Detection of crevice corrosion by optical interferometry

In the present investigation, an optical corrosion-meter has been developed for materials testing and evaluation of different corrosion phenomena. The idea of the optical corrosion-meter was established based on principles of 3D-holographic interferometry 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. In the present study, an early stage of crevice corrosion of a stainless steel and a carbon steel in seawater was monitored in situ by the optical corrosion-meter during the cyclic polarization test. The observations of crevice corrosion were basically interferometric perturbations detected only on the surface of the carbon steel underneath a crevice assembly, made of Teflon bolt, Teflon nut, and Teflon washer. The interferometric perturbations interpreted as a localized corrosion in a form of an early crevice corrosion of a depth ranged between 0.3 μm to several micrometers. On the other hand, observations of uniform fringe patterns were detected on the sample of the stainless steel indicating that the stainless steel sample was subjected only to uniform (general) corrosion attack during the cyclic polarization test. Consequently, results of the present work indicate that holographic interferometry is a very useful technique as a 3D-interferometric microscope for monitoring crevice corrosion at the initiation stage of the phenomenon for different metallic samples in aqueous solutions.