materials Article
The Preparation, Characterization and Formation Mechanism of a Calcium Phosphate Conversion Coating on Magnesium Alloy AZ91D Dong Liu 1,2,3 , Yanyan Li 1 , Yong Zhou 2 and Yigang Ding 2, * 1 2 3
*
School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan 430205, China;
[email protected] (D.L.);
[email protected] (Y.L.) Key Lab for Green Chemical Process of Ministry of Education, Wuhan Institute of Technology, Wuhan 430205, China;
[email protected] State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, China Correspondence:
[email protected]; Tel.: +86-132-9669-0986
Received: 21 April 2018; Accepted: 22 May 2018; Published: 28 May 2018
Abstract: The poor corrosion resistance of magnesium alloys is one of the main obstacles preventing their widespread usage. Due to the advantages of lower cost and simplicity in operation, chemical conversion coating has drawn considerable attention for its improvement of the corrosion resistance of magnesium alloys. In this study, a calcium phosphate coating was prepared on magnesium alloy AZ91D by chemical conversion. For the calcium phosphate coating, the effect of processing parameters on the microstructure and corrosion resistance was studied by scanning electron microscope (SEM) and electrochemical methods, and the coating composition was characterized by X-ray diffraction (XRD). The calcium phosphate coating was mainly composed of CaHPO4 ·2H2 O (DCPD), with fewer cracks and pores. The coating with the leaf-like microstructure provided great corrosion resistance to the AZ91D substrate, and was obtained under the following conditions: 20 min, ambient temperature, and no stirring. At the same time, the role of NH4 H2 PO4 as the coating-forming agent and the acidifying agent in the conversion process was realized, and the formation mechanism of DCPD was discussed in detail in this work. Keywords: magnesium alloy; formation mechanism
calcium phosphate coating;
corrosion resistance;
SEM;
1. Introduction Due to their low density, high strength/weight ratio, good machinability, and excellent recyclability, magnesium (Mg) and its alloys are becoming increasingly popular for use in various industries, including transportation, communication, and personal electronics [1–4]. Unfortunately, the poor corrosion and high chemical reactivity of Mg and its alloys, especially in chloride-containing environments, restrict their widespread usage [5–8]. Hence, it is critical to develop an effective protection technology in order to enhance the corrosion protection to magnesium and its alloys. At present, anodizing [9,10], micro-arc oxidation [11], chemical conversion coatings [12–15], and electroless plating [16] are the common surface treatments for improving the corrosion resistance of magnesium alloys, of which phosphate conversion coatings are regarded as one of the most effective and cheapest methods [17,18]. Up to now, a series of phosphate conversion coatings containing zinc-based [19], manganese-based [20], barium-based [21], and calcium-based phosphate conversion coatings [22,23] has been developed. Due to the intrinsic bioactivity and biocompatibility of calcium-based coating, and the environmentally friendly conversion technology leading to widespread Materials 2018, 11, 908; doi:10.3390/ma11060908
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application in the medical field [24,25], calcium-based phosphate conversion coating is becoming the hot topic in surface treatments of magnesium alloys. Kumta et al. [26] reported that a calcium phosphate coating was deposited on AZ31 magnesium alloy through acid etching, after which it was rinsed, polished, subjected to sonication and alkaline treatment, and finally immersed for 72 h at 37 ◦ C. However, this pretreatment process and immersion time are too intricate and too time consuming to become popular. Su et al. [27,28] selected the ternary system containing Ca(NO3 )2 , CaO, and H3 PO4 to deposit the CaHPO4 ·2H2 O coating on AZ61 magnesium alloy at the necessary pH adjusted by NaOH solution. The problem is that the difficulty of preparing the calcium phosphate bath and the given temperature still hinders the industrialization of the process. Additionally, Song et al. [29,30] developed a simple calcium phosphate solution consisting of Ca(NO3 )2 and NH4 H2 PO4 . Although the phosphate conversion coating showed excellent corrosion resistance for Mg-8.8Li alloy, the operation conditions needed the specific temperature and pH, as well as complicated pretreatments including grinding, ultrasonic cleaning, polishing with 0.5 µm diamond paste, and etching. On this basis, Sun et al. [31] optimized this process and studied the effect of the addition of triethanolamine on the morphology and corrosion resistance of calcium phosphate coating. However, the pretreatment was still complex, and the calcium phosphate bath needed the HNO3 /NaOH solution to adjust the appropriate pH value. Hence, it is urgent to develop a calcium phosphate solution with simple components that is convenient to prepare, and a short conversion coating process at room temperature. In this work, the binary system including Ca(NO3 )2 ·4H2 O and NH4 H2 PO4 is selected as the coating-forming agent. Meanwhile, NH4 H2 PO4 acts as an acidifying agent for optimizing the composition ratio of the calcium phosphate solution. In addition, the effect of processing parameters, such as conversion time, temperature, pH, and stirring rate, on the morphology and corrosion resistance of conversion coating is investigated by means of SEM, potentiodynamic polarization curve, and electrochemical impedance spectroscopy (EIS). Finally, the formation mechanism is also proposed to reveal the changes of coating in the process of deposition. 2. Materials and Methods The experimental material used for this investigation is die-casting AZ91D magnesium alloy (10 mm × 10 mm × 18 mm) with a nominal composition (wt %) of 9.1% Al, 0.9% Zn, 0.24% Mn, 0.031% Si, 0.0023% Fe, 0.015% Cu, 0.0005% Ni, and Mg balance. The specimens were connected to lead wires and embedded in epoxy resins with an exposed surface of 1 cm2 . Before treatment, the sealed electrode was carefully polished with 2000-grit SiC papers, rinsed with water, and then dried in cold air. Various calcium phosphate (Ca-P) solutions containing 25 g/L Ca(NO3 )2 ·4H2 O and different concentrations of NH4 H2 PO4 , i.e., 5 g/L, 10 g/L, 15 g/L, 20 g/L and 25 g/L NH4 H2 PO4 , were prepared for obtaining different Ca(NO3 )2 ·4H2 O:NH4 H2 PO4 concentration ratios of Ca-P solution. Hence, the corresponding concentration ratios of the Ca-P solution are 5:1, 5:2, 5:3, 5:4, and 5:5, respectively. Meanwhile, the corresponding pH values of the different concentration ratios of Ca-P solution are 3.5, 3.0, 2.8, 2.6, and 2.5 respectively. When the concentration ratio of Ca(NO3 )2 ·4H2 O:NH4 H2 PO4 is 5:1 and 5:2, the Ca-P solution presents clear and transparent. When it comes to the concentration ratio of 5:3, the status of the Ca-P solution presents as slightly turbid, indicating the maximum solubility of Ca(NO3 )2 ·4H2 O and NH4 H2 PO4 under such conditions. However, the further increase in concentration ratio (5:4) leads to the occurrence of precipitations in such Ca-P solution, and more precipitations appeared in the higher concentration ratio of Ca-P solution (5:5). To obtain the optimum processing parameters, the pre-treated AZ91D magnesium samples were immersed in Ca-P baths with different Ca(NO3 )2 ·4H2 O:NH4 H2 PO4 concentration ratios, temperatures, immersion times, pH values, and stirring rates, and were then studied by electrochemical tests to evaluate the corrosion resistance of the coating. The corrosive medium was 3.5% (wt %) NaCl. Electrochemical tests were carried out using a classical three electrodes cell with platinum as the counter electrode, saturated calomel electrode (SCE) as the reference electrode, and the AZ91D specimen as the working electrode. The potentiodynamic polarization curves were obtained using an electrochemical
