Application of AC-impedance method on determination of corrosion rate of metals

The AC-impedance technique was experimentally evaluated as one of the in-situ measuring means to determine the corrosion rate of metals immersed in aqueous solutions. Surface impedance of corroding metals was measured by both the FET and FRA methods using the system consisting of a personal computer and digital wave memories, and the two methods were compared with respect to accuracy and error factors. The tested systems were stainless steel-sulfric acid, stainless steel-chloric acid, titanium-nitric acid and zirconium-nitric acid. The measurement permitted to estimate a very low corrosion current density of firmly passivated specimens. For the stainless steel corroding actively in chloric acid, weight change of the specimen calculated from its impedance data was coincided within 2.5 % difference with the value determined by direct weighing. This indicates the capability of the impedance technique to determine corrosion rate of sample on the basis of the previously obtained correlation between measured weight change and impedance data of the same metal-solution system as the sample. In other words, on-line sensing of corrosion rate is possible. (author)