Abstract :
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 and pitting corrosion of an aluminum brass and pure copper samples in seawater and tap water, respectively, were monitored 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 observations of crevice corrosion were basically interferometric perturbations detected on the surface of both alloys underneath a crevice assembly, made of Teflon 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 assembly in seawater. Each Teflon washer contained radial grooves and had 20 plateaus which formed crevices (shield areas) when pressed against the surface of the sample. The interferometric perturbations interpreted as a localized corrosion in a form of an early crevice corrosion or pitting 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 useful technique as a 3D-interferometric microscope for monitoring crevice corrosion and pitting corrosion at the initiation stage of the phenomena for different metallic samples in aqueous solutions.
