Modified electrochemical emission spectroscopy (MEES) as NDT Technique for detection of pitting corrosion of metallic alloys in aqueous solutions

The early stages of pitting corrosion of a pure aluminum, a stainless steel, and low carbon steel in 1 M KCl, 1 M NaCl and 1 M NaOH solutions, respectively, were investigated in situ by a new method. The new technique of pitting corrosion detection is based on the optical corrosion-meter for measuring the corrosion current density (J) and on a modified electrochemical noise (EN) technique for determining the corrosion admittance (A c) at the open circuit potential of the alloys in solutions. The observations of pitting were basically interferometric perturbations detected by the optical corrosion-meter only on the stainless steel sample in 1 M NaCl solution. The interferometic perturbations were interpreted as a localized corrosion in a form of an early pitting, of a depth ranged from 0.3 μm to several micrometers. Also, the early stage of pitting corrosion of the same alloys in the same solutions was determined in situ by a modified EN technique, called the modified electrochemical emission spectroscopy (MEES) technique, simultaneously during the optical interferometry measurements. The determination of pitting corrosion by the MEES technique were EN spectra detected on corrosion admittance (A c)-time plots of the alloys in solutions. The corrosion admittance parameter, which defined the MEES technique, is capable of indicating localized corrosion activities. In this investigation, the parameter A c was modified in which the change of the corrosion current density (ΔJ) was measured by the optical corrosion-meter rather than by the zero resistance Ammeter, which is usually used for measuring the ΔJ in EN technique. In other words, the optical interferometry was used, for the first time, as a zero resistance Ammeter for measuring activities of the corrosion current of metallic samples at the open circuit potential of the samples in aqueous solutions. Consequently, results of the present work indicate that the optical corrosion-meter, as an electromagnetic method of measuring the corrosion current density is a very useful technique as a non-destructive method for the detection of pitting corrosion at the initiation stage of the phenomenon.