In this work, the use of zinc phosphate coating to protect AZ31 magnesium alloy against corrosion is discussed. In addition, the effect of solution temperature and immersion time as two important process parameters during phosphating on the surface properties and corrosion resistance was studied. The morphology and composition of phosphate films formed on Mg alloy was investigated by using scanning electron microscopy (SEM) and x-ray diffraction (XRD). The results show that the phosphate coatings mainly contain hopeite (Zn3[PO4]2·4H2O) and have flower- and slab-like crystals at lower and higher solution temperatures, respectively. Meantime, electrochemical impedance spectroscopy (EIS) and direct current (DC) polarization were used to assess the corrosion protection property. The EIS results indicate that the phosphate film formed at 60°C during 5 min immersion can greatly improve the corrosion resistance of the AZ31 magnesium alloy.
Magnesium is one of the lightest metals with a density of 1.74 g/cm3, which offers the possibility of weight reduction up to 35% compared to steel with base materials with a density of 7.8 g/cm3, and it is the 8th most abundant element on the earth making up approximately 1.93% by mass of the earth's crust and 0.13% by mass of the oceans.1–3 Magnesium alloys have outstanding physical and mechanical properties, such as high specific strength, high conductivity, good castability, very good electromagnetic features, high damping capacity, and easily being recycled. These properties make them ideal candidates for lightness engineering applications, especially in the automotive industry, computer parts, aerospace industry, and cellular phones.4–7 Magnesium alloys are considered to be an excellent material for reducing vehicle weight, which, in turn, decrease fuel consumption and thereby reduce carbon dioxide (CO2) emission.8 However, the application of magnesium alloys has been limited because of the undesirable properties, including poor corrosion and wear resistance.9–10 Upon atmospheric exposure, Mg rapidly develops oxide/hydroxide/carbonate films. These films are porous, poorly bonded and inhomogeneous, and unable to provide satisfactory protection to the underlying metal against corrosion.11 Therefore, it is very important to improve anti-corrosion performances of magnesium alloys in industrial applications. Toward this purpose, many surface modification treatments including conductive polymer films,12–13 hybrid organic-inorganic sol-gel coatings,14 chemical conversion coatings,15–24 and electroless nickel plating25 have widely been studied. Among the surface modification treatments, chemical conversion coatings are a cost-effective method to assist in the corrosion protection of Mg and its alloys.26 Commonly, the chemical conversion coating with chromate-containing solution is widely used for its low cost,15–18 but some studies showed that the hexavalent chromium used in solution treatments is carcinogenic.27–28 To avoid the chromate process, many new alternative methods have been developed recently, such as phosphate, phosphate-permanganate, and permanganate.19–24 Through the conversion coatings, zinc phosphate coating is micro-porous and paint can sink in it.1 Therefore, a properly performed zinc phosphate coating prior to paint application can create good bonding of paint film and phosphate coating on metal surface.29 Based on this consideration, phosphating process on magnesium alloy has been studied recently. Zhou, et al.,21 studied the structure and formation mechanism of zinc phosphate film on AZ91D magnesium alloy. Lian and coworkers1,20 applied a zinc phosphate conversion coating on AZ91D magnesium alloy through a phosphate solution containing mainly zinc oxide (ZnO), phosphoric acid (H 3PO4), and hydrofluoric acid (HF), and the growth and formation mechanism were assessed. Cheng, et al.,24 discussed the properties of the phosphate film formed on AZ31 magnesium alloy. Kouisni, et al.,22–23 investigated the influence of zinc ions (Zn2+) and nitrate on the phosphating process and mechanism. In our previous work,30 we studied the effect of sodium dodecyl sulfate (SDS) as an eco-friendly accelerating agent on the corrosion behavior of phosphate coating on the AZ31 magnesium alloy. In this work, we report the influence of phosphating solution temperature and immersion time on morphology and corrosion resistance of zinc phosphate coatings, which are formed on AZ31 magnesium alloy. Therefore, magnesium samples were immersed in a phosphating solution comprising H3PO4, ZnO, zinc nitrate (Zn[NO3]2), sodium fluoride (NaF), and organic amine with various temperatures and immersion times. The films were characterized by comprehensive methods such as scanning electron microscopy (SEM), x-ray diffraction (XRD), direct current polarization, electrochemical impedance spectroscopy (EIS), and salt spray.
