Detection of localized corrosion of stainless steels by optical interferrometry

Abstract

An optical corrosion meter has been developed for materials testing and evaluation of different corrosion phenomena. The meter was based on principles of 3D holographic interferrometry for measuring microsurface dissolution, i.e. mass loss, and on those of electrochemistry for measuring the bulk electronic current, i.e. corrosion current of metallic samples in aqueous solutions. The early stage of pitting corrosion of UNS No. 304 stainless steel and crevice corrosion of UNS 316 stainless steel in natural seawater were monitored, in situ by the optical corrosion meter during the cyclic polarization tests. The observations of pitting corrosion were basically interferrometric perturbations detected on the surface of the UNS No. 304 stainless steel in seawater. In contrast, the observations of crevice corrosion were basically interferrometric perturbations detected on the surface of the UNS No. 316 stainless steel underneath a crevice assembly. Eventually, the interferrometric perturbations were interpreted as localized corrosion in the form of an early pitting or of an early crevice corrosion, of a depth ranging from 0.3 μm to several micrometers. Consequently, holographic interferrometry is shown to be a useful technique as a 3D-interferrometric microscope for monitoring pitting corrosion or crevice corrosion at the initiation stage of both phenomena for different metallic samples in aqueous solutions.