Formation mechanism of a self-sealing pore micro-arc oxidation film on AM60 magnesium alloy

doi:10.1016/j.surfcoat.2015.02.041

Surface and Coatings Technology

Volume 266, 25 March 2015, Pages 188-196

https://doi.org/10.1016/j.surfcoat.2015.02.041 Get rights and content

Highlights

•
A novel self-sealing pore MAO film is developed on AM60 Mg alloy.
•
Fluorides are beneficial in improving the compactness of initial passive film.
•
Different melting points of film constituents are key factor for pore self-sealing.
•
High titanium oxide content greatly enhances the compactness of MAO film.

Abstract

Ceramic films are formed on AM60 magnesium alloy by micro-arc oxidation (MAO) using a new fluorotitanate electrolyte system. Compared to the films obtained with traditional electrolytes, the film has the characteristics of self-sealing pores and different chemical compositions. To investigate the film's growth mechanism, X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) were used to characterize the phase structure and chemical composition. Mott–Schottky (M–S) curve measurements were used to study the electrical properties of the passive layer and the influence of the fluorine during initial film deposition. The NaF content in the new electrolyte plays an important role in improving the compactness of the initial film. By increasing oxidation voltages in the MAO process, an increase of titanium oxides in the film is produced. Different melting points of film constituents and high titanium oxide content in the MAO film are key factors for forming self-sealing pores.

Introduction

Mg alloys are the lightest metal structural materials in widespread industrial application. Due to their unique combination of low density (2/3 that of aluminum and 1/4 that of steel) and unique physical and mechanical properties, such as high specific strength, good electrical and thermal conductivity, recyclability and high vibration absorption, Mg alloys are ideal materials in fields where weight reduction is critical [1], [2], [3]. They are widely used in many industries such as aerospace, automobile and communications [4], [5]. However, poor corrosion resistance is a significant factor limiting their development [6], [7], [8]. There are numerous methods to improve the corrosion resistance of Mg alloys, such as electroless plating, conversion films [9], [10], laser surface melting and organic coatings. Micro-arc oxidation (MAO) is another efficient method to improve the properties of Mg alloys by producing ceramic films on their surface [11]. The MAO films have a strong adhesion to Mg substrate, controllable thickness, and other excellent properties, such as wear resistance, thermal shock resistance and electrical insulation. MAO shows promise as a new surface modification technique. Typical MAO electrolyte systems include silicate [12], [13], phosphate [14], aluminate [15], [16], fluorozirconate [2], [17], and mixed system [18]. MAO films formed using these traditional electrolyte systems have a ceramic structure with relatively high porosity [19], resulting in limited protection for the films. The pore number may be decreased by changing electrical parameters and adjusting solution composition [20], [21], [22], [23], but these methods cannot eliminate the pores entirely. Pore-sealing post treatment is an effective way to decrease porosity and improve corrosion resistance [15]. However, an additional process is needed. Generally, these additional processes are complex, of high cost, and present environmental problems.

To overcome the shortcomings of traditional MAO films, a new fluorotitanate electrolyte system has been developed. The MAO film obtained in the new electrolyte system has the characteristic of self-sealing pores. As a result, corrosion resistance of the film is enhanced 3–5 times over that obtained with traditional electrolyte systems. However, the nature of the unique self-sealing pore mechanism is presently undefined. The aim of this paper is to discover the formation mechanism of self-sealing pores in MAO film in order to further improve the corrosion resistance of Mg alloys.

Section snippets

Experimental

AM60 Mg alloy (5.89 wt.% Al, 0.53 wt.% Mn, 0.08 wt.% Zn, 0.003 wt.% Si, 0.004 wt.% Fe, 0.002 wt.% Cu, the balance of Mg) [24] with a dimension of 70 mm × 15 mm × 5 mm was cut from a plate. The samples were first polished using sandpapers from 400 to 2000 grit, washed with distilled water and alcohol, then dried before MAO treatment. A WHD-20 MAO AC pulse power supplied a near square output waveform. Mg alloy samples and a stainless steel sheet were used as anodes and cathode, respectively. Solution

Comparison of self-sealing pore MAO film to traditional MAO film

The surface morphology of a self-sealing pore MAO film on AM60 Mg alloy is shown in Fig. 1a. There are several pores observed in the film; the diameters of these pores are greatly different, ranging from below 1 μm to approximately 20 μm for the largest. It is worth noting that these pores have been sealed by some constituents. EDX analysis of the self-sealing pore film shows that the fluorine content is 34.8 at.% in the sealed pore regions, but 14.2 at.% in other regions. Compared to the

Conclusions

(1)
A novel Ti-containing electrolyte solution was used to obtain a MAO film with self-sealing pores. Compared to traditional MAO films, the new MAO film contains Ti₃O₅ in addition to MgO and MgF₂, and presents a blue appearance. The chemical composition of the main pore-sealing constituents is MgF₂ and MgO.
(2)
In the initial MAO process, a relatively compact passive film consisting of MgO, MgF₂ and AlPO₄ is formed. The presence of fluorine ions in the electrolyte solution improves the compactness of

Acknowledgment

Thanks for the financial support of the National Natural Science Foundation of China (No. 51171198) and National Key Basic Research Program of China (No. 2013CB632205).

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View full text

Formation mechanism of a self-sealing pore micro-arc oxidation film on AM60 magnesium alloy

Highlights

Abstract

Introduction

Section snippets

Experimental

Comparison of self-sealing pore MAO film to traditional MAO film

Conclusions

Acknowledgment

Appl. Surf. Sci.

Surf. Coat. Technol.

J. Alloys Compd.

Surf. Coat. Technol.

J. Magnes. Alloys

Surf. Coat. Technol.

J. Magnes. Alloys

J. Magnes. Alloys

Mater. Chem. Phys.

Corros. Sci.

J. Magnes. Alloys

Corros. Sci.

Thin Solid Films

Surf. Coat. Technol.

Mater. Lett.

Corros. Sci.

Mater. Chem. Phys.

Mater. Sci. Eng. C

Surf. Coat. Technol.