 mg cm−2 days−1 The presence of LPSO phase lead to micro galvanic activities increasing the corrosion rate. 0.1 M NaCl Mg97Y2Zn1 trasversal Mg97Y2Zn1 longitudinal P 10(t < 5 h) mm year−1 fLPSO 25% P 40(t < 5 h) mm year−1 P 30 mm year−1 P 20 mm year−1 The corrosion resistance of extruded alloys is strongly influenced by the orientation of LPSO-phase alignments. Transversal samples showed a better long term corrosion performance associated with the corrosion front morphology. 3.5 wt% NaCl As-cast 400 °C × 10 h Ecorra −1.60 VAg/AgCl Ecorra −1.62 VAg/AgCl icorr 5.16 × 10−2 mA cm−2 icorr 8.77 × 10−2 mA cm−2 Pb 6 mm year−1 Pb 16 mm year−1 520 °C × 6 h 520 °C × 6 h + 400 °C × 10 h Ecorra −1.62 VAg/AgCl Ecorra −1.61 VAg/AgCl −2 −2 icorr 4.37 × 10 mA cm icorr 7.52 × 10−2 mA cm−2 Pb 2 mm year−1 Pb 8 mm year−1 The morphology of LPSO phase (i.e. lamellar, block-like, and rod-like) influenced the corrosion behaviour. Rodlike LPSO phases revealed a better performance with a more compact corrosion products film.
(continued on next page)
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Table 5 (continued) [Reference] Alloy
Corrosion data/Comments
Prensent study
0.5 wt% NaCl Mg–Y–Zn ΔVLPSO-matrix 60 mV Ecorr − 1.50 VAg/AgCl icorr 2.4 × 10−3 mA cm−2 P 26.46 mm year−1 Low Volta potential differences obtained for the LPSO/α-Mg (~60 mV) do not promote the micro galvanic corrosion process. Moreover, inclusions Y-rich with higher Volta Potential Differences are expected to play a minor role in corrosion processes due to their negligible contribution to the total cathode area.
P: corrosion rate. HER200h: hydrogen evolution rate during 200 h immersion. a Calculated from the reported values. b Obtained from graphical data.
term) demonstrate that a short Ce-sealing of 5 min further improves the corrosion protection offered by the PEO coating, increasing its passive range and pitting potential. These effects were attributed to the precipitation of CeO2 within the coating pores and cracks and the concomitant reduced surface porosity (Fig. 10b) and might also involve the possibility of Ce-rich compounds providing self-healing properties. The self-healing ability or active corrosion protection has been disclosed for different types of Ce-containing coatings [53–55] and can be defined as the recovery of coating integrity after some damage. In the particular case of PEO coatings with Ce compounds, it was previously reported a possible active effect of Ce ions involving their deposition and stabilization of the PEO inner layer [31]. The sealing mechanism of PEO coatings using cerium salt has been detailed in author's previous [21,31] and can be summarized as: i) Oxidation of Ce3+ ions due to the presence of H2O2 in the sealing solution (Eq. (1)) ii) Partial dissolution in acidic media of the MgO and Mg2SiO4 layer which consumes protons and leads to a local alkalinisation (Eqs. (2), (3)) iii) precipitation of insoluble oxide/hydroxide Ce compounds (mainly CeO2) due to the local pH increase (Eq. (4)).
2Ce 3 + (aq) + H2 O2 + 2OH (aq) MgO (s ) +
2H+ (aq)
Mg2 SiO4 (s ) + 4H+ (aq) Ce (OH )22+ (aq)
Mg 2 + (aq)
2Ce (OH )22 + (aq)
Ce (OH )4 (s )
(1) (2)
+ H2 O (aq)
2Mg 2 + (aq) + SiO2 (aq) + H2 O (aq)
+ 2OH (aq)
coarse and elongated α-Mg grains, strings of LPSO phases and Y-rich cuboids. The high volume fraction of LPSO phases and their cathodic activity are the main factors contributing to the high corrosion rate of the uncoated alloy. - LPSO phases are not an obstacle during PEO processing. The coating is mainly composed of MgO/Mg2SiO4 and shows a uniform threelayered structure with a < 500 nm barrier layer. The enhanced surface passivity after PEO treatment significantly reduced the corrosion rate in comparison to the untreated alloy. - Sealing of the PEO coating in an acidic Ce-containing solution for 5 min led to accumulation of CeO2 on the surface, which partially blocked the coating pores and cracks. The combined effects of insoluble Ce-rich compounds and passivity of the PEO coating resulted in a much lower corrosion rate than that of the substrate (2.4 × 10−06 A cm−2 vs 2.0 × 10−09 A cm−2). Electrochemical measurements also revealed a higher pitting potential in comparison to the unsealed coating (−1.19VAg/AgCl vs −0.63 VAg/AgCl). - Sealing for 1 h improved the short-term corrosion resistance of the PEO coating. However, long-term experiments revealed the opposite behaviour. This was associated with the excessive dissolution of the PEO coating during immersion in the acidic Ce-containing solution. This side effect of the dissolution/precipitation mechanism of Cesealings highlights the need for optimizing the sealing time in Mg/ PEO systems.
