Plasma electrolytic oxidation (PEO) coatings on Mg-alloys for improved wear and corrosion resistance

Abstract

Due to their high chemical reactivity, relatively low melting point and low hardness, magnesium and magnesium alloys have relatively poor corrosion and wear resistance. Since both corrosion and wear are surface phenomena, a number of surface engineering techniques have been used to improve corrosion and wear performance. Whilst some surface hardening/strengthening methods have led to improvements in wear properties, they have not, in themselves, significantly improved the corrosion performance. Plasma electrolytic oxidation (PEO) has the potential to produce hard, compact oxide coatings that are well adhered to the magnesium alloy substrate. Such coatings can provide both improved wear and corrosion resistance. In this paper we describe how by changing the PEO processing parameters (substrate alloy; electrolyte; current or voltage; processing time) we can change the nature of the PEO oxide coatings (thickness; microstructure; porosity; phase content; composition) which, in turn, effects the corrosion and wear performance. All PEO-coatings have a three-layer structure with a porous outer layer, and intermediate dense layer and a thin inner dense layer. From a corrosion aspect, the performance of coatings is determined by the time taken for corrosion to initiate since this is much shorter than time taken for the coating to degrade. For PEO-coated Mg-alloys, this initiation time is primarily determined by the thickness, porosity and phase content of the inner dense layer at the coating/substrate interface. With respect to tribological properties, the coefficient of friction (COF) in dry sliding wear increases with increasing surface roughness of the PEO coatings. The wear rate is primarily determined by the thickness and hardness of the intermediate dense layer.

Use of synergistic mixture of chelating agents for in situ LDH growth on the surface of PEO-treated AZ91

“…According to XRD results, the PEO coating consists mainly of MgAl 2 O 4 spinel. This result is in agreement with previously published data for Mg-PEO systems formed in aluminate electrolytes [55][56][57] . However, the respective XRD peaks of PEO layer, which correspond to MgAl 2 O 4 , disappear completely after 3 h of treatment in a DTPA containing bath, while no LDH is formed yet.…”

PEO of AZ31 Mg Alloy: Effect of Electrolyte Phosphate Content and Current Density

“…Melted oxide flux flow out similarly to the "lava" effect observed in volcanic eruptions. This process is repeated several times, and coating gradually increases its thickness [49][50][51]. Places through which the discharges responsible for the distribution of melted flux passed the oxide layer represent pores when the melted flux is cooled down.…”

The Effect of Mechanical Pretreatment on the Electrochemical Characteristics of PEO Coatings Prepared on Magnesium Alloy AZ80

“…After the critical voltage, the PEO process enters the third stage (dielectric breakdown), which lasts until the end of the entire PEO treatment. The onset of this stage itself is essentially characterized by a flattening of the voltage-time curves with a gradual transformation of the discharges not only in colour appearance, from white to dazzling bright yellow, but also the increase in size and acoustic emission was evident [31,32].…”