Elsevier

Materials Letters

Volume 106, 1 September 2013, Pages 111-114
Materials Letters

Hydrothermal synthesis of corrosion resistant hydrotalcite conversion coating on AZ91D alloy

https://doi.org/10.1016/j.matlet.2013.05.018Get rights and content

Highlights

  • MgAl hydrotalcite coating is prepared by hydrothermal treatment in NH4NO3 solution.

  • MgAl hydrotalcite coating confers superior corrosion resistance on AZ91D alloy.

  • Corrosion current density of AZ91D alloy is decreased by one order of magnitude.

Abstract

MgAl hydrotalcite conversion coating was synthesized in situ on AZ91D alloy by hydrothermal treatment in ammonia nitrate aqueous solution. The structure and property of hydrotalcite conversion coating were investigated by Scanning electron microscopy (SEM), Energy-dispersive X-ray (EDX), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and electrochemical measurements. The results show that the hydrothermally synthesized hydrotalcite conversion coating effectively confers good corrosion resistance on AZ91D alloy. The corrosion current density of treated AZ91D alloy is decreased by one order of magnitude as compared to untreated one. It provides a facile way to synthesize chromium-free conversion coating for the corrosion protection of magnesium–aluminum 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).

References (34)

  • M.Q. Tang et al.

    Mater Lett

    (2011)
  • R.F. Zhang et al.

    Surf Coat Technol

    (2012)
  • P. Liu et al.

    Mater Lett

    (2012)
  • H.W. Shi et al.

    Prog Org Coat

    (2009)
  • A. Araghi et al.

    Vacuum

    (2013)
  • C.D. Gu et al.

    Surf Coat Technol

    (2006)
  • S.Y. Zhang et al.

    Mater Charact

    (2010)
  • X.L. Luo et al.

    Acta Biomater

    (2011)
  • Y.J. Zhang et al.

    Corros Sci

    (2011)
  • X.Y. Lu et al.

    Corros Sci

    (2012)
  • T. Ishizaki et al.

    Mater Lett

    (2012)
  • R. Amini et al.

    Appl Surf Sci

    (2011)
  • A.S. Hamdy et al.

    Surf Coat Technol

    (2012)
  • Y.B. Ma et al.

    Appl Surf Sci

    (2012)
  • H.F. Gao et al.

    Surf Coat Technol

    (2012)
  • D.L. Song et al.

    Corros Sci

    (2011)
  • A.S. Hamdy et al.

    Electrochim Acta

    (2011)
  • Cited by (46)

    • One-step hydrothermal synthesis of 2,5-PDCA-containing MgAl-LDHs three-layer composite coating with high corrosion resistance on AZ31

      2023, Journal of Magnesium and Alloys
      Citation Excerpt :

      Takahiro et al. [42] prepared a MgAl-CO32− LDH coating with a thickness of 80 µm on the AZ31 substrate using the method of steam coating. Wang et al. [43] prepared MgAl-CO32− LDH coating with a thickness of 25 µm on the AZ91D substrate by hydrothermal treatment. Lin et al. [40] prepared an LDH coating with a thickness of 5–8 µm on the AZ91D substrate via one-step in situ growth method.

    View all citing articles on Scopus
    View full text