Novel Ni-P-Tribaloy Composite Protective Coating

Abstract

Oil and gas pipelines are subject to various forms of damage and degradation during their operation. Electroless Nickel (Ni-P) coatings are widely employed as protective coatings due to their ease of application and unique properties, including high wear and corrosion resistance. However, they are not ideal for protecting pipelines due to their brittleness and low toughness. Composite coatings of higher toughness can be developed through the co-deposition of second-phase particles into the Ni-P matrix. Tribaloy (CoMoCrSi) alloy possesses excellent mechanical and tribological properties making it a potential candidate for a high-toughness composite coating. In this study, Ni-P-Tribaloy composite coating consisting of 15.7 vol.% Tribaloy was successfully deposited on low-carbon steel substrates. Both the monolithic and the composite coatings were studied to evaluate the effect of the addition of Tribaloy particles. The micro-hardness of the composite coating was measured to be 6.00 GPa, 12% greater than that of the monolithic coating. Hertzian-type indentation testing was carried out to investigate the coating’s fracture toughness and toughening mechanisms. The 15.7 vol.% Tribaloy coating exhibited remarkably less severe cracking and higher toughness. The following toughening mechanisms were observed: micro-cracking, crack bridging, crack arrest, and crack deflection. The addition of the Tribaloy particles was also estimated to quadruple the fracture toughness. Scratch testing was performed to evaluate the sliding wear resistance under a constant load and a varying number of passes. The Ni-P-Tribaloy coating exhibited more ductile behavior and higher toughness, as the dominant wear mechanism was identified as material removal, as opposed to brittle fracture in the Ni-P coating.