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An early stage of crevice corrosion of an epoxy coated sample of pure aluminium, a stainless steel, and low carbon steel in 1M, KCl, 1M NaCl, and 1M NaOH solutions, respectively, was investigated in situ by a new method. The new method of crevice corrosion detection is based on the optical corrosion-meter for measuring the corrosion current density and on a modified electrochemical noise technique for determining the corrosion admittance at the open circuit potential of the alloys in solutions. The observations of crevice corrosion were basically interferometric perturbations detected by the optical corrosion-meter only on the coated steel and the stainless steel samples in 1 M NaOH and 1M NaCl solutions, respectively. The interferometric perturbations interpreted as a localized corrosion underneath the coating in a form of an early crevice, of a depth ranged between 0.3 micrometers to several micrometers. Also, the early stage of crevice corrosion of the same alloys in same solutions was determined in situ by a modified electrochemical noise technique, called the modified electrochemical emission spectroscopy (MEES) technique, simultaneously during the optical interferometry measurements. Determinations of crevice corrosion by the MEES technique were electrochemical noise spectra detected on corrosion admittance (Ac)-time plots of the alloys in solutions. The corrosion admittance parameter, Ac equals (dJ/dV), which defined the MEES technique, is capable of indicating localized corrosion activities. In this investigation, the parameter Ac was modified in which that the change of the corrosion current density was measured by the optical corrosion-meter rather than by the zero resistance Ammeter, which is usually used for measuring the dJ in electrochemical noise technique. Consequently, result of the present work indicate that optical corrosion-meter as an electromagnetic method of measuring the corrosion current density, and MEES technique, as an electronic method for determining the Ac, are very useful techniques as non- destructive methods for detection of crevice corrosion at the initiation stage of the phenomenon.
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