Detection and differentiation between cracking processes based on electrochemical and mechanical measurements

Abstract

Electrochemical voltage and current noise were measured during slow-rate-load-tensile (SRLT) tests on sensitized austenitic stainless steel specimens in a diluted aqueous solution of sodium thiocyanate. Elongation of tensile test specimen was measured also. It has been found that cracking processes caused simultaneous transients of electrochemical voltage and current. Two different types of electrochemical noise transients were observed: sharp transients of short duration (Type I), and smooth fluctuations that lasted over a much longer period (Type II). Based on the expected cracking mechanism (the slip-dissolution model) and visual observations, it is believed that the sharp transients were generated by the propagation of cracks by mechanical fractures of material ligaments, whereas the smooth fluctuations were generated by propagation of crack via intergranular dissolution. Additional analysis of current transients was performed, and the ratio between the current amplitudes and the charge assessed. Both types of transients were distinguished from the results of this analysis. Based on that, it can be assumed that during the cracking processes which generate Type I transients only the surface of the specimen reacts, whereas in the case of Type II transients the reactions occur inside the specimen.