Effect and Mechanism of PECVD-Deposited DLC Films on the Corrosion Resistance of PEO Composite Coatings on Magnesium Alloys

A PEO composite coating was fabricated on a magnesium alloy using CO₂-assisted oxidation and hydrothermal treatment, followed by PECVD carbon deposition to further improve corrosion resistance. The effects of deposition time on coating barrier properties and corrosion behavior in simulated body fluid (SBF) were investigated. The results revealed that a deposition time of 60 min produced a uniform carbon-based layer that effectively sealed the micro-pores of the PEO surface, resulting in a compact and integrated composite structure. The optimized coating exhibited markedly improved electrochemical performance, with a significantly reduced corrosion current density and a polarization resistance of 2.3 × 10⁷ Ω cm², which was approximately three orders of magnitude higher than that of the untreated substrate (3.06 × 10⁴ Ω cm²). The superior corrosion protection is attributed to the carbon-enriched outer layer, which provides an enhanced barrier effect and pore sealing ability, thereby suppressing the ingress of corrosive species and improving the corrosion-protection performance of the coating in SBF. This study provides a feasible route and mechanistic insight for designing multi scale synergistic protection systems for magnesium alloy surface engineering.