Modified electrochemical emission spectroscopy (MEES) As technique of NDT for detection localized corrosion of copper alloys in seawater

An early stage of localized corrosion, i.e., pitting and crevice corrosion, of a pure copper and a brass in uncoated and coated conditions in seawater was investigated in situ by a new non-destructive testing (NDT) method. Th e new method of localized corrosion detection is based on the optical corro sion-meter for measuring the corrosion current density (J) and on a modifie d electrochemical noise technique for determining the corrosion admittance (A(c)) at the open-circuit potential of the alloys in solutions. The observ ations of localized corrosion were basically interferometric perturbations detected by the optical corrosion-meter only on the uncoated copper and the coated brass at the beginning of the tests. The interferometic perturbatio ns interpreted as a localized corrosion in the form of an early pitting and crevice corrosion, of a depth ranging from 0.3 mu m to several micrometers , of the uncoated copper and the coated brass, respectively. Also, the earl y stage of localized corrosion of the same alloys in the same conditions wa s determined in situ by a modified electrochemical noise (EN) technique, ca lled the modified electrochemical emission spectroscopy (MEES) technique, s imultaneously during the optical interferometry measurements. Determination s of localized corrosion by the MEES technique were electrochemical noise s pectra detected on corrosion admittance (A(c))-time plots of the alloys in solutions. The corrosion admittance parameter, A(c) = (dJ/dV), which define d the MEES technique, is capable of indicating localized corrosion and unif orm corrosion activities. In this investigation, the parameter A(c) was mod ified in which the change of the corrosion current density (dJ) was measure d by the optical corrosion-meter rather than by the zero resistance ammeter , which is usually used for measuring the dJ in electrochemical noise techn ique. Consequently, results of the present work indicate that optical corro sion-meter as an electromagnetic method of measuring the corrosion current density, and MEES technique, as an electronic method for determining the A( c), are very useful techniques as non-destructive methods for the detection of localized corrosion at the initiation stage of the phenomenon. (C) 2000 Elsevier Science Ltd. All rights reserved.