Corrosion protection of DLC coatings on magnesium alloy

Abstract

Anti-corrosion properties of DLC films deposited on magnesium alloy, AZ31 (Mg–3%Al–1%Zn) were investigated. The DLC and Si-incorporated DLC (Si–DLC) films were prepared on sputter-deposited AZ31 films using a bi-polar type plasma based ion implantation and deposition (PBII&D). The AZ31 films were prepared on glass plates using an ECR-type ion beam sputter with an AZ31 target. The potentiodynamic polarization curves were recorded in aqueous 0.05 M NaCl electrolyte solution. The corrosion potential of the AZ31 film shifted to more positive value due to the protection of the DLC film. The Si-incorporation in the DLC films further improved the ability of corrosion protection of the DLC film, and the performance of corrosion protection improved with increasing Si content in the DLC films. X-ray photoelectron spectroscopy measurements indicated that a thick oxide layer exists on the Si–DLC film surfaces. The insulating oxide layer results in high anti-corrosion properties of the Si–DLC films. Very low internal stress in the Si–DLC films is one of the reasons for the high anti-corrosion performance of the films. The oxygen plasma treatment on the AZ31 film formed a thick magnesium oxide layer on the film and increased electrical resistivity, which resulted in improved corrosion performance of the AZ31. The oxygen plasma treatment will be a prospective pre-treatment method before the protective DLC coatings.