Plasma electrolytic oxidation of AZ31 magnesium alloy in aluminate-tungstate electrolytes and the coating formation mechanism

Highlights

• PEO of AZ31 magnesium in aluminate-tungstate electrolytes.

• PEO coating is not formed by an ejection of molten oxide.

• Penetrating discharges caused the inward coating growth and anion deposition.

• Anodic current density is estimated to be ∼10⁴ A cm⁻² within discharge channels.

• Thermal decomposition of water causes anomalous gas emission.

Abstract

Plasma electrolytic oxidation (PEO) of AZ31 magnesium alloy under pulsed bipolar regimes has been carried out in an aluminate electrolyte with the addition of 0–25 g l⁻¹ Na₂WO₄·2H₂O. Black coatings are formed with the addition of tungstate. Sequential anodizing has also been adopted to investigate the coating formation mechanisms by tracing the elemental distribution of W and Al in the coatings. The coatings develop an outer layer, inner layer and a barrier layer after a certain period of PEO. At the later stage of the PEO, the coating grows inwardly, which was accompanied by the strong penetrating discharges. The penetrating discharges have caused significant anion deposition, and the electrolyte species, such as W and Al, can be transported to the coating/substrate interface instantly. The anodic current density within the penetrating discharge channels is estimated to be ∼10⁴ A cm⁻², which is high enough to melt the coating materials beneath the pancake structure and cause the direct thermal decomposition of water and hence the anomalous gas evolution reported for PEO. X-ray photoelectron spectroscopy (XPS) denies that free state W exists in PEO coatings.

Introduction

Owing to their lightness, high strength to weight ratio, abundant raw material resources, good electrical conductivity and high thermal conductivity, magnesium and its alloys have been widely used in automobile, aerospace, electronic communication and biomedical industries [1], [2], [3], [4], [5], [6]. However, magnesium alloys also have a few notable shortcomings such as high chemical reaction activity, low corrosion resistance, poor creep and wear resistance, which greatly restrict their extensive applications in technical and biomedical industries [4], [5], [6], [7], [8], [9]. In order to avoid such disadvantages, surface treatment is indispensable for the practical application of magnesium alloys [10], [11]. A various methods can be used for surface modification of magnesium and its alloys, which include anodizing [12], PVD or CVD depositing [13], [14], electroplating [15] and plasma electrolytic oxidation (PEO) [16], [17], [18], [19]. Among these methods, PEO, also called micro-arc oxidation (MAO), is viewed to be the most effective to improve the surface properties of magnesium alloys [4], [17], [20], [21], [22], [23].

PEO is developed from conventional anodizing but works under higher voltages, which causes the dielectric breakdown of the oxide films, manifested by moving plasma discharges at the surface of the treated workpieces. Due to high temperatures of the plasma, several physical-chemical processes, typically electrochemical, plasma chemical and thermal diffusion, occur simultaneously during PEO, which result in rather complex coating formation mechanisms [24], [25]. A recent review has gathered the most important advancements on the mechanisms of PEO coating formation, however, a complete explanation for the coating formation mechanisms has not yet been proposed [4]. PEO is normally viewed to be significantly related with the plasma discharges and coatings grow discontinuously, with cycles of coating formation, dielectric breakdown, and material deposition in the discharge channel after its termination [23], [26], [27]. However, a recent study of the PEO on Al by Zhu et al., suggested that the thin amorphous alumina layer at the coating base was grown by an ionic migration mechanism and hence, “PEO was not an abrupt ejection of a molten material but a gentle growth process” [28]. Various attempts have been made to investigate the coating growth mechanism and associated species transportation process, for example, the using of ¹⁸O [26] and oxide particles as tracers [20].

The microstructures of PEO coatings are determined by many factors, such as the substrate metal, the electronic properties of the formed oxides, electrolyte concentration and the forming electrical regimes [25], [29], [30]. In a recent study, the high insulating and semiconductor properties of ZrO₂ and TiO₂, respectively, were attributed to the formation of “coral reef” and pancake structures on the coatings on the respective titanium and zirconium alloys [29]. The electrolyte concentration, and hence the anion deposition process, have much effect on plasma discharge behavior and coating morphologies, too [30]: Energetic penetrating discharges were found for PEO with less anion deposition, favoring the formation of pancake structures, however, the PEO in concentrated electrolytes with heavy anion deposition was accompanied by weak discharges and an absence of pancake features. There are also other special structures on the PEO coatings, such as “the characteristic solidification structure” on the PEO coatings on zirconium alloys [31], [32], [33], [34], [35]. The characteristic solidification structure, which is thought to be related with the low thermal conductivity of zirconia [31], [33], is featured by a cluster of equiaxed grain in the central region, surrounded by a ring of radially orientated, elongated grains. Up to the present, this structure is only found with the PEO of zirconium alloys.

The PEO of magnesium and its alloys is normally carried out in electrolytes based on silicate, phosphate and aluminate [36], [37], [38], [39], [40], [41], [42]. The coatings from these electrolytes are normally white in color, however, coatings with other colors are attractive for decorative and also other functional purposes [43], [44], [45], [46]. As an example, black coatings with high emittance and high absorptance are highly desired for the internal components of the spacecraft [44]. In recent years, tungsten-containing oxide layers on Al and its alloys have been explored due to their catalytic, semiconducting and corrosion resistant properties [47], [48], [49], [50], [51], [52], [53], and the incorporation of W is also known to result in black coatings on Al alloys [44], [49]. However, as compared with Al alloys, there are less works on the PEO of magnesium alloys using sodium tungstate as a component of the electrolyte [54], [55], [56], [57]. Furthermore, Zhao et al. [57] reported that sodium tungstate can lighten the coating color on AZ91 magnesium alloy, which seems to contradict to the role of tungsten for PEO coating on Al alloys.

In the present study, PEO of an AZ31 magnesium alloy has been carried out under pulsed bipolar regime in aluminate based electrolytes with the addition of 0–25 g l⁻¹ Na₂WO₄·2H₂O. The coating morphology, microstructure and phase compositions before and after the addition of Na₂WO₄·2H₂O have been characterized in detail. Sequential anodizing has also been performed to explore the coating formation mechanism. By tracing the distribution of the tungsten species in the coatings, the mechanism for the coating formation has been discussed.