Detection of crevice corrosion of metallic alloys 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 micro surface 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 titanium alloy, a carbon steel, and a pure aluminum 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 titanium alloy and the carbon steel underneath a crevice assembly, made of Teflon bolt, Teflon nut, and Teflon washer. The crevice assembly was used on all tested samples in order 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 interferometic perturbations interpreted as a localized corrosion in a form of an early crevice corrosion of a depth ranged between 0.3 and several micrometers. Consequently, results of the present work indicate that holographic interferometry is a very useful technique as a 3D-interferometric microscopy for monitoring crevice corrosion at the initiation stage of the phenomenon for different metallic samples in aqueous solutions.