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electrochemical analyzer (CS 310, Wuhan, China)voltage at a constant voltage scan rate of− 1 200 mV/s from −200 analyzer (CS 310, Wuhan, China) at a constant scan rate of 1 mV/s from mV OCP to an −3 A/cm2. Electrochemical impedance spectroscopy (EIS) mV OCP to an anodic current −density of 10 3 2 anodic current density of 10 A/cm . Electrochemical impedance spectroscopy (EIS) measurements measurements were conducted open circuit potential overrange a frequency rangetoof10100 kHz to were conducted at open circuitatpotential (OCP) over a (OCP) frequency of 100 kHz mHz with 10 mHz with a sinusoidal amplitude of 5 mV. Prior to each electrochemical test, a stabilization period a sinusoidal amplitude of 5 mV. Prior to each electrochemical test, a stabilization period of 1800 s of 1800 s was applied. was applied. The Thetreated treatedsurface surfacewas wascharacterized characterizedrespectively respectivelyby byX-ray X-raydiffraction diffraction(XRD, (XRD,D8, D8,Karlsruhe, Karlsruhe, Germany) using a Cu Kα radiation, and by field-emission scanning electron microscopy (SEM, (SEM, JSMGermany) using a Cu Kα radiation, and by field-emission scanning electron microscopy 5510LV, Kyoto, Japan) equipped with with an energy dispersion X-rayX-ray spectrometry (EDS). JSM-5510LV, Kyoto, Japan) equipped an energy dispersion spectrometry (EDS). 3.3.Results Resultsand andDiscussion Discussion
3.1.Ca-P Ca-PBath BathComponent Component 3.1. Generally, the phosphate conversion takes takes place in acid solution, needs HCl/NaOH Generally, thecalcium calcium phosphate conversion place in acid which solution, which needs solution to adjust the pH of the Ca-P solution. This process is complicated for the preparation of HCl/NaOH solution to adjust the pH of the Ca-P solution. This process is complicated for the the Ca-P bath. study, Ca(NO substance) NH4 Hsubstance) preparation of In thethis Ca-P bath. In this study, Ca(NOneutral 3)2·4H2O (a nearlyand neutral and 3 )2 ·4H 2 O (a nearly 2 PO4 (an acidic substance) were selected as coating-forming agents. Meanwhile, NH H PO (as acidifying agent) with NH 4H2PO4 (an acidic substance) were selected as coating-forming agents. Meanwhile, NH 4 H 2 PO 4 (as 4 2 4 different masses was added into the 25was g/Ladded Ca(NO 4H225 O solution, in3order tosolution, reach theinoptimal acidifying agent) with different masses into g/L Ca(NO )2·4H2O order 3 )2 ·the pH value optimal ratio ofcomponent the Ca-P bath highbath corrosion of the to reach theand optimal pH component value and optimal ratioto of form the Ca-P to formresistance high corrosion · calcium phosphate coating. The effect ofcoating. Ca-P solution Ca(NO ) 4H O:NH H resistance of the calcium phosphate The with effectdifferent of Ca-P solution with different 3 2 2 4 2 PO4 concentration ratios on the corrosion resistance coating was investigated bythe potentiodynamic Ca(NO 3)2·4H2O:NH 4H 2PO 4 concentration ratios ofonthethe corrosion resistance of coating was polarization by curve and EIS. Figure polarization 1 shows the EIS plots and potentiodynamic polarization curves of investigated potentiodynamic curve and EIS. Figure 1 shows the EIS plots and bare AZ91D alloy and the calcium phosphate coatings obtained from different concentration ratios potentiodynamic polarization curves of bare AZ91D alloy and the calcium phosphate coatingsof Ca-P solution in 3.5% NaCl solution. ratios of Ca-P solution in 3.5% NaCl solution. obtained from different concentration
Figure 1. The Theelectrochemical electrochemical impedance spectroscopy (EIS)(a)plots (a) and potentiodynamic Figure 1. impedance spectroscopy (EIS) plots and potentiodynamic polarization polarization curves (b) of bare AZ91D alloy and the calcium phosphate coatings obtained from curves (b) of bare AZ91D alloy and the calcium phosphate coatings obtained from different different concentration ratios of Ca-P solution in 3.5 wt % NaCl solution. concentration ratios of Ca-P solution in 3.5 wt % NaCl solution.
According to Figure 1a, the EIS plot of bare AZ91D shows one high frequency capacitive loop According to Figure 1a, the EIS plot of bare AZ91D shows one high frequency capacitive loop and and one low frequency inductive loop accompanied with dispersive points, while there are two one low frequency inductive loop accompanied with dispersive points, while there are two capacitive capacitive loops in the high and low frequencies existing in the EIS plot of concentration ratio 5:1. loops in the high and low frequencies existing in the EIS plot of concentration ratio 5:1. According to According to recent research, the appearance of low frequency inductive loops is mainly attributed recent research, the appearance of low frequency inductive loops is mainly attributed to the fact that to the fact that the oxidation of magnesium proceeds faster than hydrogen evolution at the corrosion the oxidation of magnesium proceeds faster than hydrogen evolution at the corrosion front [32,33], front [32,33], and the accompanying dispersive points are related to the high activity of magnesium and the accompanying dispersive points are related to the high activity of magnesium alloys [34]. alloys [34]. The EIS plot indicates that the bare AZ91D alloy shows poor corrosion resistance, and the The EIS plot indicates that the bare AZ91D alloy shows poor corrosion resistance, and the conversion conversion coating formed in the concentration ratio 5:1 of Ca-P solution cannot provide enough coating formed in the concentration ratio 5:1 of Ca-P solution cannot provide enough protection effect. protection effect. In addition, only one capacitive loop existed in the rest of the EIS plots, which In addition, only one capacitive loop existed in the rest of the EIS plots, which indicates that the indicates that the conversion coating is undamaged. Therefore, the conversion coating can conversion coating is undamaged. Therefore, the conversion coating can remarkably improve the remarkably improve the corrosion resistance of Mg substrate. In these EIS plots, the diameter of the capacitive loop is largest at the condition of concentration ratio 5:3, indicating that the conversion coating formed on AZ91D alloy at such a condition realizes its best corrosion protection performance.
