Crevice corrosion of copper alloys by optical interferometry

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 elec tronic current, i.e. corrosion current of metallic samples in aqueous solut ions. In the present work, an early stage of crevice corrosion of an alumin ium brass and a pure copper sample in seawater and tap water, respectively, was monitored in situ by the optical corrosion-meter during the cyclic pol arization test. The observations of crevice corrosion were basically interf erometric 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 different ial aeration cell between the surface of the sample and areas underneath th e crevice assembly in seawater. Each Teflon washer contained radial grooves and had 20 plateaus which formed crevices (shield areas) when pressed agai nst the surface of the sample. The interferometric perturbations were inter preted as a localized corrosion in the form of an early crevice corrosion o f depth ranging between 0.3 mu m and several micrometers. Consequently, res ults of the present work indicate that holographic interferometry is a very usefull technique as a 3D-interferometric microscope for monitoring crevic e corrosion at the initiation stage of the phenomenon for different metalli c samples in aqueous solutions. (C) 1999 Elsevier Science Ltd. All rights r eserved.