Investigation into the Incidence of Severe Rusting and Pitting Corrosion in Imported Hot-Rolled AISI 430 Ferritic Stainless Steel Coils

Abstract

Metallurgical investigations were directed to probe into the incidence of inordinate rusting and pitting in imported AISI 430 grade hot-rolled ferritic stainless steel sheet coils. Visual examination, electron microprobe analyses (EPMA), scanning electron microscopy (SEM) and electrochemical potentiokinetic reactivation (EPR) were concomitantly employed to investigate the problem. Studies revealed that the unprecedented degree of corrosion in ferritic stainless steel coils, during the short span of shipment time, was attributable to the ingress of sea water and its retention within the tight folds/ wraps of the steel coils during their shipment. The abundance of moisture and chloride (from the entrapped saline electrolyte) on the steel surface together with depleted O2 supply within the tight folds are presumed to have created conditions akin to an actively-corroding crevice, by way of passive film instability and its eventual breakdown on the stainless steel surface. As a consequence, the coils are believed to have suffered an accelerated and intensified chloride-induced corrosion attack and damage within the short span of shipment time. The investigations also revealed that the corrosive conditions were further exacerbated by the vulnerability and susceptibility of ferritic stainless steel to intergranular corrosion (IGC) due to its inherent sensitized condition. The paper thus throws light on an unusual precedent of chloride-induced corrosion in ferritic stainless steel and highlights the investigative metallographic work and corrosion failure analysis that led to above revelations.