According to the corrosion process and referring to the literature [29,32–36], three equivalent circuits in Figure 2 are presented to fit the EIS results, where RS is the solution resistance, Rct is the charge-transfer resistance, CPEdl is the constant phase element of double layer at the metal/electrolyte, Rcoat is the conversion coating resistance, and CPEcoat is the constant phase element of the coating, respectively. LA accounts for the variation of the extension of active anodic regions during Materials 2018, 11,Finally, 908 4 of 15 the sinusoidal polarization, and RA represents the resistances associated with local environmental changes (precipitation of gels, presence of bubbles) in the vicinity of the anodic and cathodic regions corrosion resistance of Mg substrate. these EIS plots, the diameter of the thehigher capacitive largest at [33]. The fitting parameters for FigureIn1a are listed in Table 1, in which Rcoat loop valueisrepresents the condition of concentration ratio 5:3, indicating that the conversion coating formed on AZ91D alloy better corrosion resistance [37–39]. In Table 1, it can be seen that the value of Rcoat representing the at such a condition its best coating corrosion protection performance. corrosion resistancerealizes of conversion increases with the increase of the concentration ratio. This According to the process and referring [29,32–36], three equivalent is attributed to the factcorrosion that the increased content of NHto 4Hthe 2POliterature 4 in the coating-forming agent ensures circuits in Figure 2 are presented to fit the EIS results, where R is the solution resistance, the ct is that S the occurrence of a coating-forming reaction by supporting enough coating-forming agent,Rand charge-transfer resistance, CPEdl isthe thetriggering constant phase of double layer at the metal/electrolyte, the decreased pH value ensures of theelement corrosion reaction of the magnesium alloy to Rinduce is the conversion coating resistance, and CPE is the constant phase element the coating, coat coatthe magnesium alloy; this then of the alkaline environment near the surface of accelerates the respectively. LA accounts for the variation of thesufficient extensiondeposition of active anodic regions the formation ofFinally, the conversion coating, which also offers sites, and thusduring enhances sinusoidal polarization, andconversion RA represents the resistances with local environmental the adhesive force of the coating [27,30,40].associated The conversion coating provides changes the best (precipitation of gels, presence of bubbles) in the vicinity of the anodic and cathodic regions [33]. protective performance for AZ91D alloy when the pH value and the concentration ratio of Ca-P The fitting parameters for Figure 1a are Meanwhile, listed in Table 1, incase, which the is higher Rcoat value in represents solution equal 2.8 and 5:3, respectively. in this there no precipitation the Ca-P better corrosion resistance [37–39]. In Table 1, it can be seen that the value of R representing the coat solution, which realizes the optimum utilization of raw material. However, precipitation did appear corrosion resistance of conversion coating increases with the increase of the concentration ratio. This is in the Ca-P solution with a further increase of the concentration ratio, leading to the waste of raw attributed to the that the increased of NH the coating-forming agent 2 PO4isin material and thefact decrease in the valuescontent of pH and Rcoat4 H . This mainly attributed to the fact ensures that the the occurrence of a coating-forming reaction by supporting enough coating-forming agent, that corrosion reaction of magnesium alloy surface induced by the decrease of pH becomes theand control the pH value ensures the the corrosion reaction of the magnesium alloy to stepdecreased suppressing the formation of triggering conversionofcoating, and that the appearance of precipitation induce the alkaline environment near the surface of the magnesium alloy; this then accelerates theit decreases the effective content of the coating-forming agent in the Ca-P solution. From the above, formation of the conversion coating, which also offers sufficient deposition sites, and thus enhances can be seen that the AZ91D alloy immersed into the Ca-P solution with a concentration ratio of 5:3 the of the the conversion coatingresistance, [27,30,40]. realizing The conversion provides the best andadhesive a pH of force 2.8 obtains best corrosion the aimscoating of being the lowest-cost protective performance for AZ91D alloy when the pH value and the concentration ratio option, and of having NH4H2PO4 act as the coating-forming agent and the acidifying agent. of Ca-P solution equal 2.8 and 5:3, respectively. Meanwhile, in this case, there is no precipitation in the Ca-P solution, which realizes Table the optimum utilization ofEIS rawshown material. However, precipitation did appear 1. The fitting results of in Figure 1a. in the Ca-P solution with a further increase of the concentration ratio, leading to the waste of raw Rs CPEcoat Rcoat CPEdl Rct LA RA χ2 n1 n2is mainly attributed material and the decrease in the values of pH and R . This to the fact that the coat 2) −2 2 −2 2 2 2 −3 (Ω·cm (μF·cm ) (kΩ·cm ) (μF·cm ) (kΩ·cm ) (kΩ·cm ) (H·cm ) 10 corrosion reaction of magnesium alloy surface induced by the decrease of pH becomes the control step none 5.86 12.7 0.91 1.26 4.84 888 4.8 suppressing the formation coating, and of precipitation 5:1 7.88 12.4 of conversion 0.90 2.87 707 that the 0.82appearance 1.38 - decreases 0.8 the5:2 effective content of16.1 the coating-forming 12.4 0.82 18.3agent in the - Ca-P -solution.- From the -above, it can - be seen 4.1 5:3the AZ91D 17.8 alloy immersed 13.2 0.80 the Ca-P 56.3 solution- with a concentration - 5:3 and a- pH of4.0 that into ratio of 2.8 5:4 12.3 17.1 0.80 29.2 2.9 obtains the best corrosion resistance, realizing the aims of being the lowest-cost option, and of having 5:5H PO11.9 15.2 0.82 25.6 3.5 NH 4 2 4 act as the coating-forming agent and the acidifying agent.
