THE EFFECT OF ELECTROLYTE STRENGTH AND GRAIN-SIZE ON THE TRANSIENT CURRENT RESPONSE IN FE-CR-NI ALLOYS DURING A SCRATCH TEST

Two alloys, Fe-16%Cr-16%Ni (16/16) and Fe-25%Cr-25%Ni (25/25), from th e Fe-Cr-Ni system have been used to study the effect of electrolyte st rength on the depassivation and repassivation kinetics and the develop ment of near surface microstructures using a scratch technique. The co ncentration of sulfuric acid in the solution 0.01 M H2SO4-0.01 M KCl w as varied and the transient current response for different electrolyte s shows that it increases systematically with increasing load in 16/16 alloy while the values at lower concentrations of sulfuric acid remai n almost unaffected in the case of 25/25 alloy. The electrolyte streng th effect is manifested in the form of strain rate during the process of scratching in an electrolyte (corrosive wear). The decrease in defo rmation band widths, after scratching, as a function of increase in co ncentration of the sulfuric acid in the electrolyte has been observed in 16/16 alloy. In contrast, the 25/25 alloy, with a higher stacking f ault energy, shows long planar array of parallel dislocations at highe r concentrations. The scatter in the data for the transient current re sponse usually associated with this technique has been also studied.