Composite magnesium phosphate coatings for improved corrosion resistance of magnesium AZ31 alloy

Accepted Manuscript Title: COMPOSITE MAGNESIUM PHOSPHATE COATINGS FOR IMPROVED CORROSION RESISTANCE OF MAGNESIUM AZ31 ALLOY Author: Jithu Jayaraj Raj. Amruth S A. Srinivasan S. Ananthakumar U.T.S Pillai Nanda Gopala Krishna Dhaipule U.Kamachi Mudali PII: DOI: Reference:

S0010-938X(16)30952-0 http://dx.doi.org/doi:10.1016/j.corsci.2016.10.010 CS 6908

To appear in: Received date: Revised date: Accepted date:

4-5-2016 10-9-2016 13-10-2016

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COMPOSITE

MAGNESIUM

PHOSPHATE

COATINGS

FOR

IMPROVED

CORROSION RESISTANCE OF MAGNESIUM AZ31 ALLOY Jithu Jayaraj1, Amruth Raj. S2, A. Srinivasan1*, S. Ananthakumar1, U. T. S Pillai1, Nanda Gopala Krishna Dhaipule3 and U. Kamachi Mudali3 1

CSIR-National Interdisciplinary Institute for Science and Technology, Trivandrum, Kerala National Institute of Technology, Calicut, Kerala 3 Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamilnadu 2

*Corresponding author

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Highlights    

A composite coating of struvite and newberyite on AZ31 alloy was obtained Effect of pH (4.5-7.5) of the coating bath on corrosion resistance was investigated Highest corrosion resistance was obtained at coating bath pH of 7.5 Uniform coating with more struvite improved the corrosion performance at pH 7.5

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Abstract Corrosion performance of magnesium phosphate composite coatings consisted of struvite (MgNH4PO4.6H2O) and newberyite (MgHPO4.3H2O) on magnesium AZ31 alloy obtained in a high concentrated NH4H2PO4 phosphating bath at different pH ranges in 1 wt.% NaCl aqueous solution were analyzed using potentiodynamic polarization test, EIS analysis and immersion test. The test results revealed that the corrosion performance increased with increase in the coating bath pH which could be attributed to the uniform and dense magnesium phosphate coating with fewer defects and increase in the content of insoluble struvite phase.

Keywords: A. Magnesium; B. EIS; B. Polarization; B. SEM; B. Weight loss.

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1. Introduction Magnesium and its alloys find their applications in transportation and electronic industries due to their higher strength to weight ratio, electromagnetic shielding properties, dent resistance properties, etc. [1]. In addition, magnesium alloys are suitable for biodegradable and biocompatible implant applications [2-3]. However, the main curse of magnesium and its alloys are their poor corrosion properties, which restrict their wide spread applications in many fields [3-5]. Developing high purity alloys, providing protective films/coatings, surface modifications, grain refinement, etc. are some of the most commonly used methods to improve the corrosion resistance [1, 6]. Among them, providing protective coatings is one of the effective ways of improving the corrosion resistance of magnesium alloys. Different techniques such as anodizing, electrochemical plating, conversion coatings, hydride coatings, ion implantation, polymer coatings, and sol-gel coatings are used to develop protective layers on magnesium alloys[6-8].

Conversion coatings on magnesium alloys are the major area of interest as they are cheap, easily viable, and environment friendly. Chemical conversion coatings are formed by chemically treating the surface of Mg alloys to produce a thin outer coating of metal oxides, phosphates, or other compounds that are chemically bonded to the surface. Traditional conversion coatings are based on chromates and fluorides, which are proved to be effective [9, 10]. However, the presence of Cr (VI) ions in chromate coatings is toxic, carcinogenic, and remains in the environment for a long period [11]. Hence the development of environmental friendly conversion coatings for Mg alloys is warranted. Formation of phosphate coatings that are nontoxic and have good corrosion resistance in comparison to traditional chromate coatings are proposed as an alternative coating for Mg alloys [7]. Over the years, various corrosion resistant phosphate conversion coatings have been developed for Mg alloys [12-15]. Pretreatment technique, coating time and temperature, and pH and concentration of the phosphate bath are the main factors influencing the quality of the coatings. The performance of the coating does not only depend on its thickness, but also on its adherence with the parent alloy surface and uniformity with fewer porosities and cracks [16-17].

Magnesium phosphate coatings (MPC) exhibit good corrosion resistance properties and have 4

been tried mainly on mild steel but to a little extent on magnesium alloys [12, 16, 18]. Unlike other phosphate coatings, the readily available Mg2+ ions from the alloy itself assist the easy formation of magnesium phosphate coatings. Different types of magnesium phosphates known as newberyite, struvite, and farringtonite are reported in literature. Most of the magnesium phosphate coating studies reported the formation of newberyite phase [16-19]. For instance, Ishizaki et al. [18] achieved an improved corrosion resistant newberyite (MgHPO4.3H2O) film on AZ31 alloy using steam-curing assisted chemical conversion method. Phong et al. [20] showed that magnesium phosphate (newberyite) coatings performed better than zinc phosphate coating in 3.5 wt.% NaCl solution. However, in general, the phosphate formation is found to be slow and it requires longer immersion time in the phosphate solution. The problem of slow coating can be overcome by the addition of Mg(OH)2 to the phosphate bath. Zhao et al. [19], reported the formation of a composite coating consisting of struvite and Mg(OH)2 in about 6 h. However, the addition of Mg(OH)2 leads to a mud-crack surface due to the evaporation of H2O during drying process that reduces the corrosion performance of the coatings. In general, all the above reported studies used low concentrated phosphate baths (in the range of 0.1 to 0.5 mol/l) and coatings were done at lower coating bath pH (more towards acidic region).

On the other hand, only a few studies are reported on the formation of struvite on magnesium substrate and its corrosion behavior [18, 21-23]. Recently, Zhao et al. [23] developed a two layer coating based on newberyite (MgHPO4.3H2O) and struvite (MgNH4PO4.6H2O) in relatively lesser time (90 min.), and demonstrated a better corrosion performance of the composite coating in simulated body fluid (SBF). Moreover, it has been shown that struvite exhibits good biocompatibility in in-vitro and in-vivo studies [21]. However, it is difficult to obtain a complete struvite based phosphate coating as the formation of newberyite is unavoidable due to its easy formation [23].

In the present study, corrosion behavior of composite coatings of magnesium phosphate consisting of struvite (MgNH4PO4.6H2O) and newberyite (MgHPO4.3H2O) on magnesium AZ31 alloy obtained in a high concentration NH4H2PO4 phosphating bath was investigated. The effect of pH of the coating bath on corrosion performance of the coating was also analyzed in 1 wt.%

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NaCl solution using potentiodynamic polarization test, electrochemical impedance spectroscopy (EIS) and immersion tests. 2. Experimental Procedure 2.1 Alloy preparation AZ31 alloy (3 wt.% Al-0.8 wt.% Zn-0.2 wt.% Mn) was selected as substrate for the present investigation and was prepared in a resistance furnace under a protective gas mixture of Ar + 0.2% SF6. Pure Mg, pure Zn, and Al-10Mn master alloy were used to prepare the alloys. Initially, Mg and Zn were melted in a boron- nitride (BN) coated mild steel crucible, and then Al-Mn master alloy was added into the melt at around 750 oC. After the addition, the melt was stirred manually for a few seconds to achieve a complete dissolution of alloying elements and uniform composition in the melt. The melt was poured into a preheated (250 oC) metallic plate shaped mould with dimensions of 200 mm x 250 mm x 20 mm. The chemical composition of the cast alloy was analyzed using optical emission spectroscopy (OES, Spectromaxx, Ametek Materials Analysis Division) and the compositions are provided in Table 1. 2.2 Coating procedure Samples for all the coating experiments were mechanically ground with up to 1000 grit SiC paper to obtain a uniform surface roughness. These samples were then washed in distilled water and ultrasonicated in ethanol for 10 min. before coating. All chemical used for coating were of reagent grade and used without further purification. Surface pretreatment was carried out for 5 min. in a solution consisted of sodium fluoride (NaF: 2g/l) and sodium nitrite (NaNO2: 4g/l). Ammonium dihydrogen phosphate (NH4H2PO4:1 mol/l) was used as the coating medium. The pH of the solution was adjusted to 4.5, 5.5, 6.5, and 7.5 by adding required amount of NH3. The pre-treated and dried samples were then immersed in the phosphating bath for 20 min. at 50 0C. After the coating, the samples were cleaned with distilled water and dried in a hot oven at 50 0C.