Figure (a) magnesium magnesium substrate; Figure2. 2. The The equivalent equivalent electrical electrical circuits circuits used used for for the the fitting fitting of of the the EIS EIS data: data: (a) substrate; (b) (b)double doublecapacitance capacitanceloop loopand and(c) (c)single singlecapacitance capacitanceloop. loop. 1. The fitting results of EIS shown in Figure Figure 1b shows theTable potentiodynamic polarization curves of the1a.bare AZ91D alloy and the calcium phosphate coatings obtained from different concentration ratios of Ca-P solution in 3.5 wt % Rs CPEcoat Rcoat CPEdl Rct LA RA NaCl solution. It can be clearly both(µF the cathodicn2and anodic current density are depressed n1 seen that χ2 10−3 (Ω·cm2) (µF·cm−2 ) (kΩ·cm2 ) ·cm−2 ) (kΩ·cm2 ) (kΩ·cm2 ) (H·cm2 ) with the 5.86 increase in- concentration ratio, and the passivation behavior is4.84observed at the anodic none 12.7 0.91 1.26 888 4.8 process of7.88 coated AZ91D alloy, that 707 the conversion coating provides good 5:1 12.4 0.90 suggesting 2.87 0.82 1.38 - protection 0.8 for 5:2 12.4 16.1 0.82 18.3 4.1 as the magnesium substrate. Meanwhile, the corresponding electrochemical parameters, such 5:3 17.8 13.2 0.80 56.3 4.0 corrosion potential17.1 (Ecorr), corrosion current density (icorr), -and cathodic Tafel 5:4 12.3 0.80 29.2 - slopes -(bc) determined 2.9 11.9extrapolation 15.2 0.82 - Table 2,- compared - with the 3.5bare by5:5 the Tafel method are25.6 listed in Table 2. According to
Figure 1b shows the potentiodynamic polarization curves of the bare AZ91D alloy and the calcium phosphate coatings obtained from different concentration ratios of Ca-P solution in 3.5 wt % NaCl solution. It can be clearly seen that both the cathodic and anodic current density are depressed with
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the increase in concentration ratio, and the passivation behavior is observed at the anodic process of coated AZ91D alloy, suggesting that the conversion coating provides good protection for the magnesium substrate. Meanwhile, the corresponding electrochemical parameters, such as corrosion potential (Ecorr ), corrosion current density (icorr ), and cathodic Tafel slopes (bc ) determined by the Tafel extrapolation method are listed in Table 2. According to Table 2, compared with the bare AZ91D alloy, the Ecorr of AZ91D alloy with the conversion coating presents the obvious positive shift, and the Ecorr at the concentration ratio of 5:3 is the most positive, implying a decrease in the tendency of corrosion initiation due to thermodynamics [35]. In addition, the corrosion current density of the conversion coating obtained from the concentration ratio of 5:3 is the lowest, and depressed two orders of magnitude compared with the corrosion current density of the bare AZ91D alloy, which is consistent with the results of EIS. Table 2. The fitting results of polarization curves shown in Figure 1b. Ecorr (VSCE )
icorr (µA/cm2 )
bc (mV/dec)
−1.632 −1.574 −1.528 −1.493 −1.509 −1.515
11.8 8.60 4.41 0.610 2.01 2.93
−159 −167 −166 −140 −171 −156
none 5:1 5:2 5:3 5:4 5:5
In short, when the ratio of Ca(NO3 )2 ·4H2 O to NH4 H2 PO4 is 5:3, the conversion coating realizes the best corrosion protection performance for the bare AZ91D alloy, where there is no precipitation in the Ca-P solution. Hence, it can ensure the minimum consumption of chemical reagent and the lowest cost, which is favorable for its extension to industrial production. 3.2. Conversion Time Too short a conversion time will result in the incomplete conversion coating [27]; however, too long a conversion time will again contribute to the dissolution of the conversion coating [41], which decreases its corrosion resistance. Hence, conversion time is an important factor influencing the corrosion resistance of the conversion coating. As shown in Figure 3, the diameter of the capacitive loop increases with the prolongation of time up to 20 min, however the diameter of the capacitive loop decreases after 20 min, which indicates that 20 min is the optimal conversion time. The Rcoat fitted by the equivalent electrical circuit in Figure 2 is listed in Table 3. From Table 3, it can be seen that the Rcoat at 20 min is the biggest, further indicating that 20 min is the optimal conversion time, obtaining the best protective performance. However, with further prolongation of the conversion time, the value of Rcoat decreases, suggesting that the compact conversion coating formed on the surface of the AZ91D alloy may partially dissolve. Table 3. The fitting results of EIS shown in Figure 3. Time (min)
Rs (Ω·cm2 )
CPEcoat (µF·cm−2 )
n1
Rcoat (kΩ·cm2 )
CPEdl (µF·cm−2 )
n2
Rct (kΩ·cm2 )
χ2 10−3
5 10 15 20 25
9.28 8.57 14.2 17.8 14.2
0.987 16.1 14.6 13.2 16.7
0.92 0.84 0.81 0.80 0.81
2.66 6.04 27.4 56.3 39.8
941 553 -
0.82 0.98 -
1.57 2.35 -
0.8 1.4 3.4 4.0 2.4
loop decreases after 20 min, which indicates that 20 min is the optimal conversion time. The Rcoat fitted by the equivalent electrical circuit in Figure 2 is listed in Table 3. From Table 3, it can be seen that the Rcoat at 20 min is the biggest, further indicating that 20 min is the optimal conversion time, obtaining the best protective performance. However, with further prolongation of the conversion time, the value of2018, Rcoat11, decreases, suggesting that the compact conversion coating formed on the surface of the Materials 908 6 of 15 AZ91D alloy may partially dissolve.
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In order to further understand the above results, the SEM of the conversion coating at different times was performed as shown in Figure 4. According to the results shown in Figure 4, it is obvious that the leaf-like conversion coating gradually covers the whole surface of AZ91D, and the scratches become increasingly indiscernible from 5 min to 20 min. However, when the immersion time increases to 25 min, as shown in Figure 4f, the coating formed at 25 min is sparser and less compact than the coating formed at 20 min. It is apparent that the coating formed at 20 min is the most compact, and thus presents the best protection for bare AZ91D, which is in accordance with the Figure 3.3.Experimental Nyquist plots of coated different Figure Experimental Nyquist plots of AZ91D coated samples AZ91D obtained samples from obtained fromconversion different analysis of EIS. times. conversion times.
Table 3. The fitting results of EIS shown in Figure 3.
