Three-body tribocorrosion of high-chromium cast irons in neutral and alkaline environments
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Two high-chromium cast irons (HCCIs) with different microstructure and Cr:C ratio of 9 and 6 were tested using a new three-body abrasion-corrosion rig incorporating electrochemical techniques. Coarse garnet particles acted as abrasive grits and the effect of mechanical parameters (i.e. load, sliding distance) was investigated. Electrolytes with different pH, neutral and alkaline were used and the effects of chloride ions and elevated temperature were also tested. SEM and optical profilometry were used to analyse the extent of abrasion-corrosion damage and the surface morphology.In the corrosion-only tests it was found that the corrosion rate increased with increasing pH, and both the chloride ions and elevated temperature had a detrimental effect. Increasing alkalinity has shifted the anodic behaviour from the matrix to the carbides. The influence of key alloying elements on the corrosion behaviour is also addressed.Electrochemical measurements revealed that the concurrent three-body abrasion had accelerated corrosion for both HCCIs in most environments tested. The following abrasion-corrosion mechanism for HCCIs in highly alkaline solutions is proposed: (i) removal of a thin oxidized layer from the carbide phase; (ii) plastic deformation and extrusion of the Fe-Cr matrix over the carbide-depleted regions. SEM images revealed repeated plastic deformation with the wear morphology similar to the three-body abrasion of ductile materials, and carbides were not visible. In neutral environments the carbide/matrix interface was the most vulnerable to corrosion initiation, with the matrix dissolving preferentially and the unsupported carbides fracturing. The influence of materials characteristics on the abrasion-corrosion synergistic action is also described in this paper.
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