CeO2 (s ) + 2H2 O
(3) (4)
Author contributions
Increasing the sealing time (Fig. 10c) lead to an excessive dissolution of the oxide layer in acidic media (Eq. (2)) and deterioration of the inner barrier layer which negatively influence the corrosion performance. The lower corrosion resistance of PEO layers after long immersion time in acidic solution is in agreement with results obtained by Phuong et al. [32] and differ from author's previous work [31], pointing out the importance of the individual characteristics of the PEO layer (thickness, microstructure and composition) for achieving the optimum sealing time in Ce solutions which, due to their low pH, can become quite aggressive for MgO and other phases that typically constitute PEO coatings.
Conceptualization: M. Mohedano, G. Garces and R. Arrabal. Formal Analysis: M. Mohedano, P. Perez, E. Matykina and B. Pillado. Methodology: M. Mohedano, G. Garces and R. Arrabal. Resources: M. Mohedano, P. Pérez, E. Matykina, R. Arrabal and G. Garces. Investigation: B. Pillado, M. Mohedano and P. Perez. Writing - Original Draft: M. Mohedano, P. Perez and B. Pillado. Writing - Review & Editing: E. Matykina, R. Arrabal and G. Garces. Funding acquisition: M. Mohedano and R. Arrabal. Project Administration: R. Arrabal and M. Mohedano.
5. Conclusions
Declaration of competing interest
- The studied Mg–Y–Zn alloy shows fully recrystallized α-Mg grains,
None.
11
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Fig. 10. Schematic illustration for (a) PEO, (b) PEO_SCe_5min, (c) PEO_SCe_1h. Adapted from [21].
Acknowledgements
RYC-2017-21843). Data statement The data corresponding to the surface characteristics, XRD diffractograms, potentiodynamic curves and hydrogen evolution measurements of the studied materials are available.
The work was supported by MINECO (Spain) Project MAT201566334-C3-3-R and MAT2015-73355-JIN. M. Mohedano gratefully acknowledges the support of Ramon y Cajal Programme (MICINN, Spain, 12
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PEO coating with Ce-sealing for corrosion protection of LPSO Mg–Y–Zn alloy a,⁎
b
a
a
b
M. Mohedano , P. Pérez , E. Matykina , B. Pillado , G. Garcés , R. Arrabal a b
a
T
Departamento de Ingeniería Química y de Materiales, Facultad de Ciencias Químicas, Universidad Complutense, 28040 Madrid, Spain Departamento de Metalurgia Física, CENIM-CSIC, Avenida Gregorio del Amo 8, E-28040 Madrid, Spain
ARTICLE INFO
ABSTRACT
Keywords: LPSO Mg-Y-Zn magnesium alloy Plasma electrolytic oxidation Protective coating Corrosion Ce-based sealing
Plasma Electrolytic Oxidation coatings (PEO), without and with Ce-based sealings, were developed on a Mg–Y–Zn alloy containing long period stacking order (LPSO) phases in order to improve its corrosion behaviour. Specimens were investigated in terms of morphology, composition and corrosion behaviour (polarization and hydrogen evolution measurements). PEO improved the corrosion resistance compared to the bulk material. Sealing for 5 min plugged the coating pores and cracks with Ce-rich compounds, resulting in improved long-term corrosion behaviour. Sealing for 1 h resulted in excessive dissolution of the coating and deterioration of the inner barrier layer.
1. Introduction Magnesium alloys with long period stacking ordered (LPSO) phases have been recently developed to improve the low strength of conventional Mg-based materials at room and elevated temperatures [1]. An LPSO phase can be described as a long range stacking of basal planes with periodic enrichment of transition metals and Y or RE elements. Several variants of the Mg hexagonal close-packed structure (rhombohedral and hexagonal Bravais lattices) have been reported for LPSO phases depending on the stacking sequence of the close-packed atomic layers [2–4]. It is believed that LPSO phases behave as alloy-strengthening components due to their high Young's modulus and promote the grain refinement effect during dynamic recrystallization [5]. Among LPSO Mg alloys, particular attention has been drawn to the Mg–Y–Zn system due to its superior mechanical properties, which can be tailored through alloy design [6–8] and thermomechanical treatments [1,6,9–13]. Accordingly, most of the studies have focused on microstructure development and deformation mechanisms with the aim of improving the mechanical properties of these alloys (e.g. tensile strength, creep resistance, fracture toughness and fatigue strength) [14]. However, the corrosion behaviour and suitability for surface treatment of LPSO Mg alloys are much less studied fields, with only a few works recently published. Previous studies have shown that the distribution and volume fraction of LPSO phases play an important role in corrosion related phenomena. Zhang [15] reported that Mg–Zn–Y cast alloys with a continuous distribution of LPSO phase show higher corrosion resistance than those with a discontinuous network. Li [16] reported that the
⁎
LPSO phase could serve as a cathode, significantly accelerating the degradation of the α-Mg matrix for high volume fractions of LPSO phases. Results obtained by Perez [17] revealed that the orientation of the LPSO phase also influences the corrosion performance with the longitudinal section revealing worse corrosion resistance than the transversal one. Up to date, there is no simple alloying or casting/forging strategy capable of reducing the corrosion rate of LPSO Mg alloys down to the values usually reported for conventional AZ alloys (0.1–15 mm/year) [18]. This brings up the necessity for surface treatment in order to exploit the outstanding mechanical properties of these alloys. Surface modification is an effective way for improving the corrosion resistance of Mg-based materials. However, this strategy has not been explored so far for LPSO Mg alloys. Among the different techniques that have been applied to Mg and its alloys (e.g., conversion treatments, solgel, gas-phase deposition processes, laser surface melting and organic/ polymer coatings), plasma electrolytic oxidation (PEO) has shown to improve both corrosion and wear resistances [19–24]. PEO involves anodic polarization up to voltages above dielectric breakdown, resulting in short-lived microdischarges and formation of ceramic-like coatings. PEO coatings are typically hard and enriched in substrate- and electrolyte-derived constituents that form crystalline and amorphous phases [25]. Coatings normally exhibit a two or three-layered microstructure comprising a nanometer barrier layer close to the substrate, an outer layer with variable porosity and, in some cases, a dense intermediate layer. Sealing of the outer porous part is required when PEO coatings are exposed to highly corrosive media. Various post-treatments have been
Corresponding author. E-mail address: [email protected] (M. Mohedano).