In order to further understand the above results, the SEM of the conversion coating at different Time Rs CPEcoat Rcoat CPEdl Rct χ2 n4. 1 According to the results shown n2 in Figure 4,2 it is obvious times was performed as shown in Figure 2 −2 2 −2 (min) (Ω·cm ) (μF·cm ) (kΩ·cm ) (μF·cm ) (kΩ·cm ) 10−3 that the leaf-like conversion coating gradually covers the whole surface of AZ91D, and the scratches 5 9.28 0.987 0.92 2.66 941 0.82 1.57 0.8 become min to 20 min. immersion 10 increasingly 8.57 indiscernible 16.1 from 50.84 6.04 However, 553when the 0.98 2.35 time increases 1.4 to 25 min, 4f, the coating less compact than 15 as shown 14.2 in Figure 14.6 0.81 formed 27.4at 25 min is- sparser and 3.4 the 20 formed17.8 56.3 formed- at 20 min-is the most - compact, 4.0 and coating at 20 min. It 13.2 is apparent0.80 that the coating 25 14.2 16.7 0.81 39.8 thus presents the best protection for bare AZ91D, which is in accordance with the analysis of2.4 EIS.
Figure micrographs of the calcium phosphate coatings under different conversion times:times: (a) 5 Figure4.4.SEM SEM micrographs of the calcium phosphate coatings under different conversion min; (b) 10 min; (c) 15 min; (d) 20 min; (e) magnification of 20 min and (f) 25 min. (a) 5 min; (b) 10 min; (c) 15 min; (d) 20 min; (e) magnification of 20 min and (f) 25 min.
3.3. 3.3. Conversion ConversionTemperature Temperature The The EIS EIS of of the the conversion conversion coating coating obtained obtained from from different different temperatures temperatures of of Ca-P Ca-P solution solution is is presented in Figure 5. The fitting results according to Figure 2 are listed in Table 4. The value of R coat presented in Figure 5. The fitting results according to Figure 2 are listed in Table 4. The value of Rcoat basically decreases drastically at 50 °C, ◦ C and ◦ C, while ◦ basically keeps keeps aa similar similar level level between between 20 20 °C and 40 40 °C, while the the R Rcoat coat decreases drastically at 50 C, indicating that high temperature decreases the corrosion resistance of the conversion coating. This indicating that high temperature decreases the corrosion resistance of the conversion coating. This phenomenon is mainly mainlyattributed attributedtotothethe fact high temperature generate a corrosion rate phenomenon is fact thatthat high temperature will will generate a corrosion rate higher higher than the deposition rate of the conversion coating, and will also accelerate the mass diffusive rate of the species of deposition reaction, which leads to a decrease in probability of coating deposition. Meanwhile, the hydrogen evolution and bubble bursting occurred more easily at a high temperature, thus making the phosphate crystals difficult to nucleate [27] and decreasing the corrosion resistance of the coating.
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than the Room deposition rate of the conversion coating, rate temperature 17.8 13.2and will also 0.80 accelerate 56.3the mass diffusive 4.0 of the species of deposition reaction, which leads to a decrease in probability of coating deposition. 20 15.6 11.6 0.81 54.8 4.2 Meanwhile, the hydrogen evolution occurred temperature, 30 15.0and bubble bursting 13.1 0.80 more easily 56.7 at a high3.4 thus making the phosphate crystals difficult to nucleate [27] and decreasing the corrosion resistance of 40 15.6 12.1 0.82 60.2 4.0 the coating. 50 11.9 20.9 0.79 23.6 1.7
Figure Nyquist plots of coated different Figure 5.5.Experimental Experimental Nyquist plots of AZ91D coated samples AZ91D obtained samples from obtained fromconversion different temperatures. conversion temperatures.
3.4. pH Value
Table 4. The fitting results of EIS shown in Figure 5.
Figure 6 shows the EIS plots of the conversion coating obtained from different pH values of Ca2 2 10−3 Temperature (◦ C) n Rs (Ω·cm2 ) CPEcoat (µF·cm−2 ) coat (kΩ ·cm ) P solution adjusted by HCl or NaOH after fixing the concentration ratioRof Ca(NO3)2·4H2χO:NH 4H2PO4 Room temperature 17.8 13.2 0.80 56.3 4.0 (5:3, pH = 2.8). The EIS fitting results are listed in Table 5. According to the fitting results, the Ca-P 20 15.6 11.6 0.81 54.8 4.2 solution without any adjustment shows the13.1 best corrosion resistance, while the increase or 30 15.0 0.80 56.7 3.4 decrease 15.6 12.1 4.0 in pH value40of the Ca-P solution weakens the protection of 0.82 the conversion60.2 coating. The decrease in 50 11.9 20.9 0.79 23.6 1.7 pH value of the Ca-P solution makes the corrosion reaction on the surface of the magnesium alloy dominate the whole reaction, while the increase in pH value of the Ca-P solution results in the 3.4. pH Value appearance of precipitation, reducing the available content of coating-forming agent in the solution, shows thetoEIS of the of conversion coating obtained pH values whichFigure is not6beneficial theplots formation conversion coating. On thefrom otherdifferent hand, when the pHofofCa-P the solution adjusted by HCl2.8, or NaOH after fixing the concentration of Ca(NO H2 PO4 Ca-P solution is above the substrate etching rate may be ratio too low to induce enough 3 )2 ·4H 2 O:NH4coating (5:3, pH = 2.8). results are listed 5. According the fitting results, the Ca-P nucleation, and The formEIS thefitting compact coating. Su et in al.Table [27] also observe a to similar phenomenon. Hence, solution without any adjustment shows the best corrosion resistance, while the increase or decrease the condition without any adjustment after simply fixing the Ca(NO3)2·4H2O:NH4H2POin4 pH value of the Ca-P(5:3) solution weakens the protective protection of the conversion coating. Theand decrease in pH concentration ratio realizes the best effect for the AZ91D alloy, the role of value the Ca-P solution makes the reaction on the surface of the magnesium alloy dominate NH 4Hof 2PO 4 acting simultaneously ascorrosion a coating-forming agent and acidifying agent is also realized. the whole reaction, while the increase in pH value of the Ca-P solution results in the appearance of Table 5. The fitting results of EIS shown in Figure 6. in the solution, which is not precipitation, reducing the available content of coating-forming agent beneficial to the2) formation of conversion On the when the the Ca-P solution 2) other −2) pH Rs (Ω·cm CPEcoat (μF· cm−2) n1 coating. Rcoat (kΩ·cm CPEdlhand, (μF·cm n2 pHRof ct (kΩ·cm2) χ2 10−3 is3.6 above 2.8, etching rate may be too enough coating and0.9 form 8.7 the substrate 18.7 0.81 10.7low to induce345 0.99 nucleation, 2.40 the et al. [27] also a similar phenomenon. Hence, without 3.2compact 11.3coating. Su 20.9 0.77observe29.1 - the condition 0.9 any after simply )2 ·4H2 O:NH4 H2 PO ratio 2.8 adjustment 17.8 13.2 fixing the 0.80Ca(NO356.3 - 4 concentration - (5:3) realizes 4.0 2.4best protective 11.5 0.81 alloy, and 20.1 the role of NH - 4 H2 PO4 acting 3.9as a the effect15.6 for the AZ91D simultaneously 2.0 8.66 14.2acidifying 0.85 4.96 realized. 522 0.96 1.56 1.3 coating-forming agent and agent is also
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Figure ofof conversion coatings obtained from different pH pH values of CaFigure 6. 6. Experimental ExperimentalNyquist Nyquistplots plots conversion coatings obtained from different values of PCa-P solution. solution.
