Elsevier

Corrosion Science

Volume 51, Issue 10, October 2009, Pages 2483-2492
Corrosion Science

Comparison of electrochemical corrosion behaviour of MgO and ZrO2 coatings on AM50 magnesium alloy formed by plasma electrolytic oxidation

https://doi.org/10.1016/j.corsci.2009.06.034Get rights and content

Abstract

Two types of PEO coatings were produced on AM50 magnesium alloy using pulsed DC plasma electrolytic oxidation process in an alkaline phosphate and acidic fluozirconate electrolytes, respectively. The phase composition and microstructure of these PEO coatings were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The corrosion behaviour of the coated samples was evaluated by open circuit potential (OCP) measurements, potentiodynamic polarization tests, and electrochemical impedance spectroscopy (EIS) in neutral 0.1 M NaCl solution. The results showed that PEO coating prepared from alkaline phosphate electrolyte consisted of only MgO and on the other hand the one formed in acidic fluozirconate solution was mainly composed of ZrO2, MgF2. Electrochemical corrosion tests indicated that the phase composition of PEO coating has a significant effect on the deterioration process of coated magnesium alloy in this corrosive environment. The PEO coating that was composed of only MgO suffered from localized corrosion in the 50 h exposure studies, whereas the PEO coating with ZrO2 compounds showed a much superior stability during the corrosion tests and provided an efficient corrosion protection. The results showed that the preparation of PEO coating with higher chemical stability compounds offers an opportunity to produce layers that could provide better corrosion protection to magnesium alloys.

Introduction

More attention is being paid, in recent times, to magnesium and its alloys for a wide range of industrial applications, e.g. automotive, aerospace and communication, etc., owing to their low density, high strength to weight ratio, high dimensional stability, good electromagnetic shielding and damping characteristics, and good machining and recycling ability [1], [2]. Unfortunately, magnesium alloys, in general, exhibit a poor corrosion resistance, which is primarily attributed to the high chemical activity of magnesium and the lack of a protective passive oxide film [3]. This disadvantage is restricting its widespread use in the aforementioned applications, especially in aggressive environments [4].

In order to improve the corrosion resistance of magnesium alloys, it is necessary to employ proper surface treatments to produce anti-corrosion protection films on the substrate. In the past several decades, many surface modification techniques have been developed for the protection of these alloys, which include electrochemical plating, conversion coatings, anodizing, gas-phase deposition processes, laser surface alloying and organic coatings [5]. Among these techniques, anodizing is one of the most popular methods to provide protection to magnesium alloys [5], [6], [7]. Based on the principle of anodizing, a relatively new process, called plasma electrolytic oxidation (PEO), has also been developed in the last decade [8], [9]. By the PEO process, a relatively thick, compact and less porous oxide coating than conventional anodizing can be produced on the surface of magnesium alloys to improve their corrosion resistance remarkably [10], [11], [12]. The PEO coatings were found to be mainly composed of magnesium oxide with some of other electrolyte-borne elements (Mg2SiO4, Mg3(PO4)2 or MgAl2O4, etc.) [13], [14], [15]. For some applications, these PEO coatings cannot provide sufficient corrosion protection to the substrate especially during long-term exposures, if the environment is aggressive, as some of the constituents of the oxide coating are not stable in neutral and acidic environments [16], [17]. Thus, many attempts have been made to produce PEO coatings with more stable oxides and compounds by modifying the constituents of the electrolytes [18], [19], [20], [21], [22], [23]. However, most of these investigations seemed to change the composition of PEO coating only to a small extent and magnesium oxide still remains as the main phase. Recently, Yao et al. [24] found that the MAO coating which was composed of t-ZrO2 and c-ZrO2 could be produced on AZ91D magnesium alloy in a zirconate electrolyte. Mu et al. [25] also reported that MgF2/ZrO2 composite PEO coating was prepared on pure magnesium. The distinct characteristic feature of this PEO coating was that its phase composition mainly consisted of relatively inert compounds (ZrO2 and/or MgF2) without MgO or with a little of MgO. This kind of PEO coating was expected to be beneficial to providing a favorable long-term protection to magnesium alloys. In order to confirm this hypothesis, in the present investigation two kinds of PEO coatings with different phase composition, i.e. one coating with predominantly MgO phase and the other coating with predominantly ZrO2 phase were prepared on an AM50 magnesium alloy. Their corrosion/deterioration processes were investigated systematically by electrochemical techniques, and an attempt to correlate the microstructure and phase composition of PEO coatings with the corrosion behaviour was made.

Section snippets

Experimental

Coupons of size 15 mm × 15 mm × 4 mm from an AM50 magnesium alloy (mass fraction: 4.4–5.5% Al, 0.26–0.6% Mn, max 0.22% Zn, max 0.1% Si, and Mg balance) were used as the substrate in this investigation. The specimens were ground with different grit (up to 2500 grit) emery sheets before the PEO treatment.

The plasma electrolytic oxidation processes were carried out using a pulsed DC power source with a frequency of 50 Hz in alkaline phosphate electrolytic solution and acidic fluozirconate electrolytic

Phase composition and microstructure

X-ray diffraction (XRD) analyses were used to investigate the chemical composition of the PEO coatings prepared from the two different electrolytes. The XRD pattern, as shown in Fig. 1a, reveals that the PEO coating produced in alkaline phosphate electrolyte was mainly composed of MgO. Interestingly, chemical compounds associated with the phosphate, the main anions composition in electrolytic solution were not detected in the coating. Instead, a broad amorphous peak could be identified in the

Discussion

Based on the examination of surface and cross-section morphologies, characteristics of the corroded surface and the electrochemical corrosion results, the evolution of the deterioration processes of the MgO and the ZrO2 coatings in 0.1 M neutral NaCl solution could be explained as follows:

For the MgO coating on AM50 magnesium alloy, the deterioration process could be divided into two stages according to the EIS behaviour and OCP measurements as a function of immersion time, i.e. degradation

Conclusions

  • 1.

    PEO coatings with different main phase compositions, i.e. MgO and ZrO2, were prepared by pulsed DC plasma electrolytic oxidation processes in two different electrolytes. While the MgO coating was relatively compact, the ZrO2 coating had a large volume of fine pores like a sponge.

  • 2.

    The large fluctuations that were observed in the potential measurement in the 50 h exposure test depicted the failure of MgO coating. The steep drop and fluctuations in the potential are attributed to the exposure of

Acknowledgements

J. Liang and P. Bala Srinivasan express their sincere thanks to the Hermann-von-Helmholtz Association, Germany and DAAD, Germany for the award of fellowship and funding. The technical support of Mr. V. Heitmann, Mr. U. Burmester and Mr. V. Kree during the course of this work is gratefully acknowledged. The authors acknowledge the support of Dr. Zhiguang Guo, Laboratory of Inorganic Materials Chemistry (CMI), University of Namur (FUNDP), Belgium, in performing the XRD analysis.

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