EFFECT OF MICROSTRUCTURE ON THE CORROSION BEHAVIOR OF 6-PERCENT CR WHITE IRON ALLOYED WITH MN AND CU

The effect of microstructure on the corrosion behavior of a 6% chromiu m (Cr) white iron containing 5.5 to 6% manganese (Mn) and 1% copper (C u) was studied to evaluate its suitability as a corrosion-resistant ma terial. Heat treatments involved holding at 900, 950, 1000, and 1050-d egrees-C for 4, 6, 8, and 10 h followed by oil quenching. Corrosion re sistance was assessed using the weight loss method in a 5% NaCl soluti on under stagnant conditions at ambient temperature for 168 and 720 h, respectively. On heat treating at 900 and 950-degrees-C, the corrosio n rate increased over that in the as-cast state (characterized by a mu ltiplicity of the matrix microconstituents) despite the matrix being p redominantly austenitic. Increasing the soaking period at 900 and 950- degrees-C had little effect on the corrosion rate. A marginal increase followed by a decrease was observed despite a decrease in the level o f massive carbides. This reflected adversely on the presence of disper sed carbides. On heat-treating at 1000-degrees-C, the expected improve ment in corrosion resistance with soaking period was stalled by unfavo rable morphology of the newly forming M7C3 carbide. This effect persis ted even on heat treating at 1050-degrees-C up to 4 h soaking period. The volume fraction of the second phase decreased with time, leading t o improved corrosion resistance.