https://doi.org/10.1016/j.surfcoat.2019.125253 Received 8 October 2019; Received in revised form 2 December 2019; Accepted 11 December 2019 Available online 12 December 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
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phosphate-cerium based solution led to a decrease in the corrosion resistance due to the dissolution of the PEO film during the sealing treatment. In the present study, PEO coatings sealed in cerium nitrate solution are developed and characterized for the first time on an LPSO Mg alloy. The corrosion behaviour was evaluated in a NaCl solution by electrochemical and hydrogen measurements. The influence of the Ce-based sealing duration on the corrosion resistance was also investigated.
Table 1 Composition in weight percent of the Mg-Y-Zn alloy determined by X-ray fluorescence analysis. Element
Y
Zn
Fe
Mn
Mg
wt%
6.8 ± 0.1
2.2 ± 0.1
0.02 ± 0.002
< 0.01
Bal.
evaluated in the past, most of them adapted from sealing processes used in conventional anodizing [21,26–28]. Among them, sealings in Cecontaining solutions are quite promising due to the efficient blockage of the coating pores and defects by Ce-rich hydroxides and oxides. For instance, Lim [29] and Laleh [30] reported an improvement in the corrosion resistance of PEO coated AZ31 alloy after post-treatment in cerium chloride solutions. Similarly, Mingo [21] and Sun [27] demonstrated the superior corrosion performance of PEO coatings sealed in cerium nitrate solutions when compared to other sealing procedures. Author's previous work [31] revealed that the corrosion performance of PEO coated specimens increases with increasing Ce concentration and sealing time due to a higher accumulation of Ce products within coating pores and cracks. However, this is not always the case as reported by Phuong [32], who found that long post-treatments in a
2. Material and methods 2.1. Material The LPSO Mg–Y–Zn alloy was cast in a resistance melting furnace and extruded at 400 °C employing an extrusion ratio of 18:1 and a rectangular profile of 4 × 20 mm2. Specimens of size 30 × 20 × 3 mm3 were ground successively with silicon carbide papers to P1200 grit size for a final roughness of Sa 0.42 ± 0.02 μm and cleaned with iso-propanol prior to corrosion measurements and PEO treatment. The composition of the alloy is given in Table 1.
Fig. 1. (a) 3D SEM reconstruction and (b, c) higher magnification micrographs along the extrusion direction of the Mg-Y-Zn alloy. 2
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Fig. 2. Surface potential maps and potential profiles of the (a,b) LPSO phase and (c,d) Y-rich inclusion.
2.2. Plasma electrolytic oxidation and Ce-sealing
were prepared by grinding through successive grades of silicon carbide paper, with final polishing to 1 μm diamond finish. Image analysis of coatings plan views was performed with ImageJ software in three different SEM micrographs. Volta potential maps were obtained with a Nanoscope IIIA MultiMode scanning kelvin force microscope (SKPFM) operating with a 100 nm tip-to-sample distance. Temperature and relative humidity were kept at 20–23 °C and 40–65%, respectively. Phase composition was examined by X-ray diffraction (XRD) using a Philips X'Pert diffractometer (Cu Kα = 1.54056 Å). The XRD patterns were recorded using Bragg-Brentano geometry (0.01° per second, 2θ range from 10° to 90°) and grazing angle of 0.5° (2θ range from 20° and 45°).
