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 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 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 interferometic perturbations interpreted as a localized corrosion in a form of an early crevice corrosion of a depth ranged between 0.3 pm to several micrometers. Consequently, results of the present work indicate that holographic interferometry is very useful technique as a 3D-interferometric microscope for monitoring crevice corrosion at the initiation stage of the phenomenon for different metallic samples in aqueous solutions.

INTRODUCTION

In a sequence of research work conducted by the author during the last 10 years-b, a novel 1 ?optical transducer was innovated for research investigations in electrochemical studies and related reasons The optical transducer was applied not only as an electrometer for measuring different Electrochemical parameters-5, but also the optical transducer was applied as a 3D-interferometrio microscope for detecting different micro-alterations, i.e., corrosion, at a metal surface in aqueous solution at a microscopic scale. For instance, the optical transducer was used for monitoring the echanochemical behavior, i.e. stress corrosion cracking, corrosion fatigue, and hydrogen embrittlement, of metallic electrodes in aqueous solutions. The optical transducer was developed by I corpora ting holographic interferometry for measuring microscopic deformation and electrochemical t techniques for determining the corrosion current of metallic samples. In other words, the optical transducer can be used as an optical corrosion-meter 6. As a result, the optical transducer has many potential applications in the field of electrochemistry, yet to be explored.

The objective of the present work was to monitor crevice corrosion of a titanium, a carbon steel ~and a pure aluminum in seawater in situ during cyclic polarization, without any physical contact, b 1 ?sing the optical corrosion-meter 6 The reason behind the selection of the titanium and the carbon steel in this study was that the titanium alloy and the carbon steel are known to be susceptible t I crevice in seawater71 if both alloys are exposed to seawater for a period of timeline, 30,60,and 90 days. In contrast, the pure aluminum is known to have less tendency to crevice corrosion in seawater. s far as this investigation is concerns, this is the first time an attempt to detect Crevice corrosion of a titanium alloy, a carbon steel and a pure aluminum in situ by the optical corrosion-meter6 during the clic polarization test, an electrochemical test, in seawater.

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