2.3 Surface characterization The surface morphology, composition and the cross-section of the conversion coatings were observed using a scanning electron microscope (SEM, Carl Zeiss EVO18) equipped with energydispersive X-ray spectroscopy (EDAX) and optical microscope (LEICA DM2500M). To identify the phase composition of the coating, X-ray diffraction (XRD, X‟Pert PRO PANalytical) studies were carried out with Cu-Kα radiation (wavelength λ = 0.15406 nm) at a voltage and current of 6

40 kV and 40 mA respectively. The samples were scanned in the range of 100-900 with a step interval of 0.0330 and at a scan rate of 0.050 /s.

2.4 Corrosion measurements 2.4.1 Electrochemical measurement Potentiodynamic polarization and electrochemical impedance measurements were conducted in an electrochemical system (CH Instruments Inc., Model 680A) with three electrode cells of saturated calomel electrode (SCE) as reference electrode, a platinum mesh as counter electrode, and the sample as working electrode. All electrochemical experiments were carried out at room temperature (25±1 0C) with 1 wt.% NaCl solution as electrolyte. The surface area of the sample exposed to the electrolyte was 1 cm2. The samples were immersed in the electrolyte for 20-35 min. to monitor their open circuit potential (OCP) till they reached a relatively stable value. The samples were scanned from cathodic region to anodic region at a rate of 1 mV/s, in the range of 200 mV to +300 mV with reference to their corresponding OCP values. The corrosion current density (icorr) and corrosion potential (Ecorr vs. SCE) were measured from the Tafel plot. The impedance measurements on the samples were performed at their open circuit potential with a peak-to-peak amplitude of 10 mV in the frequency range from 105 to 10-1 Hz. At least three tests were conducted for each condition to confirm the validity of the polarization and EIS measurements. The impedance data was fitted with equivalent circuits and values of the circuit elements were extracted using EC-Lab software. To check the OCP variations of the samples in 1 wt.% NaCl for a long time immersion, the OCP of the samples were also monitored separately for 5 days.

2.4.2 Immersion test Thin slabs of dimension 15 mm x 15 mm x 3 mm were used for immersion test. Samples were hanged in the solution to ensure that all the surfaces are equally exposed to the solution. Each cleaned and weighed samples were immersed separately in 250 ml of 1 wt.% NaCl solution for a period of 120 h (5 days) at room temperature (25±1 0C). For each condition, three samples were tested. After the immersion test, the samples were cleaned using distilled water and then using ethanol. The dried samples were immersed in a solution containing 200 g/l chromic acid at room temperature (25±1 0C) for 10 min. to remove the corrosion products. Finally samples were again 7

washed with distilled water and then ethanol before drying properly. Weight loss of each sample after the immersion test was calculated from the difference between initial and final weights. The following formula was used for calculating the corrosion rate in millimeter per year (mm/y):

where w is weight loss in g, ρ is density of the alloy in g/cm3, A is exposed area in cm2, t is exposure time in h. 3. Results 3.1 Composition and morphologies of the coatings 3.1.1 XRD analysis XRD analysis of the bare AZ31 alloy sample as well as coated samples are shown in Fig. 1.As the major peaks for the phosphates occur in the lower angles, the spectra up to 40o, i.e. 2θ < 40o is presented for more clarity. The intensity of the major peak corresponding to the α-Mg phase (JCPDS no. 00-035-0821) reduced with the coated substrates, in particular for sample coated at pH 4.5, indicating the presence of phosphates on the surface. In addition to the peaks of α-Mg phase, several weakly diffracted peaks were obtained with the coated substrates. Some of these weakly diffracted peaks correspond to the struvite phase (MgNH4PO4.6H2O) (JCPDS no. 01071-2089) [19, 24]. However, the majority of the weakly diffracted peaks correspond to the newberyite phase (MgHPO4.3H2O) (JCPDS no.00-035-0780) [18]. Hence, the XRD results suggested the presence of both newberyite and struvite phases in all the samples coated at different pH.

3.1.2 SEM and EDAX analysis The SEM micrographs of the coated samples (Fig. 2) revealed that the morphology of the coatings significantly differed with respect to the pH of the phosphating bath. The samples coated at pH 4.5 composed of coarse crystals of polygonal shaped phosphates distributed on the substrate [Fig. 2(a)]. Relatively fine and uniformly distributed phosphate crystals were seen in the microstructure of the sample coated at pH 5.5 [Fig. 2(c)]. In addition, the sharp edged polygonal crystals observed with sample coated at pH 4.5 were considerably changed with higher pH. The high magnification micrographs of the coatings obtained in both the pH [Fig.

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2(b&d)] suggested that the formation and growth of phosphate occurred layer by layer. Just below the top phosphate crystals, marked as „A‟ in Fig. 2(b&d), a cracked continuous phosphate bottom layer, marked as „B‟ in Fig. 2(b&d), could be seen. The cracks observed on the coated surfaces might be due to the presence of Mg(OH)2. Such cracks could have also formed due to the hydrogen evolution during the coating process [23].

In contrast, the SEM micrographs of coatings formed at pH 6.5 [Fig. 2(e&f)] and pH 7.5 [Fig. 2(g&h)] revealed a significant change in the morphology of coatings. The morphology of the coating obtained at pH 6.5 showed a uniform and densely packed growth of phosphates [Fig. 2(e)]. The sample surface was completely covered by the coating. Further vertical growth of phosphates on the already formed phosphates at few places could also be seen. The high magnification micrograph of the coating [Fig. 2(f)] illustrated that fine crystals of phosphates coalesced to form a continuous layer of coating with comparatively fewer cracks between the individual particles. There was not much change in the morphology of the coating obtained at pH 7.5 [Fig. 2(g&h)] except that the coating was denser and continuous with fine coalesced phosphate crystals. The cracks on the coating surface were also minimized to a greater extent at higher pH of the phosphating bath.

Table 2 presents the elemental compositions of the coatings obtained from the EDAX analysis. All the four coatings consisted of Mg, P, O and N suggesting that the phase composition of coatings should be newberyite and struvite. Atomic weight percentage of P and O in all the cases remained almost same, whereas the atomic weight percentage of N increased with corresponding reduction in that of Mg as the pH of the coating bath increased. The above observation suggested that the content of ammonia based struvite phase increased in the coating with the increase in pH of the coating bath.

