Surface Characterization of Atmospherically Corroded and Blast Cleaned Steel Available to Purchase

Surfaces of steel samples corroded in an industrial atmosphere, abrasive blasted, and environmentally tested, were characterized by optical and electron (SEM, EDXA, EPA) microscopy. At temperatures of approximately 25 C and in various relative humidity environments, it was found consistently that industrially corroded and blasted steel quickly formed new corrosion product. For the same exposures, new mill or corroded/ground/blasted steel showed no signs of new corrosion development. It was found that atmospheric corrosion proceeds heterogeneously, and the corrosion product contained atmospheric chlorine and sulfur. When blasted, the surface was layered by plastic deformation of the pit walls and trapped scale product in surface folds. In the presence of moisture, these sites became active nuclei for the rapidly formed new corrosion. In contrast, the normal passive oxide on uncontaminated steel retarded new corrosion processes. Sites contaminated with sulfur or chlorine were responsible for the initiation of new corrosion and were the likely sites for ultimate paint and coating failures.