NDT technique for detecting 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. The new method of localized corrosion detection is based on the optical corrosion-meter for measuring the corrosion current density (J) and on a modified electrochemical noise technique for determining the corrosion admittance (Ac) at the open circuit potential of the alloys in solutions. The observations 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 perturbations interpreted as a localized corrosion in a form of an early pitting and crevice corrosion, of a depth ranged between 0.3 µm to several micrometers, of the uncoated copper and the coated brass, respectively. Also, the early stage of localized corrosion of the same alloys in same conditions was determined in situ by a modified electrochemical noise (EN) technique, called the modified electrochemical emission spectroscopy (MEES) technique, simultaneously during the optical interferometry measurements. Determinations of localized 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 = (dJ/dV), which defined the MEES technique, is capable of indicating localized corrosion and uniform corrosion activities. In this investigation, the parameter Ac was modified in which that the change of the corrosion current density (dJ) 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, results 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 localized corrosion at the initiation stage of the phenomenon.

The following conclusions are drawn from the present investigation: 1. The new method is found to be a very useful technique for quantitatively distinguishing of localized corrosion and uniform corrosion of alloys in aqueous solutions. 2. The new method of detection of pitting corrosion is based on incorporating optical interferometry and modified electrochemical noise techniques. 3. The corrosion potential, corrosion current density (J), and the corrosion admittance (Ac) were directly measured without any physical contact for copper and brass in uncoated and coated conditions as a function of the elapsed time of the experiment. 4. Comparison between the corrosion activities from the Ac spectra of the two alloys, Figs. 4a,b and Figs. 7a,b, indicates that the uncoated copper (Ac = −7) and the coated brass (−1) were found to suffer from pitting corrosion and crevice corrosion, respectively, at the beginning of the tests. Then both samples were observed to passivate (Ac = 0) during the rest of the test. 5. On the other hand, the coated copper (Ac = 1.5) and uncoated brass (Ac = 10) were found to suffer uniform attack at the beginning of the test. Then both samples were observed to passivate (Ac = 0) during the rest of the test. 6. Values of J and Ac of the two alloys correspond relatively well with the corrosion potential even though the corrosion potential and the J and Ac values were measured by different techniques.