The cross sections of MPCs obtained in various pH presented in Fig. 3 show the average film thickness of the coatings. At pH 4.5, a higher coating thickness of around 48 µm [Fig. 3(a)] was obtained. As the pH of coating bath increased, the coating thickness decreased and became more or less same. The coating obtained at pH 4.5 was highly non-uniform irrespective of the high thickness value. Irregular metal-coating interface as shown in Fig. 3(a) suggested a fast and non9

uniform metal dissolution occurred at pH 4.5. The average coating thickness obtained at the bath pH 5.5 [Fig. 3(b)] was about 30 µm, and it was much more uniform than that obtained at the bath pH 4.5. On the other hand, both the coatings obtained at pH 6.5 [Fig. 3(c)] and pH 7.5 [Fig. 3(d)] had an average thickness of ~ 32 µm and ~ 35 µm respectively with smooth metal-coating interfaces.

3.2 Corrosion tests 3.2.1 OCP measurement Fig. 4(a) shows the OCP of bare AZ31 and samples coated at varying pH of the coating bath in 1 wt.% NaCl solution. Overall the coated samples showed more noble potential compared to that of bare sample. The time required to obtain the steady state OCP varied from around 20 min. to 35 min. for different samples. Bare AZ31 reached steady state OCP (-1.585 V vs. SCE) after 20 min., while all coated samples took a relatively longer time to reach steady state. The results indicated that OCP of coated samples depended on pH of the coating bath. The initial OCP of all the coated samples were relatively negative which was a result of some coating defects that exposed some magnesium substrate to electrolyte. The accumulation of corrosion products in the pores and cracks of the coatings during the measurement restricted the further corrosion of the substrate and hence quickly increased the OCP to the positive direction. The OCP curve for the sample coated in pH 5.5 showed a different trend: after the initial increase in OCP for a few seconds, there was a sudden drop in the OCP. This rapid drop in potential could be related to the severe localized dissolution. The defects [Fig. 3(b)] on the relatively thin coating were the reason for such dissolution [25]. The OCP values of each sample are provided in Table 3. As the corrosion resistance of the coatings was evaluated by immersion test for 5 days in the present study, the OCP variations of the samples in 1 wt.% NaCl solution during long time immersion over the period of 120 h (5 days) were also monitored [ref. Table 3] and presented in Fig. 4(b). The readings were taken at a regular time interval of 4 h. For bare AZ31, after a relatively steady OCP for first 32 h, there was a sudden dip in OCP to the negative direction. This negative shift can be related to the formation of pits on the exposed surface. From 32 h to 48 h, the potential increased considerably to reach a steady state again. The self-limiting effect of pitting corrosion, associated with the cathodic reaction helped in bringing the potential to the positive direction again. Corrosion of Mg in aqueous medium results in the reduction of water as 10

a cathodic reaction. As the byproduct produced during cathodic reaction is OH-, the local pH value increases. This increase in pH accelerates the formation and stabilization of the corrosion products, mainly Mg(OH)2 [25] . Hence pitting corrosion depresses gradually with time. Though some signs of pitting were also observed after 64 h, 72 h and 92 h, they were not severe as the surface contained already formed hydroxide layer. For the coated sample at pH 4.5, signs of pitting were observed after 4 h, 12 h and 28 h which might be due to the penetration of electrolyte through the pores and cracks in coatings. After 34 h, the coating seemed to have attained a much more stable OCP due to the deposition of corrosion products in the pores. For samples coated at pH 5.5, after an initial stabilization (after 40 min.), the OCP values slowly shifted to the negative direction for first 24 h. Then the OCP was stable for the next 12 h. A sudden drop in potential was observed after 36 h which could be related to coating peel off. However, the OCP values were considerably steady from 48 h to 88 h before a fall in potential from -1.542 V vs. SCE to -1.558 V vs. SCE. The OCP values remained almost steady after 92 h showing no signs of further severe pitting occurring in the test sample till 120 h. Conversely, OCP values remained more or less steady for samples coated at pH 6.5 and pH 7.5. The above observations confirmed the coating quality obtained at these higher pH ranges of the coating bath. Like the OCP values obtained for short time [ref. Fig. 4(a)], all the coated samples showed relatively higher OCP values compared to bare sample during long term immersion. Though the sample coated in pH 5.5 showed a relatively positive OCP compared to all other samples during the short time measurement, increased immersion time damaged the coating which resulted in relatively lower OCP after 5 days [ref. Fig. 4(b)]. This phenomenon was not observed in higher pH ranges due to better coating quality obtained. 3.2.2 Polarization test The polarization curves for all the samples scanned from -200 mV on the cathodic side to +300 mV on the anodic side from OCP are shown in Fig. 5(a), and corresponding electrochemical data (Ecorr, Ebreak, βc and icorr)obtained from the polarization curves are presented in Table 3. The cathodic polarization region was only used for the calculation of corrosion current density (icorr). The anodic region was not considered due to the abnormal behavior (negative difference effect) exhibited by magnesium alloys during anodic dissolution [25-27]. To estimate icorr, a slope was drawn in the cathodic branch at +100 mV from the meeting point of anodic and cathodic curves (Ecorr) of the Tafel plot [ref. Fig. 5(b)]. The corrosion current of the point at which the slope line 11

intersect with the horizontal line drawn from the Ecorr was considered as icorr. The Ecorr values of the samples changed with respect to the pH of the coating bath. The Ecorr of the coated sample at pH 4.5 (-1.541 V vs. SCE) did not change much with that of bare sample (1.533 V vs. SCE) whereas there was a significant shift in Ecorr to the positive direction (-1.463 V vs. SCE) with samples coated at pH 5.5. In contrast, the Ecorr of the sample coated at pH 6.5 moved to -1.577 V vs. SCE whereas, the Ecorr of the sample coated at pH 7.5 shifted by 0.047 mV towards the positive side (-1.530 V vs. SCE). Moreover, the Tafel plots of the coated samples, except that of the sample coated at pH 5.5, shifted towards the left side to that of bare sample. This shift resulted in reduced icorr values for the coated samples [ref. Table 3] which is the most important parameter used for defining the corrosion resistance of a material or coating. In general, the reduced icorr values of the coated samples were due to the coatings obtained on the substrate which suppressed the diffusion of ions during corrosion. Moreover, the reduction in icorr values for the samples coated at higher pH 6.5 and 7.5 were more predominant compared to that for samples coated at lower pH 4.5 and pH 5.5. In spite of defective coating, a slightly better icorr value (48.9 µA/cm2) obtained for sample coated at pH 4.5 than that of the samples coated at pH 5.5 (63 µA/cm2) could be related to the higher thickness of the coating at pH 4.5. Dense and uniform coating obtained at pH 7.5 resulted in a significant reduction of the icorr value of the bare sample from 140 µA/cm2 to 9.02 µA/cm2. The anodic polarization curves of bare AZ31 as well as coated samples at pH 4.5 and pH 5.5 exhibited an active dissolution whereas the samples coated at both the pH 6.5 and pH 7.5 showed a passivation behavior. Moreover, the passivation was more predominant with sample coated at pH 7.5 [Fig. 5(a)]. The measured breakdown potentials (Ebreak) were around -1.553 V vs. SCE and -1.424 V vs. SCE for samples coated at pH 6.5 and pH 7.5 respectively (Table. 3) The occurrence of passivation with coated samples indicated that the coatings obtained at pH 6.5 and pH 7.5 were more stable than that obtained at pH 4.5 and pH 5.5. The uniform and less defective coatings (Fig. 2(e-h)] with the presence of the more struvite phase (Table. 2) probably increased the stability of the coatings, and hence provided an effective protection.

