Chromium and tantalum oxide nanocoatings prepared by filtered cathodic arc deposition for corrosion protection of carbon steel

Combined analysis by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), X-ray Photoelectron Spectroscopy (XPS), polarization curves and Electrochemical Impedance Spectroscopy (EIS) of the relation between chemical architecture of thin (10 and 50 nm) chromium and tantalum oxide coatings grown by filtered cathodic arc deposition (FCAD) on carbon steel and their corrosion protection properties is reported. Pre-etching in the deposition process allows reducing the substrate native oxide layer to traces of iron oxide. A carbidic interlayer is then formed by reaction between the first deposited metallic particles and the residual carbon surface contamination of the alloy. The bulk coatings mostly consist of Cr2O3 or Ta2O5 with no in-depth variation of the stoichiometry. Surface and bulk of the coatings are contaminated by hydroxyl and organic groups. The 50 nm coating has a relatively large porosity assigned to a columnar growth preventing good sealing at grain boundaries. The duplex structure (Ta/Ta–C) of the carbidic interlayer promotes a less defective growth of tantalum oxide than the single Cr–C interlayer for chromium oxide, thereby improving the sealing properties. The dielectric constants suggest poor insulating properties in line with a defective and porous nanostructure of the coatings. No dissolution was observed for both oxide nanocoatings in neutral 0.2 M NaCl. Penetration of the electrolyte and access to the interface with the carbon steel surface cause the dissolution of the Cr–C interlayer, but not that of the Ta/Ta–C interlayer, and a more rapid initiation of localized corrosion.