Exploring new W–B coating materials for the aqueous corrosion–wear protection of austenitic stainless steel

Abstract

The material loss of metallic surfaces through corrosion–wear is a serious concern in many application sectors, ranging from bio-medical implants to marine, oil and gas field components to transport vehicle and nuclear reactor devices. In principle, self-passivating alloys, like stainless steels, can be protected from surface degradation caused by corrosion–wear through the application of protective thin, hard surface coatings. In this work the suitability of using W matrix coating materials supersaturated with varying levels of boron were applied to austenitic stainless steel substrates (Ortron 90) and assessed for this purpose. These materials were compared to a highly corrosion–wear resistant “datum” surface engineered material (CrN coated Ti–6Al–4V) in sliding contact tests against a chemically inert aluminium oxide ball, whilst immersed in 0.9% NaCl solution at 37 °C. The work demonstrated that all the coated materials to be very much more resistant to material loss through corrosion–wear (by nearly an order of magnitude) compared to uncoated stainless steel, and two coatings, W–13%B and W–23%B coated Ortron 90 were similarly resistant as CrN coated Ti–6Al–4V. Three fundamental types of corrosion–wear were discovered that represented differing levels of passive film durability. The total material loss rate (TMLR) during corrosion–wear testing showed linear proportionality with the change in open circuit potential δOCP which obeyed the governing equation: TMLR = m δOCP + C.