AZ31 magnesium alloy samples with a specimen size of 30 by 30 by 3 mm were used as substrate material. The chemical composition of AZ31 magnesium alloy used in this study is presented in Table 1.
The samples were polished with 400, 1000, and 1200 grit abrasive paper, and then the samples were degreased in 10 wt% potassium hydroxide (KOH) and rinsed in deionized water to remove all the alkali before the zinc phosphate treatment. The composition of the solution is summarized in Table 2. The phosphating treatments were carried out at four different temperatures of 40, 50, 60, and 70°C during 5 min and for three various immersion times of 3, 5, and 10 min at 60°C.
Structure and Composition Analysis
Morphology was observed using SEM. The energy used was 16 kv. Phase composition of the phosphate coating was analyzed by XRD with a diffractometer operating at 40 kV, 40 mA, using a CuKα target.
The corrosion resistance of the phosphate samples were studied in 3.5 wt% NaCl aqueous solution with a scan rate of 0.01 V s−1 and at potential range of ±100 mV around open-circuit potential (OCP). The test was performed on a 1 cm2 area of each sample exposed to 3.5 wt% NaCl solution. The corrosion current densities (icorr) of the samples were obtained from direct current (DC) polarization results using General Purpose Electrochemical System (GPES) software. The anticorrosion performance of the phosphated samples was also studied using EIS. The test was done on 1 cm2 area of each sample exposed to 3.5 wt% NaCl solution. The electrochemical system, used in this study, includes silver/silver chloride (Ag/AgCl) electrode (as reference electrode), platinum electrode (as auxiliary electrode), and phosphate magnesium sample (as working electrode). Measurements were carried out at a frequency range and perturbation of 10 kHz to 10 mHz and ±10 mV, respectively. Using frequency response analysis (FRA) software, the results obtained by EIS were analyzed.
Salt Spray Test
According to the ASTM B11731 standard, specimens for the salt spray test were exposed in the salt spray cabinet for 24 h.
RESULTS AND DISCUSSION
The α-Mg and β (Mg17Al12) are two main phases in the microstructure of AZ31. α-Mg is a solid solution of Mg-Zn-A1 with the same crystal structure of pure magnesium. The β (Mg17Al12) is an intermetallic phase containing more than 40% Al. As soon as the alloy sample is soaked in the phosphating solution, the surface of the sample divided into micro-anode sites (lower electron density sites) and micro-cathode sites (higher electron density sites) and the reactions on the surfaces should be thought to take place on different local polarization sites correspondingly. As to the AZ31 magnesium alloy, β phase in the magnesium alloys is regarded as the micro-cathode sites32 and magnesium the micro-anode sites. Therefore, the following reactions can occur at micro-anode sites. Magnesium dissolved and released metal ions at the micro-anode sites:
There is also a small quantity of aluminum dissolved into the solution:
ZnO reacts with H3PO4 to form Zn(H2PO4)2·2H2O:
The product Zn(H2PO4)2·2H2O of the above reaction is soluble and it dissolved in the solution to produce the following:
The complex ion ZnPO4− has the ionization reaction as follows:
In the phosphating solution, Zn(NO3)2 has two actions. First, it provides Zn2+ in the solution:
Because of the difference in the standard potential of zinc (−1.76 V) and magnesium (−2.36 V), magnesium on the magnesium alloy surface dissolved in solution to give out electrons, which reduced some Zn2+ near the surface to Zn deposited on the surface on the micro-cathode sites of some α phases and become the composition of the phosphate coating:
At the micro-cathode sites, hydrogen ions reduced simultaneously:
The reduction of the hydrogen ions leads to the increase of local pH at the metal-solution interfaces, which facilitate the precipitation of insoluble zinc phosphate.22 The formation of insoluble phosphate film may have followed the reaction:
Here, Zn3(PO4)2·4H2O is the main ingredient of the phosphate film. After the original phosphate film has formed, it is still regarded as the micro-cathode. Reaction (9) progresses continuously, until the nucleation and growth of phosphate crystals to form the integrated phosphate film and all of the micro-anodes are covered. There should be metallic zinc crystals in the zinc phosphate coating as a result of the reaction of Equation (7).
During phosphating, the NO3− ions reacts with hydrogen ions:
This reaction consumes hydrogen ions and makes the pH of the solution near the magnesium alloy surface rise rapidly, and it is beneficial to the formation of phosphate coating.22
Figure 1 depicts the simple model of deposition of phosphate conversion coating on AZ31 magnesium alloy.
