Abstract
Plasma electrolytic oxidation coatings were prepared in aluminate, phosphate, and silicate-based electrolytic solutions using a soft-sparking regime in a multi-frequency stepped process to compare the structure, corrosion, and wear characteristics of the obtained coatings on AZ31 magnesium alloy. The XRD results indicated that all coatings consist of MgO and MgF2, while specific products such as Mg2SiO4, MgSiO3, Mg2P2O7, and MgAl2O4 were also present in specimens based on the selected solution. Surface morphology of the obtained coatings was strongly affected by the electrolyte composition. Aluminate-containing coating showed volcano-like, nodular particles and craters distributed over the surface. Phosphate-containing coating presented a sintering-crater structure, with non-uniform distributions of micro-pores and micro-cracks. Silicate-containing coating exhibited a scaffold surface involving a network of numerous micro-pores and oxide granules. The aluminate-treated sample offered the highest corrosion resistance and the minimum wear rate (5 × 10−5 mm3 N−1 m−1), owing to its compact structure containing solely 1.75% relative porosity, which is the lowest value in comparison with other samples. The silicate-treated sample was degraded faster in long-term corrosion and wear tests due to its porous structure, and with more delay in the phosphate-containing coating due to its larger thickness (30 µm).
Highlights
Magnesium (Mg) and its alloys are among the top choices for materials selection due to the need for weight reduction in the automotive industry or biocompatibility of implants in the human body applications [1,2]
It is suggested that the conductivity of each solution significantly affects the and silicate solutions, respectively
AlO2 ions are incorporated affects the current density level achieved during the3−Plasma Electrolytic Oxidation (PEO) process [7]
Summary
Magnesium (Mg) and its alloys are among the top choices for materials selection due to the need for weight reduction in the automotive industry or biocompatibility of implants in the human body applications [1,2]. This is because of their unique characteristics such as high strength-to-weight ratio, low density, damping capacity, non-toxicity, and the ability to be recycled, which have drawn the attention of other industries as well [3,4]. The low hardness of magnesium alloys cannot provide enough resistance while contacting with a counterpart, thereby resulting in high wear loss [6] Due to these limitations, a proper anti-corrosion and wear-proof coating must be situated on Mg substrate to make its application possible. Researchers have found Plasma Electrolytic Oxidation (PEO) more interesting than other methods to overcome magnesium alloys weaknesses, which has caused a meaningful increase in the number of Coatings 2020, 10, 937; doi:10.3390/coatings10100937 www.mdpi.com/journal/coatings
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