Localized corrosion of copper alloys by optical interferometry and other techniques

An optical corrosion meter has been developed for materials testing and eva luation 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 e lectrochemistry for measuring the bulk electronic current, i.e., corrosion current of metallic samples in aqueous solutions. In the present work an ea rly stage of crevice corrosion and pitting corrosion of an aluminium brass and pure copper samples in seawater and tap water, respectively, were monit ored in situ by the optical corrosion meter during the cyclic polarization test. The observations of pitting corrosion were basically interferometric perturbations detected on the surface of both alloys. However, the observat ions of crevice corrosion were basically interferometric perturbations dete cted on the surface of both alloys underneath a crevice assembly, made of T eflon bolt, Teflon nut, and Teflon washer. The crevice assembly used on all tested samples to crevice corrosion to create a differential aeration cell between the surface of the sample and areas underneath the crevice assembl y in seawater. Each Teflon washer contained radial grooves and had 20 plate aus which formed crevices (shield areas) when pressed against the surface o f the sample. The interferometric perturbations interpreted as a localized corrosion in a form of an early crevice corrosion or pining corrosion of a depth ranged between 0.3 mu m to several micrometers. Consequently, results of the present work indicate that holographic interferometry is a very use ful technique as a 3D-interferometric microscope for monitoring crevice cor rosion and pining corrosion at the initiation stage of the phenomena for di fferent metallic samples in aqueous solutions.