A contribution to the surface analysis and characterisation of HVOF coatings for petrochemical application

The appropriate selection of bulk materials and coatings of valve component s is an important factor for the economic success of oil and gas production activities in the petrochemical field. Materials and coatings are importan t because particle erosion and surface wear are associated to corrosion by hydrogen sulphide during oil and gas flow. The wear of high pressure valves of gas system will lead to pollution, safety problems and cost increases. The most common solution of these problems is the deposition of hard materi als as tungsten carbide or chromium carbide by thermal spray. These coating s are deposited by high velocity oxygen fuel (HVOF) thermal spray process t o obtain a very high hardness with excellent cohesion and adhesion. Tungste n carbide cobalt-chromium based coating, chromium carbide nickel-chromium c oating as well as Inconel 625 have been adopted in the specifications of pe trochemical companies and their behaviour and wear, erosion and corrosion p roperties are reported in the literature. This paper addresses the experimental study, surface analysis and functiona l characterisation of HVOF coatings innovative for the specific application such as NiAl and composite material WC/intermetallic compounds containing Ni, Cr, Co and Mo. These coatings have been systematically submitted to cor rosion and functional tests based on the determination of the behaviour of the coatings in H2S and CO2 atmosphere and to wear and erosion according to standard ASTM G75-95 (slurry test); material loss and surface damage have been determined; the coatings have been completely characterised from the p oint of view of the structure (morphology, porosity, hardness, wear) and of the surface properties by means of a prototype 3-dimensional (3-D) stylus micro-geometrical surface analysis system, their corrosion and functional b ehaviour have been compared with the behaviour of the above mentioned coati ngs. The slurry test allows a clear discrimination among, the performances of an alysed coatings. Namely, WC/Mo compound, because of its carbide content, sh ows fairly good behaviour in an erosive environment and higher erosion resi stance than Inconel 625 and NiAl; all the tested coatings show similar beha viour in a corrosive environment. (C) 2001 Elsevier Science B.V. All rights reserved.