Effect of electrochemical parameters on wear and tribocorrosion capabilities of the PEO coatings generated through pulsed waveforms on AZ91 magnesium alloy

Abstract

In this study, PEO coatings were produced on AZ91 Mg alloy using pulsed waveforms at different electrical parameters such as type (unipolar and bipolar), cathodic duty cycle (20 and 40 %), and voltage amplitude (350 and 400 V) to assess wear and tribocorrosion resistance. The coating bath contained NaAl2O4 and NaF. The coating phase composition, microstructure, and chemical composition of the coatings were evaluated using GXRD, SEM, and EDS. Dry wear behavior was studied using the reciprocating ball-on-flat device, while tribocorrosion behavior was evaluated by immersing the coated specimens in 3.5 wt% NaCl solution and reading the OCP before, during, and after sliding. The worn area in both cases was also examined. The results showed that the bipolar waveform with the higher cathodic duty cycle (40 %) provided the best tribocorrosion and wear performance by forming MgF2 (as a corrosion-resistant phase) and MgAl2O4 (as a hard phase). At all three waveforms, the higher coating voltage (i.e. 400 V) enhanced the wear and tribocorrosion resistance by forming thicker coatings with higher amounts of MgF2 and MgAl2O4 phases.

Keywords

• PEO coatings
• Wear
• Tribocorrosion
• Magnesium alloy
• MgAl2O4
• MgF2