Specimens were masked with Lacquer 45 resin (McDermid plc.) to isolate a working area of ~3 cm2. PEO treatments were conducted parallel to the extrusion direction in an alkaline silicate-based electrolyte (0.086 mol/L sodium silicate, 0.15 mol/L sodium hydroxide and 0.046 mol/L sodium fluoride) at (20 ± 1) °C using a voltage-controlled 2 kW power supply (EAC-S2000, ET Systems electronic) and a 2 L double jacketed cell with a stainless steel (AISI 316) counter-electrode. The electrical parameters were as follows: 60 s ramp, 400 V/−30 V square signal, 500 Hz and 400 mA cm−2 rms current density limit. Two sealing post-treatments were conducted for 5 and 60 min at (30 ± 1) °C by immersion in 10 g/L cerium(III) nitrate hexahydrate, 0.3 g/L hydrogen peroxide and 1 g/L boric acid aqueous solution. Hydrogen peroxide and boric acid were used as accelerators [33,34]. Previous experiments by the authors revealed that short sealing times (< 10 min) yielded better results for this alloy system than longer ones (> 40 min). The selected times, 5 and 60 min, were chosen as representative extremes. The sealing solution was prepared using high purity chemicals and magnetic stirring for at least 30 min. A total volume of 250 mL of sealing solution, with a very low stirring rate, was used for treating batches of 3 specimens. Fresh solutions were prepared after treatment of 15 specimens. After sealing the specimens were dried in warm air.
2.4. Corrosion tests Hydrogen evolution tests were carried out by triplicate in 0.5 wt% NaCl solution at room temperature, (22 ± 2) °C, during 15 days of immersion. Details of the procedure, design and the relationship between the volume of hydrogen and the mass loss of the specimen can be found elsewhere [35]. Potentiodynamic polarization measurements were performed in naturally aerated 0.5 wt% NaCl solution at room temperature by using a computer-controlled potentiostat (GillAC, ACM Instruments) and a three-electrode cell with a graphite counter-electrode and a silver-silver chloride 3 M KCl reference electrode. The cathodic and anodic branches were obtained separately after 1 h of immersion at 0.3 mVs−1 from the OCP (open circuit potential) to −100 mV and + 3000 mV, respectively, with a 5 mA cm−2 current limit.
2.3. Characterization Specimens were examined by scanning electron microscopy (SEM) using a JEOL JSM-6400 instrument equipped with Oxford Link energydispersive X-ray (EDS) microanalysis hardware. Coatings cross-sections
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Fig. 3. Plan view SEM micrographs of the coatings: (a,b) PEO, (c,d) PEO_SCe_5 min and (e,f) PEO_SCe_1h.
Table 2 Surface characteristics of the studied coatings.
PEO PEO_SCe_5min PEO_SCe_1h
Surface porosity (%)
Pore size (μm)
Number of pores, (Pores μm−2)
1.9 ± 1.0 0.8 ± 0.2 1.8 ± 0.9
0.38 ± 0.05 0.19 ± 0.02 0.28 ± 0.09
(4.29 ± 0.09) × 10−02 (0.38 ± 0.09) × 10−02 (4.1 ± 0.1) × 10−02
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Fig. 4. Cross-section examination of the unsealed PEO coating. (a,b) BSE micrographs and (c, d, e, f) X-ray elemental maps.
3. Results
shaped Y-rich intermetallics (46.5Al, 0.9Zn, 52.6Y, in at.%) are also observed (Fig. 1c). Recrystallized fine α-Mg grains (~1 μm in diameter and ~60% area fraction) are the result of partial dynamic recrystallization during high temperature processing [36]. 3D characterization of this alloy in a previous study revealed that the volume fraction of the LPSO phase is close to 25% and that strings of this phase are composed of independent fibers of 200–300 μm in length [17]. Previous studies have pointed out the cathodic behaviour of LPSO phases based on the preferential corrosion of the surrounding α-Mg matrix, particularly for lamellar LPSO phases [16,37], but only a few
3.1. Characterization 3.1.1. Mg–Y–Zn alloy Fig. 1 shows the 3D SEM reconstruction and higher magnification micrographs along the extrusion direction for the Mg–Y–Zn alloy. The material shows fully recrystallized α-Mg grains surrounded by coarse and elongated α-Mg grains (98.3Al, 0.6Zn, 1.1 Y, in at.%) and bright contrast strings of LPSO phases (95.3Al, 2.1Zn, 2.6Y, in at.%). Cuboid-
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Fig. 5. Cross section examination of the PEO_Ce_5min coating. (a,b) BSE micrographs and (c, d) X-ray elemental maps.
works have presented SKPFM results [38]. Our results show Volta potential differences of the order of ~60 mV, confirming the cathodic activity of the LPSO phase (Fig. 2 a,b). This value is much lower than that reported by Li [16] in a Mg–Zn–Y cast alloy (250 mV). The authors believe the latter result to be misleading as it was determined right at the LPSO/α-Mg interface, where SKPFM artefacts are quite common [39]. Y-rich cuboid intermetallics reveal higher potential values (~140 mV) than the LPSO phase (Fig. 2c,d). This might be due to the higher Y + Zn/Mg ratio (calculated based on at.%) of cuboids (~1.15) compared with that of LPSO phase (~0.05) which leads to a more noble reduction potential [40,41].