3.2.3 EIS analysis The electrochemical impedance behavior of the studied samples is represented as Nyquist plot in 12

Fig. 6(a). The bare and coated samples processed at pH 4.5 and pH 5.5 exhibited capacitive loops at high and medium frequencies, and inductive loops at low frequencies. On the other hand, the coated samples processed at pH 6.5 and pH 7.5 showed capacitive loops at all frequencies with weak inductive regions in the lower frequencies. The weakening of the inductive loop at the tested low frequency ranges in the samples coated at higher pH indicated the better performance of the coatings. In general, for bare substrates, the high frequency capacitive loop represents the formation of oxides or hydroxides at the metal-solution interface as the result of corrosion occurring during the test. Similarly in coated samples, the protective barrier formed on the substrate surface as a result of coating results in a capacitive loop in the high frequency region [28]. The intermediate-frequency loop represents the region which attributes to the mass transfer in the solid phase caused by the diffusion of Mg2+ ions coming out through oxides, hydroxides or phosphates in the barrier layer formed in coated samples, or through the corrosion products on the bare sample [29, 30]. The low frequency inductive loops are normally attributed to the relaxation of the adsorbed species. In the present study, low frequency inductive loop was prominent in bare AZ31 and coated samples processed at pH 4.5 and pH 5.5. The inability of phosphate layers formed on these specimens to hinder the exposure of AZ31 substrate was the reason for the inductive behavior at lower frequencies [31]. The presence of inductive loop also indicated the occurrence of pitting corrosion in the samples. Cracked and porous coatings [ref. Fig. 2(a-d)] obtained at lower pH 4.5 and pH 5.5 could not protect the surface effectively, and hence active corrosion occurred through those cracks and pores. On the other hand, the completely covered phosphate coatings [ref. Fig. 2(e-h)] on the samples coated at pH 6.5 and pH 7.5 provided a better protection even at tested lower frequencies and delayed the pitting corrosion which resulted in the weakening of the inductive loops.

The electrochemical results are also represented in Bode plots, log Zmod vs. log f (Fig. 6(b] and phase angle vs. log f (Fig. 6(c)] respectively. The high frequency region in Fig 6(b) represents solution resistance (Rs). A sudden dip in the curves at low frequency region could be interpreted as the effect of inductive loop for bare AZ31 and coatings formed at pH 4.5 and pH 5.5. In contrast, the plots for samples coated at pH 6.5 and pH 7.5 showed resistive behavior in the low frequency region. Moreover, Zmod of samples at lower frequency region (0.1 Hz) increased in the following order: bare AZ31 (120 Ω.cm2) < pH 4.5 (389 Ω.cm2) < pH 5.5 (1047 Ω.cm2) < pH 6.5 13

(2511 Ω.cm2) < (2582 Ω.cm2). Phase angle vs. log f curves [Fig. 6(c)] showed one time constant in the medium frequency region for bare AZ31 and coating obtained at pH 4.5 and pH 5.5. For samples coated in bath pH 6.5 and pH 7.5, two time constants were clearly seen: one at high frequencies and another one at medium frequencies. Normally, the high frequency capacitive loop corresponds to the barrier effect formed by the uniform and less porous coating. The time constant in the medium frequency appears due to double layer coating capacitance and the corresponding charge transfer resistance during the corrosion process. The experimental data obtained during EIS measurement was numerically fitted for detailed interpretation. Two sets of circuits were used for the analysis [Fig. 7(a&b)]. For bare as well as the samples coated at pH 4.5 and pH 5.5, one time constant circuit with an inductor [Fig. 7(a)] was used whereas the impedance data for the samples coated at pH 6.5 and pH 7.5 was fitted with two time constant circuit without the use of an inductor [Fig. 7(b)]. In both circuits, Rs represents the solution resistance between the specimen and reference electrode. In these equivalent circuits, a constant phase element (CPE) was used instead of a pure capacitance for the coating capacitance and the double layer capacitance between the coating and the metal substrate. The reason for the use of CPE was to obtain a good fit with considering the electrode surface roughness, inhomogeneous reaction rates on the electrode surface, varying thickness and non-uniform current distribution during the corrosion process [32]. The CPE is represented by the following relation

⁄

where T is a proportionality constant, ω is the angular frequency which can be represented as ω=2πf with f as the frequency, “n” represents the independent parameter and j = (-1)1/2. The “n” value varies from 0 to 1; for an ideal capacitor behavior, the value of n=1, for an ideal resistance behavior the value of n=0 and if n=0.5, then it represents Warburg impedance. Apart from Rs, the equivalent circuit of bare AZ31, and coated samples at pH 4.5 and pH 5.5 included a charge transfer resistance (Rct) in parallel with a double layer capacitance (CPE1) represented the corrosion process, and a resistance (RL) with an inductance (L) in series, represented the relaxation process of Mg+ and Mg(OH)2 occurring on the substrate surface. The time constant, Rct//CPE1, in the equivalent circuit for coatings obtained at pH 6.5 and pH 7.5 represented the medium frequency region and the second time constant (RH //CPE2) represented the high 14

frequency region [ref. Fig. 7(b)].

The fitted values of each element used in the equivalent electrical circuit were simulated using EIS spectrum analyzer and are shown in Table 4. The value of Rs of the bare sample was smaller than that of coated samples. The variation in Rs was mainly due to the reaction between the substrate and the ions in the electrolyte solution which decreased the ionic concentration and thereby causing an increase in the solution resistance for coated samples. In general, the charge transfer resistance (Rct) is correlated to the corrosion process i.e. high Rct means high resistance. The Rct values of the samples increased in the following order: bare AZ31 (178.7 Ω.cm2) < pH 4.5 (251 Ω.cm2) < pH 5.5 (1080 Ω.cm2) < pH 6.5 (1483 Ω.cm2) < pH 7.5 (1649 Ω.cm2). Similarly, CPE is related to corrosion reaction area i.e. low CPE implies relatively low area of the exposed surface is corroded. The fitted values of CPE1 showed continuous decrease in the double layer coating capacitance in the following order: AZ31 (51.3×10-6 sn/Ω.cm2) > pH 4.5 (46.3×10-6 sn/Ω.cm2) > pH 5.5 (27.5×10-6 sn/Ω.cm2) > pH 6.5 (5.98×10-6 snΩ.cm2) > pH 7.5 (5.69×10-6 sn/Ω.cm2). The values of n for all the coatings were closer to unity, indicating a capacitance behavior. However the deviation observed in the n values was normally due to nonuniformity in the coating, which led to non-uniform current distribution. The above impedance analysis suggested that the corrosion performance of the coating improved with increase in bath pH, which is in line with the Tafel analysis.

3.2.4 Immersion test As the electrochemical tests are short time measurements, a long time immersion corrosion test would provide more realistic information on the stability of the coatings. Average corrosion rates of both bare AZ31 as well as the coated samples calculated after 5 day immersion test in 1 wt.% NaCl solution presented in Fig. 8 showed a similar trend as observed with the electrochemical tests. In comparison with bare samples, coatings obtained in bath pH 4.5 and pH 5.5 showed a little over 50% improvement in corrosion rate while samples coated in bath pH 6.5 and pH 7.5 showed an average improvement of 76% and 80% respectively. Moreover, the standard deviation of the corrosion rates obtained for samples coated at both pH 6.5 and pH 7.5 were considerably smaller than that of the samples coated at lower pH. The uniform and dense coating obtained at the higher pH was the reason for such small variations. 15

The photograph of corroded bare AZ31 alloy sample presented in Fig. 9(a) shows that the sample suffered from severe localized pitting corrosion, which is the most common form of corrosion occurring in uncoated magnesium alloys [33]. Macro images of magnesium phosphate coated samples at pH 4.5 and pH 5.5 [ref. Fig. 9(b&c)] showed less corroded regions compared to bare AZ31. However, corrosion in the both samples seemed to be highly non-uniform. Many of the areas corroded so rapidly while some areas were not corroded at all. In contrast, the samples coated at pH 6.5 and pH 7.5 [Fig. 9(d&e)] showed comparatively less pitting corrosion and more non-corroded regions. Non porous and dense coating obtained at higher pH prevented the penetration of NaCl solution into the surface which attributed to reduced pitting corrosion. Even though similar coating thickness and morphology was obtained at pH 6.5 and pH 7.5, slightly better corrosion performance observed at pH 7.5 was due to the presence of more struvite content in the less defective and uniform coating obtained at this pH range.

