A Contribution to the Production and Characterization of HVOF coatings for pplication in the Petrochemical Field

摘要

The appropriate selection of bulk materials and coatings of valve components, is an important factor for the economic success of oil and gas production activities in petrochemical field. Materials and coatings are important because particle erosion and surface wear is 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 problem and cost increases. The most popular solution of these problems is the deposition of hard material like tungsten carbide or chromium carbide by thermal spray. Particularly these coatings are deposed by HVOF (High Velocity Oxygen Fuel) to obtain a very high hardness with excellent cohesion and adhesion. Tungsten carbide cobalt-chromium based coating, chromium carbide nickel-chromium coating as well as Inconel 625 are adopted actually in the specifications of the industrial petrochemical companies and their behavior and wear, erosion and corrosion properties are reported in literature. This paper addresses the study and surface analysis and characterization of alternative coatings such as NiAl and composite material WC / intermetallic compounds containing mainly Ni, Cr, Co and Mo. The best parameters to produce these coatings has been found by implementing a DOE and the obtained coatings have been systematically submitted to corrosion and functional tests based on the determination of the behaviour of the thermal spray coatings in an atmosphere of H_2S and CO_2 and to wear and erosion test according to ASTM G75-95; removed material weight and usured surface damages have been determined. Furthermore the coatings have been completely characterized before and after the tests from the point of view of the structure (porosity, coating cohesion and adhesion, hardness, wear) and of the surface properties by means of a prototype 3-dimensional stylus micro-topography surface analysis system. Their corrosion and functional behaviour have been finally compared with the behaviour of the above mentioned coatings applied at present as standard in the petrochemical sector. The results state that WC/intermetallic compound could be a good substitute of IN625 for certain kind of application where good antierosion behaviour is requested.