3.5. Stirring Rate
Table 5. The fitting results of EIS shown in Figure 6.
Figure 7 shows the effect of the stirring rate on the EIS characteristic of the calcium phosphate Rs fittingCPE Rcoat CPEdl Rct 6, it can2 be − coat are listed in Table 3 coatings, results 6. According to Figuren72 and Table pH and the n1 χ 10 seen (Ω·cm2 ) (µF·cm−2 ) (kΩ·cm2 ) (µF·cm−2 ) (kΩ·cm2 ) that the stirring rate exerts an important effect on the protection of the conversion coating. As long 3.6 solution 8.7is stirred,18.7 0.81 10.7 Meanwhile, 345 the Rcoat 0.99gradually 2.40decreases0.9 as the the Rcoat decreases sharply. as the 3.2 11.3 20.9 0.77 29.1 0.9 stirring rate increases, and the conversion coating at the static condition presents the best corrosion 2.8 17.8 13.2 0.80 56.3 4.0 resistance, i.e., not beneficial formation of the coating. 2.4 11.5stirring is15.6 0.81 for the 20.1 - conversion - When the 3.9Ca-P solution to the surface alloy 2.0 is stirred, 8.66 the solution 14.2 close0.85 4.96 of the magnesium 522 0.96 is always 1.56acid. Hence 1.3 the magnesium alloy keeps dissolving continuously, while the precipitation reaction does not easily occur on theRate surface of the magnesium alloy due to the quick diffusion of the species of deposition 3.5. Stirring reaction caused by stirring, leading to the inferior protection of the conversion coating under dynamic Figure 7 shows the effect of the stirring rate on the EIS characteristic of the calcium phosphate conditions. coatings, and the fitting results are listed in Table 6. According to Figure 7 and Table 6, it can be seen that the stirring rate exerts anTable important effect results on theof protection conversion coating. As long as the 6. The fitting EIS shownofinthe Figure 7. solution is stirred, the Rcoat decreases sharply. Meanwhile, the Rcoat gradually decreases as the stirring Stirring Rate 2) s (Ω·cm CPEcoat (μF· cm−2) at n1 theRstatic coat (kΩ·cm CPEpresents dl (μF· cm−2the ) n2 corrosion Rct (kΩ·cm2resistance, ) χ2 10−3 rate(r/min) increases, R and the2)conversion coating condition best i.e., stirring is not6.95 beneficial for14.5 the formation conversion coating. When 150 0.84 of the 4.02 448 0.96 the Ca-P 1.93 solution 1.5 is 100the solution 9.01close to the23.6 8.97 810 0.87Hence 3.32 0.5 stirred, surface of 0.80 the magnesium alloy is always acid. the magnesium 11.1 18.3 0.82the precipitation 21.3 3.5the alloy 50 keeps dissolving continuously, while reaction does not easily- occur on 0 17.8 13.2 0.80 56.3 4.0 surface of the magnesium alloy due to the quick diffusion of the species of deposition reaction caused by stirring, leading to the inferior protection of the conversion coating under dynamic conditions.
Figure 7. Experimental Nyquist plots of conversion coatings obtained from different stirring rates of Ca-P solution.
Based on the above discussions, the leaf-like coating on the AZ91D alloy provides the best corrosion resistance when the AZ91D is immersed in a Ca-P bath composed of 25 g/L Ca(NO3)2·4H2O
Table 6. The fitting results of EIS shown in Figure 7. Stirring Rate Rs (Ω·cm2) (r/min) 150 6.95 100 9.01 Materials 502018, 11, 90811.1 0 17.8
CPEcoat (μF·cm−2)
n1
Rcoat (kΩ·cm2)
CPEdl (μF· cm−2)
n2
Rct (kΩ·cm2)
14.5 23.6 18.3 13.2
0.84 0.80 0.82 0.80
4.02 8.97 21.3 56.3
448 810 -
0.96 0.87 -
1.93 3.32 -
χ2 10−3 1.5 0.5 9 of 15 3.5 4.0
Figure Figure 7. 7. Experimental Experimental Nyquist Nyquist plots plots of of conversion conversion coatings coatings obtained obtained from from different different stirring stirring rates rates of of Ca-P solution. Ca-P solution.
Based on the above discussions, the leaf-like on inthe AZ91D Table 6. The fitting results ofcoating EIS shown Figure 7. alloy provides the best corrosion resistance when the AZ91D is immersed in a Ca-P bath composed of 25 g/L Ca(NO3)2·4H2O Stirring Rate (r/min)
Rs (Ω·cm2 )
CPEcoat (µF·cm−2 )
n1
Rcoat (kΩ·cm2 )
CPEdl (µF·cm−2 )
n2
Rct (kΩ·cm2 )
χ2 10−3
150 100 50 0
6.95 9.01 11.1 17.8
14.5 23.6 18.3 13.2
0.84 0.80 0.82 0.80
4.02 8.97 21.3 56.3
448 810 -
0.96 0.87 -
1.93 3.32 -
1.5 0.5 3.5 4.0
Based on the above discussions, the leaf-like coating on the AZ91D alloy provides the best corrosion resistance when the AZ91D is immersed in a Ca-P bath composed of 25 g/L Ca(NO3 )2 ·4H2 O and 15 g/LNH4 H2 PO4 for 20 min at a static condition of ambient temperature. From an electrochemical viewpoint, the corrosion resistance of calcium phosphate coating in this work is higher than that in some reported references [26–31]. Meanwhile, compared with those reported references [26–31], the preparation process of calcium phosphate coating in this work is also the simplest. 3.6. Coating Composition Figure 8 shows the micrograph of the coating under the optimal conditions. It can be seen that the leaf-like coating uniformly and compactly covers the whole sample surface. The correspondent EDS analysis shows that the main elements of the coating are O, Ca and P. The atomic ratio of Ca:P:O listed in Figure 8 is approximately 1:1:4. Given the main composition of leaf-like Ca-P conversion coating in the reported literature [27,28,31,42], it can be inferred that CaHPO4 ·2H2 O should be the main composition of the coating. Figure 9 shows the XRD patterns of the bare AZ91D alloy and the coating for further affirmation of the composition of the conversion coating. It is obvious that the substrate of the AZ91D alloy consists of α-Mg and β-Mg17 Al12 with crystalline, while the coating is mainly composed of CaHPO4 ·2H2 O (DCPD, JCDPS NO. 09-0077). In addition, the peaks of Mg substrate disappear, indicating that the DCPD coating basically covers the whole surface of the AZ91D alloy. Hence, the DCPD coating shows excellent protection for the AZ91D alloy.