4. Discussion 4.1Coating formation mechanism In the present study, composite coating of magnesium phosphate based on newberyite and struvite was obtained on AZ31 alloy substrate using chemical conversion coating using ammonium biphosphate bath at different pH. Pretreatment was carried out before coating as surface activation of the substrate plays an important role in chemical conversion coating [34]. The major alloying elements of AZ31 alloy are Al, Zn and Mn. The chemical analysis showed that the samples consisted of 0.982% of Zn and 0.108% of Mn. Normally, the added Zn goes into solid solution of Mg and also a small amount is found in the Mg17Al12 intermetallic. The purpose of Mn addition to Mg-Al alloys is to reduce the ill effect of Fe by forming dense Al-Mn-Fe intermetallics that settles in the bottom of the crucible. Moreover, these complex intermetallics are less harmful to the α-Mg matrix and thereby reduce the corrosion [4, 40]. Hence, both elements had minor role to play in the coating performance. On the other hand, the alloy consisted of about 2.838 % of Al [Table 1]. In Mg-Al alloys, most of added Al presents in the form of Mg17Al12 intermetallic at the interdendritic regions. Before any chemical conversion coatings, these intermetallics should be removed from the surface to obtain a uniform coating. The presence of intermetallics on the surface can act as active sites for galvanic corrosion as well 16

as they affect the adherence of phosphate coating. Hence substrate was treated with the bath containing NaF and NaNO2 to remove the intermetallic Mg17Al12 from the surface of AZ31. Fluoride ions from the activation bath react preferably with the Al of the Mg17Al12 and remove them by forming soluble AlF3 whereas NO2- ions in the activation bath increase the rate of the above reaction [34-35].

After the removal of Mg17Al12 intermetallic, unreacted fluoride ions present in the coating bath will react with Mg2+ present on the substrate to form MgF2 layer. Though MgF2 coating itself can provide effective corrosion protection, in the present study the influence of MgF2 layer formed on the Mg subtract during pretreatment should be negligible. Normally, longer processing time is required (24 h to 48 h) for relatively thick and compact MgF2 coatings [36, 37]. In the present study, the dipping time was about 5 min., and moreover, the F- ions concentration in the pretreatment bath was expected to be low after the formation of AlF3 which resulted in a random distribution of MgF2 layer with micro-cracks [38]. Hence 5 min. dipping was not sufficient to achieve a relatively thick and adhesive MgF2 layer. Moreover, there was a possibility that the thin layer of MgF2 might have been removed during the subsequent coating process. Fig. 10(a) shows the schematic representation of surface pretreatment of AZ31 substrate.

Followed by the surface activation, substrate was treated with the NH4H2PO4 bath to get the composite magnesium phosphate coating. The schematic representation of coating formation is represented in Fig. 10(b). The reactions that occurred in different coating bath pH and various phase formation could be explained using the following reactions. In the aqueous solution, NH4H2PO4 dissociates to form NH4+ and H2PO4- ions [ref. eq. (3)]. NH4H2PO4 NH4

H2PO4

(3)

The generation of Mg2+ ions can be understood with the basic corrosion of Mg occurring in aqueous medium which can be represented by following half-cell reactions [36, 39, 40].

17

Anodic reaction:

g2

g

Cathodic reaction:

2H

2e

2e

(4)

H2

(5)

Corrosion of Mg and its alloys results in the evolution of hydrogen gas, and will in turn increases the local pH of the solution [41]. However, the concentration of the Mg2+ ions in the solution is variable, since it is readily formed from the surface of the substrate when the substrate comes in contact with the NH4H2PO4 solution. Therefore the local concentration of Mg2+ ions near the surface of the substrate is higher than the bulk solution. With the increase in the concentration of the Mg2+ ions, MgHPO4.3H2O i.e. newberyite phase forms within the pH 3 to 9.5. Further changes in the pH of the solution results in the formation of Mg3(PO4)2 and Mg(OH)2 [39]. Zhou et al. [42] predicted the precipitation of struvite phase with software known as MINTEQ by incorporating thermodynamic database of struvite to it. Their study suggested that the precipitation of struvite dominates in the range of pH from 7.75 to 9.27. Therefore in the phosphating bath containing NH4+ ions, following reactions are expected to occur that results in the composite coating of magnesium phosphates of newberyite and struvite. g2

H2PO4

3H2O

g2

H2PO4

NH4

gHPO4.3H2O 6H2O

H

gNH4PO4.6H2O

(6) H

(7)

4.2 Effect of bath pH on coating morphology The pH of the coating bath solution affects the coatings in the following ways (i) rate of coating deposition, (ii) morphology, (iii) composition, (iv) thickness. In the present study, thick non uniform coarse phosphate coating with many defects was obtained in lower bath pH 4.5. The reaction rate or formation of coating at pH 4.5 was relatively high. Because of the aggressive nature of the magnesium dissolution reaction at this low pH, the rate of Mg2+ ion formation from the substrate should be high. In parallel, the rate of nucleation and growth of insoluble phosphates on the substrate should also be faster due to the local change in pH at the surface as the consequence of vigorous hydrogen evolution. More voids seen on the surface coated at bath

18

pH 4.5 [Fig. 2(a&b)] was mainly due to the vigorous hydrogen bubble formation during the coating. The coating reaction was comparatively slow at slightly higher bath pH 5.5. The improvement in morphology [ref. Fig. 2(c&d)] of the coating at pH 5.5 suggested that the Mg dissolution reaction and phosphate formation were slower but at a steady rate. Also, the morphology of coating changed from relatively coarse to fine particles because of the lower reaction rate at pH 5.5. In case of bath pH 6.5 and pH 7.5, the metal dissolution reaction was less aggressive and the formation of Mg2+ ions over the surface of the substrate was more uniform that resulted in the formation of phosphate coating with more uniformity and fewer defects [ref. Fig. 2(e-h)].

The thickness of the coating also varied with change in coating bath pH. In general, initial nucleation of the phosphates occurs on the substrate and then continuously grows laterally to cover the surface. Further nucleation starts on the already formed phosphates on the surface and grows again to form a continuous layer of phosphate along the thickness direction. The coating process continues as long as the Mg2+ ions are available near the coating surface. Higher coating thickness obtained at pH 4.5 could be explained with the high reactivity of magnesium substrate and availability of more Mg2+ ions at the surface at lower pH. More phosphates particularly newberyite phase forms at a faster rate, but with coarse crystal size [ref. Fig. 3(a)]. However, the coating thickness obtained in coating bath pH 5.5, pH 6.5 and pH 7.5 had relatively similar thickness of 30 µm, 32 µm and 35 µm [ref. Fig. 3(b-d)] respectively. In pH 5.5, the smaller thickness was obtained due to relatively lower reaction rate. In coating bath pH 6.5 and 7.5, even though the reaction rate was reduced, struvite formation was favorable at this pH, which helped in achieving a similar coating thickness at relatively higher pH. 4.3 Effect of bath pH on the corrosion performance of coating Both short term electrochemical and long term immersion weight loss measurements indicated that corrosion resistance of bare AZ31 improved significantly with magnesium phosphate coatings. Moreover, the corrosion performance was influenced by pH of the coating bath. As the pH of the bath increased, the coating morphology became finer with less defect and uniform thickness [ref. Fig. 2(e&h)] due to steady metal dissolution reaction. Also, the formation of stable struvite was increased at higher pH (Table 2). These factors improved the corrosion resistance of the coating. 19

