Plasma electrolytic oxidation coatings in KOH electrolyte and its discharge characteristics

• The light emitting mechanisms were different at three stages.
• At anodizing stage, the spectral peak shift due to different type of “flaws”.
• At transition stage, emitting lights were mainly from H2O+ and O2 and its intensity decreased with increasing voltage.
• At discharge stage, the light emitting resulted from the discharge sparks.
• The active plasma species of discharge sparks were Na, K, Mg, and O2+.

The ceramic coatings on the surface of AZ31 magnesium alloy were prepared via plasma electrolytic oxidation (PEO) of potassium hydroxide electrolyte. The discharge characteristics and the composition of the coatings were studied. The state evolution of the substrate surface during the PEO process were detected by optical emission spectroscopy (OES) and a high speed camera. The results of OES show that in the anodic oxidation stage, light emission could be attributed to electroluminescence because of the “flaws” of the dielectric barrier layer. The movement of the spectral peak also occurred at the anodic oxidation stage because of different “flaws” in the dielectric barrier layer. At the transition stage, light emission could be attributed to the ionization species present in the bubbles. At the discharge stage, the main active plasma species in the plasma field were Na, K, Mg, and O2+, which resulted in the light emitting. The coatings were mainly composed of MgO and Mg(OH)2.