coating in the reported literature [27,28,31,42], it can be inferred that CaHPO 4·2H2O should be the main composition of the coating. Figure 9 shows the XRD patterns of the bare AZ91D alloy and the main composition of the coating. Figure 9 shows the XRD patterns of the bare AZ91D alloy and the coating for further affirmation of the composition of the conversion coating. It is obvious that the coating for further affirmation of the composition of the conversion coating. It is obvious that the substrate of the AZ91D alloy consists of α-Mg and β-Mg17Al12 with crystalline, while the coating is substrate of the AZ91D alloy consists of α-Mg and β-Mg17Al12 with crystalline, while the coating is mainly composed of CaHPO4·2H2O (DCPD, JCDPS NO. 09-0077). In addition, the peaks of Mg mainly composed of CaHPO4·2H2O (DCPD, JCDPS NO. 09-0077). In addition, the peaks of Mg substrate disappear, indicating that the DCPD coating basically covers the whole surface of the Materials 2018, 11, 908 10 of 15 substrate disappear, indicating that the DCPD coating basically covers the whole surface of the AZ91D alloy. Hence, the DCPD coating shows excellent protection for the AZ91D alloy. AZ91D alloy. Hence, the DCPD coating shows excellent protection for the AZ91D alloy.
Ca Ca
Figure 8. SEM micrograph of the Ca-P conversion coating and thethe correspondent EDS analysis. Figure 8. SEM micrograph of the Ca-P conversion coating correspondent EDS analysis. Figure 8. SEM micrograph of the Ca-P conversion coating andand the correspondent EDS analysis.
Figure 9. XRD patterns of the bare AZ91D sample and the AZ91D samples with Ca-P conversion Figure 9. XRD patterns of the bare AZ91D sample and the AZ91D samples with Ca-P conversion coating. 9. XRD patterns of the bare AZ91D sample and the AZ91D samples with Ca-P Figure coating. conversion coating.
3.7. Formation Mechanism 3.7. Formation Mechanism 3.7. Formation Mechanism For further discussion of the formation mechanism of the DCPD coating, OCP was measured as For further discussion of the formation mechanism of the DCPD coating, OCP was measured as a function of time to monitor the formation depositionmechanism of conversion coatings as shown in Figure 10. The For further discussion of the of the DCPD [43], coating, OCP was measured as a a function of time to monitor the deposition of conversion coatings [43], as shown in Figure 10. The OCP curve can be divided the intodeposition three stages. In the initial several[43], seconds (A–B), the OCP function of time to monitor of conversion coatings as shown in Figure 10.decreases The OCP OCP curve can be divided into three stages. In the initial several seconds (A–B), the OCP decreases curve can be divided into three stages. In the initial several seconds (A–B), the OCP decreases quickly due to the activation of the surface of the Mg substrate and the dissolution of both the loose surface oxide and the Mg substrate in the acid Ca-P bath [30,41,43,44]. In the following stage (B–C), the OCP presents a sharp positive shift, indicating that the deposition of the calcium phosphate conversion coating begins, and then increasingly covers the surface. Therefore, the SEM shown in Figure 4 shows that the conversion coating gradually deposits on the AZ91D alloy from 5 min to 15 min, and thus both the diameter of the capacitive loop (Figure 3) and the corresponding Rcoat (Table 3) increase piece by piece. At the last stage (C–D), the OCP reaches a stable value, indicating that the deposition reaction at the solution/electrode interface has reached a stable state, and the dynamic equilibrium between the formation and dissolution of the DCPD coating has been established. Hence, the SEM micrograph of DCPD coating in 20 min shown in Figure 4 presents the best compactness, and thus can provide the best protection for the AZ91D alloy. With further prolongation of immersion time, the dynamic equilibrium between the formation and dissolution of DCPD coating may be destroyed, i.e., the coating dissolution rate may preponderate over the formation rate, and thus lead to the partial dissolution of the coating. Therefore, the coating at 25 min becomes sparse and thin, as shown in Figure 4, and the diameter of the capacitive loop at 25 min shown in Figure 3 reduces obviously, and
equilibrium between the formation and dissolution of the DCPD coating has been established. Hence, the SEM micrograph of DCPD coating in 20 min shown in Figure 4 presents the best compactness, and thus can provide the best protection for the AZ91D alloy. With further prolongation of immersion time, the dynamic equilibrium between the formation and dissolution of DCPD coating may be destroyed, the coating dissolution rate may preponderate over the formation rate, and thus11lead Materials 2018,i.e., 11, 908 of 15 to the partial dissolution of the coating. Therefore, the coating at 25 min becomes sparse and thin, as shown in Figure 4, and the diameter of the capacitive loop at 25 min shown in Figure 3 reduces thus cannotand effectively prevent AZ91D from corroding. Additionally, due toAdditionally, the partial dissolution of obviously, thus cannot effectively prevent AZ91D from corroding. due to the the coating, the very small area of Mg substrate will be exposed to the calcium phosphate bath, and partial dissolution of the coating, the very small area of Mg substrate will be exposed to the calcium thus the cathodic hydrogen be slowly accelerated. presentsHence, a sluggishly phosphate bath, and thus theevolution cathodicmay hydrogen evolution may beHence, slowlyOCP accelerated. OCP positive shift after the C–D stage [45]. presents a sluggishly positive shift after the C–D stage [45].
Figure Figure 10. 10. OCP OCP curve curve during during the the conversion conversion coating coating process. process.