The corrosion results also indicated that the corrosion behavior of the coating obtained at pH 5.5 showed slightly different trend as it showed slightly poor corrosion resistance in the immersion test (Fig. 8) and slightly higher icorr during polarization test (Table 3). Moreover, polarization behavior of the coating obtained at pH 5.5 also showed slightly higher cathodic behavior [Fig. 5(a)]. As explained earlier, the major factors affecting the corrosion performance of a coating are morphology, composition and thickness of the coatings. The findings of the present study indicated that the compositions, thickness and morphology of the coatings changed with respect to the change in the coating bath pH. The EDAX analysis (Table 2) indicated that the coating obtained at pH 4.5 and pH 5.5 consisted of mainly newberyite phase with slight difference in the struvite content. Even though the morphologies of these coatings were almost similar, the major difference was seen with the coating thickness. The thickness of the coating obtained at pH 5.5 (30 µm) was smaller when compared to that of pH 4.5 (45 µm) [ref. Fig. 3(a&b)]. The increased coating thickness hindered the rate of cathodic reaction, and hence corrosion performance of the coated sample in bath pH 4.5 was better than that of samples coated at bath pH 5.5. Similar trend was also noticed in the immersion test (Fig. 8). On the other hand, though coating thicknesses obtained at pH 5.5, pH 6.5 and pH 7.5 were similar, the composition and morphology of the coatings were different. Uniform coatings of densely packed and fine phosphate particles with fewer defects reduced the cathodic activities of the samples coated at bath pH 6.5 and pH 7.5. In addition, the increased formation of insoluble struvite phase improved the anticorrosive properties of the coating [23, 43].

Though the corrosion rates of Mg alloys calculated from weight loss measurement after long time immersion and icorr calculated from the short term electrochemical test shows a similar trend, the corresponding corrosion rates of samples do not match each other [26, 44]. Fig. 11 shows a comparative study on the corrosion rates obtained from potentiodynamic polarization test and long time immersion test. The icorr obtained by extrapolating the cathodic curves from the Tafel plot was used in calculating the corrosion rate. The equation used for calculating corrosion rate (mm/y) from icorr is given below [26, 44-46]:

20

where icorr is expressed in mA/cm2 Average corrosion rate obtained from both tests and % deviation between the corrosion rates are also provided in Table 5. Even though both the corrosion tests were conducted in the same corrosive medium (1 wt.% NaCl), there was a considerable deviation observed as expected. A study conducted by Shiet.et al. [44] found out a typical deviation of 48% to 96% in the corrosion rates for Mg alloys. The difference in test durations is the major reason for such deviations. Potentiodynamic polarization test is a short term technique whereas immersion test represents a long term technique. Highly localized corrosion phenomenon is common in many of the Mg alloys in NaCl solution. Corrosion starts at a localized area in the sample and spread laterally with respect to time. Thus to reach a steady state corrosion, a long time immersion in corrosive medium is required. Hence the corrosion rate obtained from Tafel extrapolation is not related to steady state corrosion. Many authors, in their studies, discussed a number of reasons for such abnormal behavior exhibited by Mg alloys in long term and short term tests [26, 41, 44, 46-48]. Potential reasons are (i) areas exposed during immersion and potentiodynamic polarization test are different (in the present study, areas exposed in potentiodynamic polarization and immersion tests were 1 cm2 and 6 cm2 respectively), which leads to difference in electrochemical reactions occurring in both tests (ii) possibility that the cathodic curve obtained from Tafel plot represents more than one cathodic reaction. Percentage deviation of the difference in corrosion rates of the tested samples varied from 49.28% for bare AZ31 alloy to 84.49% for coating obtained at pH 7.5 suggesting that the percentage deviation in corrosion rate increased as the pH of coating bath increased from 4.5 to 7.5. During instantaneous polarization test, the prolonged stability of the coating could not be accounted unlike in long term immersion test which led to the observed discrepancy. 5. Conclusions The following conclusions are drawn from the study.

1. In addition to the formation of conventional newberyite phase of Mg phosphate, the formation of a stable struvite based magnesium phosphate coating could be obtained by using NH4+ containing phosphating bath, and struvite content was enhanced by increasing the pH of the coating bath. 21

2. For a fixed phosphating time, higher coating thickness was obtained with lower pH 4.5. However, the coating consisted of coarse phosphate crystals with many cracks and pores. As the pH of the coating bath increased the coating thickness became more uniform with fewer defects. 3. The corrosion performance of the coatings improved with increase in the bath pH which was attributed to the quality of coatings and presence of more insoluble struvite phase. 4. As reported earlier, a discrepancy between the corrosion rates calculated from the short term polarization test and the long term immersion test was noticed due to the fact that the short term tests are not related to the steady state corrosion.

Acknowledgement The authors would like to thank the Council of Scientific and Industrial Research (CSIR), India for the financial support and the Director, CSIR-National Institute for Interdisciplinary Science and Technology (NIIST), Trivandrum for providing an opportunity to conduct the study. Acknowledgments are also due to the Directors of National Institute of Technology, Calicut and Indira Gandhi Center for Atomic Research (IGCAR), Kalpakkam.

22

References 1. A. S. Hamdya, D. P. Butt, Corrosion mitigation of rare-earth metals containing magnesium

EV31A-T6 alloy via chrome-free conversion coating treatment, Electrochemica Acta, 108 (2013) 852-859. 2. Y. F. Zheng, X. N. Gu, F. Witte, Biodegradable metals, Materials Science and Engineering

R, 77 (2014) 1-34. 3. Y. Chen, Z. Xu, C. Smith, J. Sankar, Recent advances on the development of magnesium

alloys for biodegradable implants, Acta Biomaterialia, 10 (2014) 4561-4573. 4. G. L. Song, Recent progress in corrosion and protection of magnesium alloys, Advanced

Engineering Materials, 7 (2005) 563-586. 5. A.S. Gnedenkov, S.L. Sinebryukhov, D.V. Mashtalyar, S.V. Gnedenkov, Localizedcorrosion

of the Mg alloys with inhibitor-containing coatings: SVET and SIET studies,Corrosion. Science, 102 (2016) 269-278. 6. H. Hornberger, S. Virtanen, A.R. Boccaccini, Biomedical coatings on magnesium alloys – A

review, Acta Biomaterialia, 8 (2012) 2442-2455. 7. J.E. Gray, B. Luan, Protective coatings on magnesium and its alloys- a critical review,

Journal of Alloys and Compounds 336 (2002) 88-113. 8. H. Wang, R. Akid, M. Gobara, Scratch-resistant anticorrosion sol–gel coating for the

protection of AZ31 magnesium alloy via a low temperature sol–gel route, Corrosion Science, 52 (2010) 2565-2570. 9. O. Lunder, J.C. Walmsley, P. Mack, K. Nisancioglu, Formation and characterization of a

chromate conversion coating on AA6060 aluminium, Corrosion Science, 47 (2005) 16041624. 10. M. Ren, S. Cai , T. Liu, K. Huang, X. Wang, H. Zhao, S. Niu, R. Zhang, X. Wu, Calcium

phosphate glass/MgF2 double layered composite coating for improving the corrosion resistance of magnesium alloy, Journal of Alloys and Compounds, 591 (2014) 34–40. 11. S. Pommiers, J. Frayret, A. Castetbon, M. P. Gautier, Alternative conversion coatings to

chromate for the protection of magnesium alloys, Corrosion Science, 84 (2014) 135–146. 12. T. Ishizaki, I. Shigematsu, N. Saito, Anticorrosive magnesium phosphate coating on AZ31

magnesium alloy, Surface & Coatings Technology, 203 (2009) 2288-2291. 23

13. S. V. Dorozhkin, Calcium orthophosphate coatings on magnesium and its biodegradable

alloys,Acta Biomaterialia, 10 (2014) 2919-2934. 14. R. Zeng, F. Zhang, Z. Lan, H. Cui, E. Han, Corrosion resistance of calcium-modified zinc

phosphate conversion coatings on magnesium–aluminium alloys, Corrosion Science 88 (2014) 452-459. 15. X. B. Chen, D. R. Nisbet, R. W. Li, P. N. Smith, T. B. Abbott, M. A. Easton, D. H. Zhang,