The cross-sections of the unsealed PEO layer along with the Si, F, O and Y X-ray elemental maps are shown in Fig. 4. Backscattered scanning electron (BSE) micrographs (Fig. 4a and b) reveal a coating with a thickness of (5.4 ± 1.2) μm and a typical three-layered structure [25,44–46], consisting of an outer part with through-going discharge channels, an intermediate section with smaller pores and a compact inner barrier layer (< 500 nm). X-ray elemental maps show Si species homogeneously distributed throughout the coating thickness (Fig. 4c) and a F-rich inner region (Fig. 4d), indicating a rapid inward migration of fluoride ions during the oxide formation [47]. LPSO phases are oxidized and incorporated into the coating in a localized manner, resulting in Y-rich regions. These regions did not significantly affect neither the coating thickness nor the morphology. This behaviour is quite different from that recently reported by Martin et al. [48]. The differences might be ascribed to the different anodizing conditions and composition of secondary phases. Fig. 5 and Fig. 6 show significant differences between the 5 min and 1 h sealed PEO coatings. After 5 min, the PEO layer does not undergo significant changes and, as marked by the brighter areas of the BSE micrograph and X-ray elemental maps, Ce-rich compounds are preferentially deposited within the crater-like pores [31] (Fig. 5a). The BSE micrograph at higher magnification (Fig. 5b) and the distribution of F (Fig. 5d) suggest that the barrier inner layer remains intact and, therefore, the anticorrosion properties ascribed to this inner layer are not compromised. Sealing for 1 h (Fig. 6), on the other hand, leads to
3.1.2. PEO coatings Surface morphologies of the PEO coated specimens before and after sealing are shown in Fig. 3. The unsealed coating reveals a crater-like morphology with microcracks and micropores located at the sites of the discharge channels due to residual stresses and gas evolution during the coating formation [42] (Fig. 3a and b). After post-treatment, a Ce-rich superficial layer is formed, mimicking the morphology of the PEO coating and decreasing the number and size of open pores (Fig. 3c, d, e and f). Cracks observed in the Ce-rich layer may be associated with its dehydration (Fig. 3d and f) [43]. The best sealing properties, based on surface porosity, pore size and number of pores, are obtained for the 5 min post-treatment (Table 2).
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Fig. 6. Cross section examination of the PEO_Ce_1h coating. (a,b) BSE micrographs and (c, d) X-ray elemental maps.
important changes in the PEO coating. These are summarized as follows; i) decrease in the coating thickness up to 40% in some areas; ii) visible large pores through the film thickness; and iii) deterioration of the barrier layer which becomes hardly visible (Fig. 6b). Indeed, the Xray map shows a depletion in F in the inner region (Fig. 6d) indicating that the barrier layer has been partially dissolved. The XRD pattern of the unsealed PEO coating obtained at BraggBrentano geometry (Fig. 7 a) shows magnesium oxide (MgO) as the main constituent. Further analysis using grazing angle geometry (Fig. 7 b) reveals the formation of forsterite (Mg2SiO4), suggesting that Si element is incorporated as crystalline compound in the outer region of the coating. No crystalline F containing phases are identified. The presence of CeO2 compound is detected after both sealing post-treatments which can be more clearly observed in the grazing angle measurements (Fig. 7 b). It is worth to note that after Ce based post-treatments the Mg2SiO4 peaks disappeared, indicating the partial dissolution of the coating in contact with the acidic media. Other Ce-rich amorphous compounds may form, but they were not identified in the present study. In all the patterns, Mg peaks correspond to the substrate.
obviously more corrosion resistant. This is evidenced by the lower current densities of the anodic/cathodic branches and the well-defined passive region of ~400 mV between Ecorr and the pitting potential, Epit (−1.19 VAg/AgCl). Ce-sealings yielded PEO coatings with higher Ecorr and Epit values (~ −1.2 VAg/AgCl and − 0.63 VAg/AgCl, respectively) and lower corrosion current densities, icorr, namely, up to three orders of magnitude lower than the uncoated alloy (10−9 vs 10−6 A cm−2). This was mainly associated with an increased surface passivity (~600 mV passive range) and further inhibition of the anodic reaction when compared to the unsealed coating. It needs to be noted that sealing for 1 h shifts the polarization curve towards higher current values for both the anodic and cathodic branches in comparison with the 5 min sealing. This is a clear indication of coating dissolution and insufficient blockage of the Ce-rich layer, which was already mentioned in Section 3.1.2. Fig. 9 shows the collected hydrogen versus time for the studied materials up to 15 days of immersion in 0.5 wt% NaCl. The kinetic laws calculated from the experimental data are gathered in Table 4 and were approximated by a linear equation y = bt + a, where “y” coordinate represents the collected hydrogen in mL cm−2 and “t” is the immersion time in days. As expected from the electrochemical measurements, the uncoated material reveals a high corrosion rate with significant amounts of hydrogen evolved from the surface. PEO coatings (without and with post-treatments) reduce considerably the hydrogen evolution when compared to the bulk material (up to ~24 times in the best case for PEO_SCe_5min). Also, two trends can be distinguished depending on
3.2. Corrosion behaviour The potentiodynamic polarization results obtained after immersion in 0.5 wt% NaCl for 1 h are shown in Fig. 8 and Table 3. The corrosion potential, Ecorr, of the bulk material is higher (−1.50 VAg/AgCl) than that of the PEO coated specimen (−1.61 VAg/AgCl), although the latter is
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Fig. 7. XRD patterns of studied materials. (a) Bragg-Brentano geometry (b) grazing angle.
