Pitting Corrosion Susceptibility of Microstructural Features in a Compositionally Complex Ferritic Steel as a Function of Titanium Concentration

Abstract

Compositionally and/or microstructurally complex alloys present multiple opportunities for achieving and optimizing desirable qualities that are not typically accessible through traditional, single-principle-component alloying methodologies, without significant compromise. FBB8+Ti, a novel ferritic steel strengthened by Heusler- and B2-strengthening phases, is one such alloy and is investigated here across a range of titanium concentrations for its corrosion performance in a chloride environment. Pitting potentials, corrosion rates, and passivation current densities were established during full immersion in 0.01 M NaCl for alloys prepared with a (a) 0.05 μm finish and (b) subsequent ion-polish, in separate experiments. A scanning electron microscopy was used to identify microstructural features that were vulnerable to metastable pitting on a sub-feature length scale, enabled by a series of potentiostatic holds in the passive region where nascent breakdown/repair occurs. No significant trend in corrosion behavior was observed with titanium content variance for specimens polished to a 0.05 μm finish within the range of compositions investigated, suggesting that alloy design may be optimized in terms of mechanical and thermal performance brought about by exploring titanium content without consideration on the effects of corrosion. Preferred sites of pit initiation were sensitive to the surface finish, highlighting the delicate balance between the susceptibility of possible favored pitting sites. Results were compared to stainless steel 316L where the passive current density for the FBB8 alloys was found to be favorable, despite slightly lower pitting potentials.