Effect of Temperature on the Phosphate Coating Properties
Effect of Temperature on the Phosphate Coating Structure — Morphologies of phosphate coatings with different phosphating solution temperatures were observed by SEM, as shown in Figure 2. Figures 2(a) and (b) show that the surface morphology of the sample immerses in the phosphating solution with the temperature of 40°C. Some white flowers are observed on the surface, but more than half of the substrate is not still covered. There are some cracks on the surface that are caused by internal stresses.21 Surface morphology of phosphate coating formed at 50°C is shown in Figures 2(c) and (d). Phosphate film precipitated on cracks; therefore, fewer cracks are observed at 50°C. Cracks emerged between the depositions because of the dissolution of magnesium in the metal matrix.21 Phosphate coating (Figures 2[a] through [d]) consists of both flowers and slab-like particles. The phase compositions in the coating were analyzed by XRD, and the result is shown in Figure 3. It is seen that the phase of crystal phosphate coating, formed in phosphating solution at 40°C and 50°C, mainly consists of hopeite (Zn3[PO4]2·4H2O) and zinc (Figures 2[a] and [b]). Phosphate coating formed at 60°C is compact and complanate. The whole substrate is coated by slab-like crystals, and no substrate can be seen (Figures 2[e] and [f]). It will be proved further by EIS testing that because of the least-exposed area, the coating formed at 60°C has the highest corrosion protection property. However, when temperature increases more, some trans cracks21 appear in the joint of the crystal clusters as presented in Figures 2(g) and (h). According to XRD analysis, the latter coatings mainly consist of hopeite (Zn3[PO4]2·4H2O), and no zinc crystal is detected (Figures 2[c] and [d]).
The influence mechanism of phosphatization on magnesium alloy could be analyzed by the relative variation of peak intensity of different phases as phosphatization process temperature increased. As shown in Figure 4, the diffraction intensity variations of (020) plane of hopeite and the (101) plane of magnesium are different with the increase of phosphating solution temperature. It is shown that the plane of magnesium was approximately linear, decreased with the augmentation of temperature until 60°C, and after that there was a slight increase in the intensity of plane of magnesium at 70°C. It is seen that with the increase of temperature, the intensity of plane of hopeite increased. It can be deduced that at 40°C and 50°C, both reactions of Equations (7) and (9) took place simultaneously, which resulted in zinc and hopeite codeposited on the surface of the magnesium alloy, but at 60°C, because the magnesium substrate was nearly covered by the phosphate film (Figures 2[e] and [f]), there was no dissolution of magnesium to ensure the deposition of metallic zinc. Therefore, only hopeite deposition maintained. Because there are no small zinc particles to interdict the growth of hopeite, many clusters of large slab-like crystals formed on the surface (Figures 2[e] through [h]).
Effect of Temperature on the Corrosion Resistance of Phosphate Coating —
Using DC polarization, the anticorrosion performances of the phosphated samples are studied. Figure 5 shows the polarization diagrams of samples. Different parameters including corrosion current density (icorr), corrosion potential (Ecorr), and Tafel polarization slops were calculated from polarization curves (Table 3). As it can be seen clearly in Table 3, an increase in temperature caused a decrease in icorr until 60°C. This indicates that, by increasing the temperature, phosphate coating with higher corrosion resistance can be produced on the magnesium surface. It seems that, using phosphate coating over the magnesium surface, the increase in corrosion resistance of the magnesium was obtained as a result of the barrier properties of the phosphate coating. Moreover, by increasing the temperature, the Ecorr was shifted to nobler values (in positive direction). This can be understood from the higher values of Ecorr of the phosphate coatings formed at 40, 50, 60, and 70°C compared to the Ecorr value of the bare sample (Figure 5, Table 3). The higher barrier property of the phosphated sample at 60°C can be attributed to the more uniformity as well as the morphology of the phosphate coating crystals observed in SEM micrographs (Figures 2[g] and [h]). The denser crystal structure of the phosphated sample at 60°C, having fewer micro-cracks, can be responsible for the lower permeability of electrolyte into this coating. A slight increase in icorr value and decrease in Ecorr value of the phosphate sample at 70°C can be attributed to cracks and appeared in the phosphate coating, which decreased the barrier property.