corrosion experiments.
the immersion time: up to 3 days of immersion (short term) all the coatings show effective anticorrosion properties with values of accumulative hydrogen lower than 3 mL cm−2. For longer immersion times, all the coated materials show some level of degradation, which is typical for this type of porous oxide layers without top layers such as paints. According to the total hydrogen collected, an opposite effect of Ce-based sealing post-treatments on the corrosion behaviour of PEO coating is observed: post-treatment conducted for 5 min reveals a beneficial effect whereas 1 h sealing has a negative influence. This failure of the PEO_SCe_1h specimen was not observed in the electrochemical tests, which shows the importance of carrying out long-term
4. General discussion LPSO Mg alloys are known to be highly susceptible to corrosion in aqueous media due to galvanic coupling between the cathodic LPSO phases and the α-Mg matrix. Therefore, the amount, composition and distribution of these phases are all factors needing consideration. Table 5 summarizes the influence of some of these factors and shows examples of literature data on the corrosion of LPSO Mg alloys together with some of the experimental results obtained in the present study
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Fig. 8. DC polarization curves of studied materials after 1 h immersion in 0.5 wt% NaCl at 20 °C. Table 3 Parameters obtained from polarization curves in 0.5 wt% NaCl. Specimen
Ecorr (VAg/AgCl)
icorr (A cm−2)
Mg–Y–Zn PEO PEO_SCe_5min PEO_SCe_1h
−1.50 −1.61 −1.20 −1.25
2.4 7.1 2.0 1.3
× × × ×
10−06 10−08 10−09 10−08
Table 4 Kinetic laws calculated from hydrogen measurements. Epit (VAg/AgCl) − −1.19 −0.63 −0.63
[16,17,37,38,49,50]. Detailing the corrosion performance of the bulk material is not the final goal of the present work, although it is worth to note that the studied Mg–Y–Zn alloy shows slightly lower corrosion rate in comparison with the available data. This is likely to be associated with the low Volta potential differences obtained for the LPSO/α-Mg (~60 mV) that do not promote the micro galvanic corrosion process. Moreover, inclusions Y-rich with higher Volta potential differences are expected to play a minor role in corrosion processes due to their negligible contribution to the total cathode area. In this particular study, the combination of PEO processing and Ce-
Material
Kinetic law: y = b t + a; [y (mL cm−2), t (days)]
Time (days)
R2
Mg–Y–Zn PEO PEO_SCe_5min PEO_SCe_1h
y y y y y
0 0 0 0 8
0.99 0.98 0.95 0.83 0.99
= = = = =
11.61 t − 11.70 0.88 t − 0.46 0.48 t − 0.38 2.17 t 6.26 t − 30.50
≤ ≤ ≤ ≤ ≤
t ≤ 15 t ≤ 15 t ≤ 15 t≤8 t ≤ 15
sealing was evaluated for the first time as a way for improving the corrosion performance of an LPSO Mg alloy. SEM/EDS and XRD characterization revealed that PEO processing can successfully be applied to a Mg alloy containing LPSO phases. The typical three-layered structure of AC PEO coatings is obtained and the intermetallics are oxidized/ incorporated into the coating without disrupting its thickness or morphology (Fig. 10a). The F-rich inner barrier layer (< 500 nm) is the main responsible for the anticorrosion properties of the coating, whereas the outer layers with their interconnected porosity only serve as a short-term corrosion barrier [25,51,52]. Electrochemical (short-term) and hydrogen measurements (long-
Fig. 9. (a) Volume of hydrogen evolved from the cathodic reaction with immersion time of studied materials in 0.5 wt%NaCl solution. (b) Detail for better comparison of PEO coated specimens.
