Measurement of low corrosion rates: Comparison of A.C. impedance and thin layer activation methods

Abstract

The very low corrosion rate (0.05–1 μm per year) of 18Cr-13Ni-1Nb stainless steel in 7.8 M HNO 3 at room temperature was determined with some confidence in a period of a few days by the combination of a.c. impedance and a radiotracer technique, Thin-Layer Activation. Both techniques were taken to their limits. It is argued that the a.c. method gives an upper estimate for the corrosion rate whilst the tracer method gives a lower estimate. Application of the a.c. method requires an estimate of the Tafel parameter relating corrosion rate and polarization resistance: an approximate method, utilizing the variation of the apparent value of the polarization resistance with the amplitude of the a.c. signal, is given in this paper. The corrosion rate decreased steadily with time over the first few days of exposure of the polished steel surface to the acid. Comparison of the results of the two methods suggested that there was a burst of active dissolution within the first few seconds of exposure, and some speculation about causes and consequences of this phenomenon is made.