N. Birbilis, Controlling initial biodegradation of magnesium by a biocompatible strontium phosphate conversion coating, Acta Biomaterialia, 10 (2014) 1463-1474. 16. M. Fouladi, A. Amadeh, Effect of phosphating time and temperature on microstructure and

corrosion behavior of magnesium phosphate coating, Electrochimica Acta, 106 (2013) 1-12. 17. H. H. Elsentriecy, H. Luo, H. M. Meyer, L. L. Grado, J. Qu, Effects of pretreatment and

process temperature of a conversion coating produced by an aprotic ammonium-phosphate ionic liquid on magnesium corrosion protection, Electrochimica Acta 123 (2014) 58-65. 18. T. Ishizaki, R. Kudo, T. Omi, K. Teshima, T. Sonoda, I. Shigematsu, M. Sakamoto,

Corrosion resistance of multilayered magnesium phosphate/magnesium hydroxide film formed on magnesium alloy using steam-curing assisted chemical conversion method, Electrochimica Acta, 62 (2012) 19-29. 19. Q. Zhao, W. Mahmood, Y. Zhu, Synthesis of dittmarite/Mg(OH)2 composite coating on

AZ31 using hydrothermal treatment, Applied Surface Science, 367 (2016) 249-258. 20. N. V. Phuong, S. Moon, Comparative corrosion study of zinc phosphate and magnesium

phosphate conversion coatings on AZ31 Mg alloy, Materials Letters, 122 (2014) 341-344. 21. F. Tamimi, D. L. Nihouannen, D. C. Bassett, S. Ibasco, U. Gbureck,, J. Knowles, A. Wright,

A. Flynn, S. V. Komarova, J. E. Barralet, Biocompatibility of magnesium phosphate minerals and their stability under physiological conditions, Acta Biomaterialia, 7 (2011) 2678-2685. 22. L. Wu, L. Zhao, J. Dong, W. Ke, N. Chen, Potentiostatic conversion of phosphate mineral

coating on AZ31 magnesium alloy in 0.1 M K2HPO4 solution, Electrochimica Acta, 145 (2014) 71-80. 23. H. Zhao, S. Cai, Z. Ding, M. Zhang, Y. Li, G. Xu, A simple method for the preparation of

magnesium phosphate conversion coatings on a AZ31 magnesium alloy with improved corrosion resistance, RSC Advances, 5 (2015) 24586-24590.

24

24. X. Lu, K. Shih, X. Li, G. Liu, E. Y. Zeng, F. Wang, Accuracy and application of quantitative

X-ray diffraction on the precipitation of struvite product, Water Research 90 (2016) 9-14. 25. Y.R. Chu, C.S. Lin, Citrate gel conversion coating on AZ31 magnesium alloys, Corrosion

Science, 87 (2014) 288-296. 26. M. C. Zhao, P. Schmutz, S. Brunner, M. Liu, G. L. Song, A. Atrens, An exploratory study of

the corrosion of Mg alloys during interrupted salt spray testing, Corrosion Science 51 (2009) 1277–1292. 27. T. R. Thomaz, C. R. Weber, T. Pelegrini Jr., L. F. P. Dick, G. Knörnschild, The negative

difference effect of magnesium and of the AZ91 alloy in chloride and stannate-containing solutions, Corrosion Science, 52 (2010) 2235-2243. 28. H. Ardelean, I. Frateur, P. Marcus, Corrosion protection of magnesium alloys by cerium,

zirconium and niobium-based conversion coatings, Corrosion Science, 50 (2008) 1907-1918. 29. G. L. Song, A. Atrens, D. StJohn, J. Nairn, Y. Li, The electrochemical corrosion of pure

magnesium in 1 N NaCl, Corrosion Science, 39 (1997) 855-875. 30. G. L. Song, A. Atrens, D. StJohn, X. Wu, and J. Nairn, The anodic dissolution of magnesium

in chloride and sulphate solutions, Corrosion Science, 39 (1997) 1981-2004. 31. S. K. Roy, M. E. Orazem, B. Tribollet, Interpretation of low-frequency inductive loops in

PEM fuel cells, Journal of The Electrochemical Society, 154 (2007) B1378-B1388. 32. D. Huang, J. Hu, G. L. Song, X. Guo, Inhibition effect of inorganic and organic inhibitors on

the corrosion of Mg–10Gd–3Y–0.5Zr alloy in an ethylene glycol solution at ambient and elevated temperatures, Electrochimica Acta, 56 (2011) 10166-10178. 33. F. Cao, G. L. Song, A. Atrens, Corrosion and passivation of magnesium alloys, Corrosion

Science, 111 (2016) 835-845. 34. T. S. N. S. Narayanan, S. Park, M. H. Lee, Surface Modification of Magnesium and its

Alloys for Biomedical Applications, Woodhead Publishing Series in Biomaterials, 2 (2015) 23-52. 35. H. Y. Yang, X. B. Chen, X. W. Guo, G. H. Wu, W. J. Ding, N. Birbilis, Coating pretreatment

for Mg alloy AZ91D, Applied Surface Science, 258 (2012) 5472-5481. 36. F. Witte, J. Fischer, J. Nellesen, C. Vogt, J. Vogt, T. Donath, F. Beckmann, In vivo corrosion

and corrosion protection of magnesium alloy LAE442, Acta Biomaterialia, 6 (2010) 17921799. 25

37. M. Thomann, C. Krause, N. Angrisani, D. Bormann, T. Hassel, H. Windhagen, A. M.

Lindenberg, Influence of a magnesium-fluoride coating of magnesium-based implants (MgCa0.8) on degradation in a rabbit model, Journal of Biomedical Materials Research, 93A (2010) 1609-1619. 38. N. V. Phuong, S. Moon, D. Chang, K. H. Lee, Effect of microstructure on the zinc phosphate

conversion coatings on magnesium alloy AZ91, Applied Surface Science, 264 (2013) 70-78. 2+

3-

39. X. B. Chen, N. Birbilis, T.B. Abbott, Effect of [Ca ] and [PO4 ] levels on the formation of

calcium phosphate conversion coatings on die-cast magnesium alloy AZ91D, Corrosion Science, 55 (2012) 226-232. 40. G. L. Song, A. Atrens, Understanding magnesium corrosion – A framework of improving

alloy performance, Advanced Engineering Materials, 5 (2003) 837-858. 41. G. L. Song, Corrosion behavior and prevention strategies for magnesium (Mg) alloys,

Corrosion Prevention of Magnesium Alloys, Woodhead Publishing, (2013) 3-37. 42. S. Zhou, Y. Wu, Improving the prediction of ammonium nitrogen removal through struvite

precipitation, Environmental Science and Pollution Research, 19 (2012) 347-360. 43. I. A. Kartsonakis, S. G. Stanciu, A. A. Matei, E. K. Karaxi, R. Hristu, A. Karantonis, C. A.

