Laser optical interferometry as NDT methods for cleaner and sustainable desalination plants: pitting and crevice corrosion

Abstract

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 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 pitting corrosion of pure aluminium, UNS No.304 stainless steel alloy and copper in seawater were monitored in situ by the optical corrosion-meter during the cyclic polarization test. Also, an early stage of pitting corrosion of pure copper and an aluminium-brass alloy in tap water were monitored in situ by the optical corrosion-meter during the cyclic polarization test. The observations of pitting were basically interferometric perturbations detected only on the surface of the aluminium in seawater, UNS No.304 stainless steel in seawater, pure copper in tap water. In addition, an early stage of crevice corrosion of a carbon steel alloy, a titanium alloy, UNS No.316 stainless steel, UNS No.304 stainless steel, pure aluminium, and an aluminium-brass alloy in seawater as well as pure copper in tap water were 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 most of the tested samples in solutions, except UNS No.304 stainless steel and the pure aluminium in seawater. The crevice corrosion was detected underneath a crevice assembly on the surface of the samples. The crevice assembly made of Teflon bolt, Teflon nut, and 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 solutions. 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 pitting corrosion or crevice corrosion of a depth ranged between 0.3 μm to several micrometers. Consequently, results of the present work indicate that holographic interferometry is very useful technique as a 3D-interferometric microscope for monitoring pitting corrosion and crevice corrosion at the initiation stage of the phenomena for different metallic samples commonly used in desalination plants.