Detection of localized corrosion of stainless steels by optical interferrometry

An optical corrosion meter has been developed for materials testing and eva luation 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 ele ctronic current, i.e, corrosion current of metallic samples in aqueous solu tions. The early stage of pitting corrosion of UNS No. 304 stainless steel and crevice corrosion of UNS 316 stainless steel in natural seawater were m onitored, in situ by the optical corrosion meter during the cyclic polariza tion tests. The observations of Fitting corrosion were basically interferro metric perturbations detected on the surface of the UNS No. 304 stainless s teel in seawater. In contrast, the observations of crevice corrosion were b asically interferrometric perturbations detected on the surface of the UNS no. 316 stainless steel underneath a crevice assembly. Eventually, the inte rferrometric perturbations were interpreted as localized corrosion in the f orm of an early pitting or of an early crevice corrosion, of a depth rangin g from 0.3 mu m to several micrometers. Consequently, holographic interferr ometry is shown to be a useful technique as a 3D-interferrometric microscop e for monitoring Fitting corrosion or crevice corrosion at the initiation s tage of both phenomena for different metallic samples in aqueous solutions. (C) 1999 Elsevier Science Ltd. All rights reserved.