The EIS of phosphate films with different phosphatization process temperatures during 5 min is shown in Figure 6. These diagrams are composed of two parts, but one loop can be seen. It can be attributed to the less immersion time in NaCl solution, which results in overlapping these two loops. Using the electrical model shown in Figure 7, different parameters including Rct (charge-transfer resistance), Rs (solution resistance), Rc (coating resistance), Cdl (double-layer capacitance), and Cc (coating capacitance) are calculated (Table 4). Rp and C are the polarization resistance (in which Rct and Rc are included) and capacitance (in which Cdl and Cc are included), respectively.33–34 Table 5 shows EIS calculated parameters for bare sample.
Polarization resistance (Rp) can be used to evaluate the corrosion resistance of zinc phosphate films. In comparison with bare magnesium alloy AZ31, zinc phosphate coating formed at 60°C has the greatest corrosion resistance. Since at 60°C coating is compact and almost whole substrate is covered (Figures 2[e] and [f]), zinc phosphate coating is propitious to avoid as much as possible reactions at its interface in NaCl solution. The corrosion protection of phosphate coating formed at 40°C is the least. Because there are some cracks and substrate is not covered perfectly by zinc phosphate coating, magnesium can dissolve in the media. These observations are completely in agreement with the results shown in Figure 5 and Table 3.
It is well known that33 phase angle (−θ) is a parameter indicating the coating intactness to the substrate. The lowest and the highest intactness can be seen at −θ near to 0 and 90, respectively. With respect to this fact, an increase in −θ may show the increase in coating intactness. Variation of θ at 10 kHz films with different phosphatization process temperatures during 5 min is also shown in Figure 6. It is observed that samples with phosphate coating shows increasing behavior of the phase angle by increasing the temperature until 60°C (Table 4). It can be attributed to uniformity of phosphate coatings formed at higher temperatures. Decreasing of −θ at 70°C can be caused by loose coating and new cracks (Figures 2[g] and [h]).
Salt Spray Tests
Figure 8 shows the images of samples with phosphate coating formed at different temperatures after salt spray tests. The surface of the specimen with a phosphating process temperature of 60°C looks smoother than others, implying its superior corrosion resistance over the rest specimens. These results are in agreement with the data obtained in the electrochemical (polarization and EIS) tests.
It can be concluded that when the temperature is lower, the heat of the reaction is not enough to drive the reaction. Therefore, the rate of phosphatation is slow, the phosphate layer is thin, and its anti-corrosion property is not good. This is why the phosphate coatings formed at 40°C and 50°C could not afford good corrosion resistance. When the reaction temperature is higher, the activation energy of the reaction declines, the rate of phosphating will be accelerated, and the phosphating film will have good corrosion resistance. At very high temperatures, the corrosion resistance is decreased, which may be caused by the dissolution of the film by free-acid ions and the film becomes loose. So, it is very important to choose a proper temperature. For 5 min immersion time, the proper temperature is 60°C.
Effect of Time on Phosphate Coating
Effect of time on the phosphate coating structure formed at 60°C (the most proper temperature) is studied. The SEM images of the phosphate coating formed with different immersion times are shown in Figure 9. After 3 min immersion in phosphating solution, the clusters of phosphate particles are not large enough to form a compact coating, so corrosive media can easily reach the substrate. This is why the corrosion resistance of phosphate coating formed after 3 min is less than that formed after 5 min (Figures 10 and 11 and Tables 6 and 7).
In Figure 9(c), the coating formed after 10 min is loose and several cracks are seen, and the corrosion resistance of coating formed after 10 min immersion decreases comparing with coating formed after 5 min (Figure 10 and Table 7). This may be caused by the dissolution of the film by the free-acid ion.24 As a result, the best phosphating time is 5 min at 60°C.
In this study, different phosphate coatings were prepared by changing phosphating solution temperatures and immersion times. The properties of coatings were investigated by different methods and the results are as following:
❖ The protective properties of zinc phosphate coating formed on the surface of the magnesium alloy were dependent on the temperature of the phosphating solution.
❖ When the duration of the immersion in this phosphating solution is 5 min, the optimum temperature of the phosphating process is 60°C.
❖ The temperature has an obvious influence on the structure of the phosphate coating crystals. At lower temperatures (40°C and 50°C), phosphate coating crystals are flower-like, but at higher temperatures (60°C and 70°C), slab-like crystals are observed on the surface.
❖ The coating formed on the surface mainly consists of hopeite (Zn3[PO4]2·4H2O).
* Faculty of Polymer and Color Engineering, Amirkabir University of Technology, PO Box 15875-4413, Tehran, Iran.