Air plasma sprayed metallic coatings for sour environments

Abstract

Among protective coatings for components used in petrochemical applications, plasma sprayed alloys containing chromium, nickel, and molybdenum have potential to provide functional surfaces with good resistance to corrosion and wear. They could offer a cost effective solution by limiting the use of expensive bulk materials such as Inconel or Hastelloy superalloys or highly alloyed chromium stainless steels, since a low cost base material could be selected to give the required mechanical strength combined with suitable corrosion resistance from its modified surface. To limit fabrication costs of coating deposition, air plasma spraying techniques can be proposed, but proper optimisation of the plasma processing parameters is required to increase coating performance. In this paper, coatings based on Ni–Cr–B–Si and Mo alloys have been deposited following a design of experiment methodology. Results of characterisation tests for preliminary spraying trials (metallographic, tribological, and corrosion tests) are reported and discussed. Following analysis of numerical results, a coating obtained by the codeposition of two commercial powders (Metco 505 and Metco 16C) was sprayed on carbon steel substrates. Sulphide stress corrosion cracking tests, according to the NACE TM 0177-90 standard, at up to 90% of the yield stress of the coated specimens did not induce any damage for test durations of up to 720 h. Tribological properties have also been evaluated in terms of the slurry abrasivity response number (SAR), friction coefficients, and mass loss after ASTM standard ball on disc tests. Good resistance to erosion damage was obtained with a final SAR number of 37.