Crevice corrosion of copper alloys by optical interferometry

Abstract

In the present investigation, an optical corrosion-meter has been developed for testing of materials 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 an aluminium brass and a pure copper sample in seawater and tap water, respectively, was monitored in situ by the optical corrosion-meter during the cyclic polarization test. The observations of crevice corrosion were basically interferometric perturbations detected on the surface of both alloys underneath a crevice assembly, made of a Teflon bolt, a 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 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 were interpreted as a localized corrosion in the form of an early crevice corrosion of depth ranging between 0.3 μm and several micrometers. Consequently, results of the present work indicate that holographic interferometry is a very usefull technique as a 3D-interferometric microscope for monitoring crevice corrosion at the initiation stage of the phenomenon for different metallic samples in aqueous solutions.