Tribocorrosion evaluation of hydrogenated and silicon DLC coatings on carbon steel for use in valves, pistons and pumps in oil and gas industry

Several components in the oil and gas industry are subjected to wear and corrosion. This work evaluated the feasibility of using Diamond-Like Carbon (DLC) coatings in Subsea Safety Control Valves (SSCV), piston and pumps. These are made from API X65 carbon steel and are subjected to wear and corrosive/saline environments. Coatings were deposited using Plasma Enhanced Chemical Vapour Deposition (PECVD). The electrochemical behaviour of Silicon-doped and Hydrogenated DLC films was evaluated before and after wear tests. Film characterisation included nano-indentation, surface roughness, micro-abrasion testing, Raman spectroscopy, atomic force microscopy and scanning electron microscopy. Electrochemical tests and electrochemical impedance was also measured. Sliding wear tests against silicon nitride were conducted with a maximum initial Hertz stress of 150 and 400 MPa under dry and wet conditions. The H-DLC had better wear performance than Si-DLC. The advantages of H-DLC were related to higher hardness increasing the wear resistance; small galvanic coupling between DLC and steel, inhibiting the localised corrosion into the DLC defects; lowest anodic current, suggesting high resistivity to use as a corrosion barrier for steel and the corrosion process on the substrate that did not affect DLC properties (adhesion and wear/ corrosion resistance).