Micro-Arc Oxidation Treatments on Zn

Micro-arc oxidation has been a hot topic in the field of surface treatment of aluminum, magnesium and titanium for about twenty years. It consists in a high-voltage oxidation beyond the dielectric breakdown occurring at the interface metal/oxide/electrolyte. Due to the high temperature reached in the resulting plasma, the grown coatings are dry ceramic layers, exhibiting interesting corrosion and wear resistance. This process is generally performed in alkaline media containing additives favoring both the growth rate and the corrosion resistance of the coated materials such as aluminates and silicates. This original paper focuses on the study of micro-arc treatments performed on zinc [1], which could lengthen the service life of sacrificial anodes.

An electrochemical study during the anodizing process revealed that the formation of the first stable insulating layer necessary to reach the dielectric breakdown (and therefore to initiate the sparks) is in competition with the dissolution of zinc. Therefore, beyond a maximum value of pH, starting with the micro-arc regime is no more possible. On the other hand, the addition of silicates or aluminates to the reference electrolytic bath (KOH 0.05 M) facilitates the occurrence of the sparking initiation: around 200 V in KOH 0.05 M, around 150 V with aluminates 0.05 M, around 100 V with silicates 0.05 M.

Post-treatment characterizations of the treated materials performed by FEG-SEM observations coupled with EDX analyses showed that the thickness, the morphology and the composition of the coatings highly depend on the composition of the electrolytic bath.

In terms of corrosion resistance, open-circuit potential measurements, potentiodynamic polarizations and electrochemical impedance spectroscopy measurements were performed in NaCl 0.1 M and showed a very slight improvement of the performances of the treated materials regarding an untreated one since the obtained coatings are cracked and dusty.

[1] TR. Tan, JR. Cheng, JH.Wang, JG Duh, HC. Shih, Surf. Coat. Tech 110, 194-199 (1998)