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Table 5 Examples of corrosion data for LPSO Mg alloys. [Reference] Alloy
Corrosion data/Comments
[38] Mg15Gd2Zn0.39Zr -As-cast -T4
3.5 wt% NaCl As-cast Ecorra −1.57 VAg/AgCl icorr 4.33 × 10−2 mA cm−2 HER200h 1.02 mL cm−2 days−1 Pa 2.32 mm year−1
[50] Mg0.75Zn2Y -Gravity-cast -Injection-cast -Ribbon consolidated alloy (Mg0.75Zn2Y, Mg0.75Zn2Y0.5Al,Mg0.75Zn2Y1.5Al)
[49] Mg15.24Gd4.75Zn eAs cast -T4 -T6 -T6F
[16] Mg0.9Zn1.6Y Mg2.1Zn5.2Y Mg3.1Zn7.6Y eAs cast
[17] Mg97Y2Zn1
[37] Mg4.59Zn8.92Y0.47Ti -As-cast −400 °C × 10 h −520 °C × 6 h −520 °C × 6 h + 400 °C × 10 h
T4 fLPSO 27% Ecorra −1.44 VAg/AgCl icorr 1.16 × 10−1 mA cm−2 HER200h 0.12 mL cm−2 days−1 Pa 0.27 mm year−1 The improved long term corrosion behavior of T4 treated alloy with LPSO structure was ascribed to: the lower VPD of LPSO vs α-Mg (in comparison with the eutectic phase) and the lower volume fraction of cathodic constituents. 0.17 M NaCl (Hydrogen evolution)/ 0.1 M NaCl (Electroquemical measuerements) Mg0.75Zn2Y Gravity-cast Mg0.75Zn2Y Injection-cast Ecorrb −1.63 VAg/AgCl Ecorrb −1.72 VAg/AgCl −1 P 2.8 mm year P 1.4 mm year−1 Mg0.75Zn2Y Ribbon-consolidated alloy Mg0.75Zn2Y0.5Al Ribbon-consolidated alloy Ecorrb −1.64 VAg/AgCl Ecorrb −1.71 VAg/AgCl P 1.0 mm year−1 P 0.8 mm year−1 Mg0.75Zn2Y1.5Al Ribbon-consolidated alloy Ecorrb −1.72 VAg/AgCl P 0.6 mm year−1 - The cooling rate strongly influence the corrosion performance. Rapidly solidified alloys showed lower corrosion rate. - The best corrosion behavior was achieved for the alloy combining aluminum addition and a rapid solidification 3.5 wt% NaCl As-cast T4 fLPSO 19.68% fLPSO 18.74% Ecorra −1.53 VAg/AgCl Ecorra −1.57 VAg/AgCl −2 −2 icorr 8.942 × 10 mA cm icorr 4.507 × 10−2 mA cm−2 P 323.39 mm year−1 P 17.30 mm year−1 T6 T6F Ecorra −1.58 VAg/AgCl Ecorra −1.63 VAg/AgCl icorr 1.604 × 10−2 mA cm−2 icorr 2.630 × 10−2 mA cm−2 P 4.98 mm year−1 P 9.75 mm year−1 LPSO phase was formed in T6 and T6F alloys and acted as corrosion barrier improving the degradation behavior. 0.1 M NaCl Mg-0.9Zn-1.6Y Mg-2.1Zn-5.2Y fLPSO 3.6% fLPSO 20.3% Ecorra −1.64 VAg/AgCl Ecorra −1.60 VAg/AgCl icorr 1.2 × 10−2 mA cm−2 icorr 6.7 × 10−2 mA cm−2 Pa 0.23 mm year−1 Pa 3.42 mm year−1 Weight lossb 0.5 mg cm−2 days−1 Weight lossb4.5 mg cm−2 days−1 Mg-3.1Zn-7.6Y ΔVLPSO-matrix 250 mV fLPSO 36.2% Ecorra −1.61 VAg/AgCl icorr 2.6 × 10−2 mA cm−2 Pa 0.91 mm year−1 Weight lossb 2.0 mg cm−2 days−1 The presence of LPSO phase lead to micro galvanic activities increasing the corrosion rate. 0.1 M NaCl Mg97Y2Zn1 trasversal Mg97Y2Zn1 longitudinal P 10(t < 5 h) mm year−1 fLPSO 25% P 40(t < 5 h) mm year−1 P 30 mm year−1 P 20 mm year−1 The corrosion resistance of extruded alloys is strongly influenced by the orientation of LPSO-phase alignments. Transversal samples showed a better long term corrosion performance associated with the corrosion front morphology. 3.5 wt% NaCl As-cast 400 °C × 10 h Ecorra −1.60 VAg/AgCl Ecorra −1.62 VAg/AgCl icorr 5.16 × 10−2 mA cm−2 icorr 8.77 × 10−2 mA cm−2 Pb 6 mm year−1 Pb 16 mm year−1 520 °C × 6 h 520 °C × 6 h + 400 °C × 10 h Ecorra −1.62 VAg/AgCl Ecorra −1.61 VAg/AgCl −2 −2 icorr 4.37 × 10 mA cm icorr 7.52 × 10−2 mA cm−2 Pb 2 mm year−1 Pb 8 mm year−1 The morphology of LPSO phase (i.e. lamellar, block-like, and rod-like) influenced the corrosion behaviour. Rodlike LPSO phases revealed a better performance with a more compact corrosion products film.
(continued on next page)
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Table 5 (continued) [Reference] Alloy
Corrosion data/Comments
Prensent study
0.5 wt% NaCl Mg–Y–Zn ΔVLPSO-matrix 60 mV Ecorr − 1.50 VAg/AgCl icorr 2.4 × 10−3 mA cm−2 P 26.46 mm year−1 Low Volta potential differences obtained for the LPSO/α-Mg (~60 mV) do not promote the micro galvanic corrosion process. Moreover, inclusions Y-rich with higher Volta Potential Differences are expected to play a minor role in corrosion processes due to their negligible contribution to the total cathode area.
