the influence of current density and treatment time on peo coatings on az91 magnesium alloy

Magnesium alloys are used in various industries due to their excellent properties. however, their susceptibility to corrosion has been a concern. to overcome this challenge, researchers have developed a surface treatment technique known as plasma electrolytic oxidation (peo) that is achieved by subjecting the alloy to a high-voltage electrical discharge in an electrolyte solution. the process creates a plasma discharge on the surface, which leads to the formation of a thick and dense oxide layer by anodic oxidation. one of the key parameters in peo is the current density, which determines the rate of oxide formation and the resulting characteristics of the oxide layer. another important factor is the treatment time, as it influences the thickness and quality of the coating. in this article, we will explore the effects of current density and treatment time and how they impact the treated surface on az91. the az91 plates with dimensions of 40 mm × 10 mm × 2 mm were polished using silicon carbide papers and ultrasonically degreased in acetone for 10 min. coatings were fabricated at five different current densities with values of 0.25 a/cm2, 0.35 a/cm2, 0.4 a/cm2, 0.5 a/cm2 and 0.6 a/cm2 in electrolyte consisting of 10g/l na2sio3.5h2o, 5g/l koh and 2g/l naf. peo process was applied at duty cycle of 50% and frequency of 100 hz for treatment time of 5 and 8 min. the results show that higher current densities lead to thicker oxide layers due to the increased rate of oxide growth. this can be beneficial when a more substantial coating is required for improved wear resistance or corrosion protection. however, excessively high current densities can lead to the formation of porous oxide layers, which may adversely affect the surface quality. these pores can act as initiation points for corrosion. it should be mention that treatment time influences the thickness of the oxide layer, but it should be optimized to prevent overgrowth. by studying sem images of coatings, we can conclude that constant current density of 0.35 a/cm2 for a treatment time of 5 min provides a uniform coating with tiny pores and has the least cracks. the eds analysis results indicates that increasing current density leads to higher o/mg and si/mg ratio which demonstrates higher oxidation rate and denser coatings. in order to determine the phases formed on peo-treated az91, xrd analysis was performed and the peo coating was composed of mgo and mg2sio4.