Based on the above discussions, the formation mechanism of the DCPD coating on the AZ91D Based on the is above discussions, formation mechanism of the on the AZ91D magnesium alloy divided into threethe parts, A, B, and C, as shown inDCPD Figurecoating 11. At the beginning, magnesium alloy is divided into three parts, A, B, and C, as shown in Figure 11. At the beginning, part part A occurs at the surface of the AZ91D alloy, in which the anodic dissolution and cathodic A occurs atevolution the surface of the AZ91D alloy, inof which anodic dissolution and cathodic hydrogen hydrogen result in the production Mg2+,the and the increase of the local pH close to the 2+ , and the increase of the local pH close to the surface of the evolution result in the production of Mg surface of the AZ91D alloy, respectively. The increase of OH− will react with H2PO4- to produce 2− AZ91D alloy, respectively. The increase of OH− will react with H2 PO produce HPO long 4 to the 4 . As HPO 42−. As long as HPO42− ions exist in the Ca-P solution, part B will occur: leaf-like DCPD coating 2 − as HPO ions exist in the Ca-P solution, part B will occur: the leaf-like DCPD coating deposits 4 deposits on the surface with some protection for the AZ91D alloy (Figures 3 and 4). With the on the surfaceofwith some protection for the alloycover (Figures 3 and 4). With of thethe prolongation of prolongation time, the DCPD coating willAZ91D gradually the whole surface AZ91D alloy time, the DCPD coating will gradually the DCPD whole surface theexcellent AZ91D alloy as described in as described in part C, which forms thecover compact coating of with corrosion resistance part C, which forms the compact DCPD coating with excellent corrosion resistance (Figures 3 and 4). (Figures 3 and 4). In this work, Ca(NO3)2·4H2O and NH4H2PO4 are selected as coating-forming agents In induce this work, Ca(NO3 )2of ·4H NH4 HHence, as coating-forming agents to induce 2 O and 2 PO4 are to the formation DCPD coating. the selected DCPD coating was obtained from a Ca-P bath the formation of DCPD coating. Hence, the DCPD coating was obtained from a Ca-P bath with with different Ca(NO 3)2·4H2O:NH4H2PO4 concentration ratios, and can protect the bare AZ91D alloy different Ca(NOas ·4H2 O:NH andcontent can protect the4Hbare AZ91D alloy from 3 )2shown 4 H2 PO1. 4 concentration from corrosion in Figure However, the ratios, different of NH 2PO4 in the Ca-P bath corrosion as shown role in Figure However,resistance the different content NH4 H2due PO4 to in its theacidifying Ca-P batheffect. plays plays an important in the1.corrosion of the DCPDofcoating an important role in the corrosion resistance of the DCPD coating due to its acidifying effect. As the Ca(NO3)2·4H2O:NH4H2PO4 concentration ratio increases, i.e., the higher the concentrationAs of · the Ca(NO ) 4H O:NH H PO concentration ratio increases, i.e., the higher the concentration of 3 2 2 4 2 4 coating-forming agent in the Ca-P bath, the pH of the Ca-P bath decreases. This decrease in pH can coating-forming agent in the Ca-P bath, the the Ca-P bath decreases. This decrease in cause pH can promote the initial coating nucleation due topH theof quicker activation of the Mg substrate, and a promote the initial coating nucleation due to the quicker activation of the Mg substrate, and cause a higher concentration of OH− at the interface between metal and solution due to the easier hydrogen higher concentration of OH− at the interface between metal and solution due to the easier hydrogen evolution [27,30], which is beneficial to the occurrence of part B, and thus forms a more compact DCPD coating. Therefore, the increase of NH4 H2 PO4 content remarkably improves the protective effect of DCPD coating for the bare AZ91D alloy. When the concentration ratio equals 5:3, the optimal pH (2.8) of DCPD formation with the best protective performance for the AZ91D alloy is realized. Yet, with the further increase of NH4 H2 PO4 content, there are precipitations existing in the bath, i.e., raw materials are wasted, the effective content of the coating-forming agent is decreased, and the pH decreases to a lower value. In this lower pH of the bath, the more rapid hydrogen evolution will retard the complete coverage of the DCPD coating, which leads to the decrease in corrosion resistance. Hence, the DCPD coating formed in the concentration ratio 5:3 of the Ca-P bath provides the better protective performance for bare AZ91D, compared to other concentration ratios.
Yet, with the further increase of NH4H2PO4 content, there are precipitations existing in the bath, i.e., raw materials are wasted, the effective content of the coating-forming agent is decreased, and the pH decreases to a lower value. In this lower pH of the bath, the more rapid hydrogen evolution will retard the complete coverage of the DCPD coating, which leads to the decrease in corrosion resistance. Materials 2018,Hence, 11, 908 the DCPD coating formed in the concentration ratio 5:3 of the Ca-P bath provides 12 of 15 the better protective performance for bare AZ91D, compared to other concentration ratios.
Figure 11. Formation mechanism of calcium phosphate conversion coating on AZ91D magnesium Figure 11. Formation mechanism of calcium phosphate conversion coating on AZ91D magnesium alloy: (A) corrosion and activation; (B) deposition of DCPD; (C) the whole coverage of DCPD coating. alloy: (A) corrosion and activation; (B) deposition of DCPD; (C) the whole coverage of DCPD coating.
4. Conclusions 4. Conclusions In this work, a calcium phosphate coating was fabricated on the magnesium alloy AZ91D by In this work, a calcium phosphate coating was fabricated on the magnesium alloy AZ91D by very simple chemical conversion. The coating, with a leaf-like microstructure, is composed mainly of very simple chemical conversion. The coating, with a leaf-like microstructure, is composed mainly of CaPO4 ·2H2 O, and compactly covers the whole electrode surface, which can obviously strengthen the corrosion resistance of the AZ91D substrate. For obtaining the calcium phosphate coating, the AZ91D alloy was immersed in a Ca-P bath composed of 25 g/L Ca(NO3 )2 ·4H2 O and 15 g/L NH4 H2 PO4 (pH = 2.8, without any pH adjustment) for 20 min at a static condition of ambient temperature, which is very simple and suitable for popularization. Under such conditions, the role of NH4 H2 PO4 as simultaneously a coating-forming agent and acidifying agent is also realized. Furthermore, the formation mechanism of calcium phosphate conversion coating is proposed and the formation process is divided to three parts, among which the alkalization surrounding the magnesium substrate induced
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by corrosion activation, and inducing the production of HPO4 − plays an important role in the formation of the calcium phosphate conversion coating. Author Contributions: Conceptualization, D.L., Y.L. and Y.D.; Funding acquisition, D.L. and Y.D.; Investigation, Y.L.; Resources, Y.Z.; Writing—original draft, D.L., Y.L. and Y.D. Funding: This research was funded by the Open Research Fund of the State Key Laboratory of Oil, Gas Reservoir Geology and Exploitation (No. PLN1115), the Key Project of the Chinese Ministry of Education (No. 213024A) and the China National Natural Science Foundation (No. 51401150, No. 51601133). Conflicts of Interest: The authors declare no conflicts of interest.
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