P: corrosion rate. HER200h: hydrogen evolution rate during 200 h immersion. a Calculated from the reported values. b Obtained from graphical data.
term) demonstrate that a short Ce-sealing of 5 min further improves the corrosion protection offered by the PEO coating, increasing its passive range and pitting potential. These effects were attributed to the precipitation of CeO2 within the coating pores and cracks and the concomitant reduced surface porosity (Fig. 10b) and might also involve the possibility of Ce-rich compounds providing self-healing properties. The self-healing ability or active corrosion protection has been disclosed for different types of Ce-containing coatings [53–55] and can be defined as the recovery of coating integrity after some damage. In the particular case of PEO coatings with Ce compounds, it was previously reported a possible active effect of Ce ions involving their deposition and stabilization of the PEO inner layer [31]. The sealing mechanism of PEO coatings using cerium salt has been detailed in author's previous [21,31] and can be summarized as: i) Oxidation of Ce3+ ions due to the presence of H2O2 in the sealing solution (Eq. (1)) ii) Partial dissolution in acidic media of the MgO and Mg2SiO4 layer which consumes protons and leads to a local alkalinisation (Eqs. (2), (3)) iii) precipitation of insoluble oxide/hydroxide Ce compounds (mainly CeO2) due to the local pH increase (Eq. (4)).
2Ce 3 + (aq) + H2 O2 + 2OH (aq) MgO (s ) +
2H+ (aq)
Mg2 SiO4 (s ) + 4H+ (aq) Ce (OH )22+ (aq)
Mg 2 + (aq)
2Ce (OH )22 + (aq)
Ce (OH )4 (s )
(1) (2)
+ H2 O (aq)
2Mg 2 + (aq) + SiO2 (aq) + H2 O (aq)
+ 2OH (aq)
coarse and elongated α-Mg grains, strings of LPSO phases and Y-rich cuboids. The high volume fraction of LPSO phases and their cathodic activity are the main factors contributing to the high corrosion rate of the uncoated alloy. - LPSO phases are not an obstacle during PEO processing. The coating is mainly composed of MgO/Mg2SiO4 and shows a uniform threelayered structure with a < 500 nm barrier layer. The enhanced surface passivity after PEO treatment significantly reduced the corrosion rate in comparison to the untreated alloy. - Sealing of the PEO coating in an acidic Ce-containing solution for 5 min led to accumulation of CeO2 on the surface, which partially blocked the coating pores and cracks. The combined effects of insoluble Ce-rich compounds and passivity of the PEO coating resulted in a much lower corrosion rate than that of the substrate (2.4 × 10−06 A cm−2 vs 2.0 × 10−09 A cm−2). Electrochemical measurements also revealed a higher pitting potential in comparison to the unsealed coating (−1.19VAg/AgCl vs −0.63 VAg/AgCl). - Sealing for 1 h improved the short-term corrosion resistance of the PEO coating. However, long-term experiments revealed the opposite behaviour. This was associated with the excessive dissolution of the PEO coating during immersion in the acidic Ce-containing solution. This side effect of the dissolution/precipitation mechanism of Cesealings highlights the need for optimizing the sealing time in Mg/ PEO systems.
CeO2 (s ) + 2H2 O
(3) (4)
Author contributions
Increasing the sealing time (Fig. 10c) lead to an excessive dissolution of the oxide layer in acidic media (Eq. (2)) and deterioration of the inner barrier layer which negatively influence the corrosion performance. The lower corrosion resistance of PEO layers after long immersion time in acidic solution is in agreement with results obtained by Phuong et al. [32] and differ from author's previous work [31], pointing out the importance of the individual characteristics of the PEO layer (thickness, microstructure and composition) for achieving the optimum sealing time in Ce solutions which, due to their low pH, can become quite aggressive for MgO and other phases that typically constitute PEO coatings.
Conceptualization: M. Mohedano, G. Garces and R. Arrabal. Formal Analysis: M. Mohedano, P. Perez, E. Matykina and B. Pillado. Methodology: M. Mohedano, G. Garces and R. Arrabal. Resources: M. Mohedano, P. Pérez, E. Matykina, R. Arrabal and G. Garces. Investigation: B. Pillado, M. Mohedano and P. Perez. Writing - Original Draft: M. Mohedano, P. Perez and B. Pillado. Writing - Review & Editing: E. Matykina, R. Arrabal and G. Garces. Funding acquisition: M. Mohedano and R. Arrabal. Project Administration: R. Arrabal and M. Mohedano.
5. Conclusions
Declaration of competing interest
- The studied Mg–Y–Zn alloy shows fully recrystallized α-Mg grains,
None.
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Fig. 10. Schematic illustration for (a) PEO, (b) PEO_SCe_5min, (c) PEO_SCe_1h. Adapted from [21].
Acknowledgements
RYC-2017-21843). Data statement The data corresponding to the surface characteristics, XRD diffractograms, potentiodynamic curves and hydrogen evolution measurements of the studied materials are available.
The work was supported by MINECO (Spain) Project MAT201566334-C3-3-R and MAT2015-73355-JIN. M. Mohedano gratefully acknowledges the support of Ramon y Cajal Programme (MICINN, Spain, 12
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