Hydrothermal synthesis of corrosion resistant hydrotalcite conversion coating on AZ91D alloy
Introduction
Magnesium alloys as the most promising green engineering material for the 21 century are widely used in aerospace, automobile and electronic industries due to its lightweight, high specific strength and stiffness, good dumping performance and recyclability. However, they are extremely susceptible to corrosion whether in wet atmosphere or acid, neutral and weak alkaline solutions due to their chemical activity. Therefore, the corrosion resistance of magnesium alloys is of primary concern for their applications. Up to now, many methods were used to enhance the corrosion resistance of magnesium alloys, such as anodization [1], [2], [3], sol–gel treatment [4], electroless plating[5], [6], electroplating [7], [8], organic coating [9], [10], [11], chemical conversion coating [12], [13], [14], [15], [16], [17], [18], [19], [20] and so on. Among these methods, chemical conversion coatings including chromium coating [13], phosphate coating [12], [14], Ti/Zr based coating [15], vanadia coating [16], [17], [21], stannate coating [22], [23], rare earth coating [18], [19], [20], organic conversion coating [24], [25], composite coating [26], [27], etc. are regarded as one of the effective measures to enhance the corrosion resistance of alloys. In contrast, chromium conversion coating is the most superior among the conversion coatings. However, chromium conversion process can induce serious environmental pollution due to their high toxicity. Thus, much work has been focused on seeking environmentally-friendly alternatives.
Layered double hydroxides (LDH) named as hydrotalcite like compounds (HT) or anionic clays represent an important class of ionic lamellar solids. They can be expressed by the general formula [M2+1−xM3+x(OH)2] An−x/n·mH2O, where the cations M2+ and M3+ occupy the octahedral holes in a brucite-like layer and the anion An− is located in the hydrated interlayer galleries [28]. The unique structure confers superior corrosion resistance on the hydrotalcite coating [29], [30], [31], [32], which makes hydrotalcite become the potential replacement for chromium conversion coating. However, it is difficult to grow hydrotalcite layer on the Mg alloys, due to the passivity of magnesium in alkaline solutions. Therefore, rare works about the synthesis of hydrotalcite coating on magnesium alloys were reported. In this paper, MgAl hydrotalcite coating was synthesized on AZ91D alloys by hydrothermal treatment in ammonia nitrate aqueous solution. Meanwhile, the corrosion resistance of chemical conversion coating was investigated in 3.5 wt% NaCl solution as well.
Section snippets
Experimental
Synthesis of MgAl hydrotalcite coating: AZ91D sheet (99.9%) with dimension of 4×1.1×0.1 cm3 served as the substrate. The AZ91D sheet was ground by emery paper #600, #800 and #1000 successively. The polished AZ91D sheets were cleaned by ultrasonication in acetone for 15 min, and then rinsed with deionized water and finally dried at room temperature. NH4NO3 (0.0036 mol) was dissolved in deionized water (50 mL), and 5% ammonia solution was then slowly added until the pH reached 10. Then the solution
Results and discussion
Fig. 1 shows the structure and composition of the prepared chemical conversion coating. It can be seen from Fig.1a that after the hydrothermal treatment, the AZ91D alloy with chemical conversion coating uniformly displays black color. SEM of the coating in Fig.1b shows that the chemical conversion coating possesses the layer micro-/nano-structures which consist of vertically cross-linked nanosheets. The corresponding EDX result shows that the as prepared chemical conversion coating is mainly
Conclusions
In summary, we have demonstrated a facile way to synthesize corrosion resistant MgAl hydrotalcite coating by hydrothermal treatment of AZ91D alloy in ammonia nitrate solution. The results show that the chemical corrosion of AZ91D alloy induces in situ deposition of MgAl hydrotalcite coating during the hydrothermal conversion treatment, which confers good corrosion resistance on AZ91D alloy. The corrosion current density of the hydrothermally treated alloy is decreased by one order of magnitude
Acknowledgments
The authors thank the special support of Marine Public Service Project no. (201005028-3) and the support of the Fundamental Research Funds for the Central Universities (No. 852012).
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