Detection of crevice corrosion of steels in seawater 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 work, an early stage of crevice corrosion of a carbon steel and a stainless steel in seawater was monitored in situ by the optical corrosion-meter during the cyclic polarization test. servations of crevice corrosion were basically interferometric perturbations detected only on the surface of the carbon steel and the stainless steel underneath a crevice assembly, made of a Teflon bolt, Teflon nut, and a Teflon washer. The crevice assembly was used on all tested samples to create a differential aeration cell between the surface of the sample and the areas underneath the crevice assembly in seawater. Each Teflon washer contained radial grooves and had 20 plateaus that formed crevices (shield areas) when pressed against the surface of the sample. The interferometic perturbations interpreted as localized corrosion in the form of an early crevice corrosion as a depth ranging between 0.3 μm and several micrometers. Consequently, results of the present work indicate that holographic interferometry is a very useful technique for monitoring crevice corrosion at the initiation stage of the phenomenon for different metallic samples in aqueous solutions.