Charitidis, Evaluation of the protective ability of typical corrosion inhibitors for magnesium alloys towards the Mg ZK30 variant, Corrosion Science, 100 (2015) 194-208. 44. Z. Shi, M. Liu, A. Atrens, Measurement of the corrosion rate of magnesium alloys using

Tafel extrapolation, Corrosion Science, 52 (2010) 579–588. 45. ASM International, Metals handbook, Corrosion, ninth ed., Vol. 13, ASM International,

1987. 46. M. C. Zhao, M. Liu, G. L. Song, A. Atrens, Influence of pH and chloride ion concentration

on the corrosion of Mg alloy ZE41, Corrosion Science, 50 (2008) 3168-3178. 47. G. L. Song, Corrosion behavior of magnesium alloys and protection techniques, Surface

Engineering of Light Alloys, Woodhead Publishing, (2010) 3-39. 48. S. Johnston, Z. Shi, M. S. Dargusch, A. Atrens, Influence of surface condition on the

corrosion of ultra-high-purity Mg alloy wire, Corrosion Science, 108 (2016) 66-75.

26

49.

Figure captions Fig. 1. XRD analysis of bare as well as coated substrates at different coating bath pH.

Fig. 2. SEM micrographs of coated AZ31 substrates in different pH of the phosphating bath: (a) and (b) pH 4.5, (c) and (d) pH 5.5, (e) and (f) pH 6.5, (g) and (h) pH 7.5

Fig. 3. Cross-sectional optical images showing coating thickness obtained at different coating bath pH: (a) pH 4.5, (b) pH 5.5, (c) pH 6.5 and (d) pH 7.5

Fig. 4. Open circuit potential measurement for bare AZ31 as well as samples coated at various bath pH in 1 wt.% NaCl solution (a) for shorter time (upto 35 min.) (b) for 5 days (each readings taken in a time interval of 4 h).

Fig. 5. (a)The potentiodynamic polarization curves of bare as well as the coated samples at different bath pH scanned at their corresponding OCP in 1 wt.% NaCl at a scan rate of 1 mV/s and (b) Illustration of determining cathodic current density (icorr) from Tafel plot. Fig. 6. EIS analysis conducted for bare AZ31 and coated samples at their open circuit potential with a peak-to-peak amplitude of 10 mV in the frequency range from 105 to 10-1 Hz. (a) Nyquist plots, (b) Bode plots of log Zmod vs. log f and (c) Bode plot of Phase angle vs. log f. Fig. 7. Equivalent circuit models used to fit the EIS data: (a) for bare AZ31 and samples coated at bath pH 4.5 & pH 5.5 and (b) for samples coated at bath pH 6.5 & pH 7.5.

Fig. 8. Corrosion rate of bare AZ31 and coated samples at different bath pH obtained from 120 h immersion test in 1 wt.% NaCl.

Fig. 9. Photographs of corroded samples after immersion test: (a) bare AZ31, samples coated at different bath pH (b) pH 4.5, (c) pH 5.5, (d) pH 6.5 and (e) pH 7.5.

27

Fig. 10. Schematic representation of (a) surface pretreatment in NaF and NaNO2 solution (b) coating formation on the AZ31 substrate in NH4H2PO4 phosphate bath . Fig. 11. Comparison on corrosion rates of samples obtained from weight loss measurement from long term immersion and corrosion current density (icorr) from potentiodynamic polarization test.

Table Captions Table 1. Chemical composition of AZ31 cast alloy. Table 2. EDAX analysis of coatings obtained at different pH of the phosphate bath. Table 3. OCP, Ecorr, Ebreak, βc and icorr of samples obtained from electrochemical tests. Table 4. The fitted values of EIS measurements of bare AZ31 and samples coated at different bath pH using equivalent circuits shown in Fig. 7. Table 5. Corrosion rates of samples obtained from immersion and potentiodynamic polarization tests, and their percentage deviations.

Table 1. Chemical composition of AZ31 cast alloy. Alloy Al composition

Zn

Mn

Fe

Cu

Ni

wt.%

0.982

0.108

0.002

0.020

0.003

2.838

28

Table 2. EDAX analysis of coatings obtained at different pH of the phosphate bath. Composition pH

N

O

Mg

P

wt.%

at.%

wt.%

at.%

wt.%

at.%

wt.%

at.%

4.5

3.38

4.78

50.59

62.70

17.40

14.19

28.63

18.33

5.5

5.69

7.78

53.56

64.14

16.97

13.37

23.78

14.71

6.5

9.70

12.99

52.71

61.78

14.87

11.47

22.72

13.75

7.5

12.94

17.11

52.24

60.47

9.81

7.47

25.01

14.95

29

Table 3. OCP, Ecorr, Ebreak, βc, icorr of samples obtained from electrochemical tests. Sample

OCP (V vs. SCE)

Ecorr (V vs.

Ebreak (V vs.

βc

icorr

SCE)

SCE)

(mV/decad

(µA/cm2)

e)

AZ31 alloy

pH 4.5

pH 5.5

pH 6.5

pH 7.5

* #

#

Short term*

Long term

-

-

-

1.585(±0.0

1.579(±0.0

1.533(±0.02

28)

34)

2)

-

-

-

1.551(±0.0

1.568(±0.0

1.541(±0.02

24)

28)

1)

-

-

-

1.501(±0.0

1.560(±0.0

1.463(±0.04

37)

41)

3)

-

-

-

-

1.557(±0.0

1.558(±0.0

1.577(±0.03

1.553(±0.011

28)

32)

1)

)

-

-

-

-

1.542(±0.0

1.539(±0.0

1.530(±0.03

1.424(±0.023

28)

35)

4)

)

N/A

-248(±107)

42)

N/A

-239(±96)

-191(±103)

63.0(±6.7 5)

-224(±78)

10.7(±2.0 6)

-258(±89)

Steady state OCP values after immersion in 1 wt.% NaCl (20-35 min.).

30

48.9(±4.2 4)

N/A

OCP values after 120 h immersion in 1 wt.% NaCl.

140.0(±6.

9.02(±3.0 8)

Table 4. The fitted values of EIS measurements of bare AZ31 and samples coated at different bath pH using equivalent circuits shown in Fig. 7.

2

Rs (Ω.cm ) Rct (Ω.cm2) CPEc (sn/Ω.cm2) nc Rox (Ω.cm2) CPEox (sn/Ω.cm2) nox L (H)

Bare AZ31

pH 4.5

pH 5.5

pH 6.5

pH 7.5

26.54 178 51.6×10-6 0.873 132 260.3

40.18 251 46.3×10-6 0.812 180 187.8

36.30 1080 27.5×10-6 0.828 485 116.1

42.46 1483 5.98×10-6 0.782 1073 15.8×10-6 0.905 -

39.22 1649 5.69×10-6 0.981 1160 21.3×10-6 0.821 -

31

Table 5. Corrosion rates of samples obtained from immersion and potentiodynamic polarization tests, and their percentage deviations. Corrosion rate Condition

Immersion Test (mm/y) Polarization Test (mm/y)

% Deviation

Bare AZ31

6.29(±0.18)

3.19(±0.14)

49.28

pH 4.5

3.35(±0.38)

1.09(±0.09)

67.46

pH 5.5

3.74(±0.23)

1.43(±0.15)

61.76

pH 6.5

1.47(±0.07)

0.23(±0.47)

84.35

pH 7.5

1.29(±0.08)

0.20(±0.70)

84.49

32

Graphical Abstract

A dense and uniform coating with more struvite phase obtained at pH 7.5 showed better corrosion resistance.

Figure1

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Figure11