The Effectiveness of Hard Coatings in Protecting Steels against Erosion-Corrosion in Oxidizing Environments at High Temperature

Abstract

Industrial processes can involve motion of hard particles that impact on metallic surfaces at high temperatures, thereby leading to erosion-corrosion damage. In this paper, results from a laboratory study of the erosion-corrosion behaviour of an Fe-1Cr-Mo steel have been compared with those of several hard commercially-applied thermal-sprayed (wire arc- and D-gun) coatings, using two test rigs that generate erosion conditions pertinent to such processes. The first is a spouting-bed rig that gives particle loadings of about 10 vol%, with impact velocities between the erodent particles and the specimens of up to 10 m s -1 ; the second is a jet-impact rig that gives lower particle loadings and higher impact velocities. Emphasis has been placed on the effects of temperature (in the range: 450 to 550°C) and impact angle (in the range: 15° to 90°) on the erosion-corrosion damage caused by silica particles in an oxidizing gas (air). Following exposures, the specimens were examined and analyzed in plan and cross section by optical and analytical scanning electron microscopy. The extents of damage, determined by measurements of thickness loss in cross section, have been compared and are discussed in terms of relevant mechanisms of erosion-corrosion. In these tests, in-situ grown oxides gave some protection to the uncoated steel while the resistance of the coated specimens to damage was determined mainly by the effect of hard phase on the extent of deformation of the surface.