Trends in the Electrochemical Polarization Potentiodynamic Reactivation Method – EPR

This method designed to examine the susceptibility to nonuniform corrosion, ranks among the more successful technique developments. One of its numerous advantages is that it allows nondestructive, on-site examination. EPR measurements are used to establish the resistance of stainless steels and alloys to intergranular corrosion and stress corrosion cracking e.g. in nuclear engineering applications as well as to study grain boundary precipitation and other minute local changes in alloy composition and structure. By the EPR test, the specimen and/or the field object (working electrode) is tested in acid solutions, most often in solutions of sulfuric acid (c = 0.01–5 mol dm–3 H2SO4) and potassium thiocyanate (c = 0.001 to 0.1 mol dm–3 KSCN). The principle of the measurements is to reactivate the sample from the incomplete passivity region. This indicates local changes in chemical composition in relation to phase transformations

Critical Pitting Temperature Measurements for Evaluating Superaustenitic Stainless Steels Resistance to Solutions Modeling Wastewater Treatment of Flue Gas Desulfurization Plants

Critical pitting temperature (CPT) determination by potenciostatic procedure is often used as a method of screening stainless steels in solutions with activity range 0.067 – 4.3. Values of activity of chloride ions as well as pH values for those solutions are quite different from the values of the same parameters in solutions modeling wastewater treatment of flue gas desulfurization plants. In those waters the soluble components, mainly calcium chloride (which must be treated by evaporation and crystallization) can give solutions with pH values below 4and activities of chloride ions much more higher. Thus, even small changes of CaCl2 concentration can give an increase in chloride ions activity. In the work reported here the effect of two different activities of chloride ions was investigated using potenciostatic CPT measurements on superaustenitic stainless steels in c = 5.685 and 5.88 mol L–1 CaCl2 solutions with pH values below 4. The results demonstrated that an increase in the activity of chloride ions causes a decrease in the potentialat which, for given a temperature, the growth of stable pits is possible

Slip-dissolution model of stress corrosion crack growth and possibilities of its application for evaluation of stress corrosion cracking susceptibility of duplex stainless steels in hydrogen sulfide-chloride environments at 120°C

Results are summarized of the feasibility of using slip-dissolution model of stress corrosion crack growth for the evaluation of stress corrosion cracking (SCC) susceptibility of duplex stainless steels in hydrogen sulfide-chloride environments at 120°C. First a brief note is presented about the historical background of the concept of discussion of SCC results received in the constant extension rate test (CERT) of stainless steels in terms of phenomenological model that incorporates a slip-dissolution mechanism and elastic-plastic fracture mechanics, introducing of the average near crack-tip strain rate, ε̄T and average crack growth rate αav as quantities measurable in CERT test has allowed a comparison to existing theoretical model of Ford. This idea has been used in our slow strain rate tests (SSRT) of duplex stainless steels (DSS) in hydrogen sulfide-chloride environments at 120°C. It is shown that experimental values of ε̄T and αav determined in SSRT experiments for several duplex stainless steels in 20%NaCl-H2S (pH=4) at 120°C can be used for calculation of input parameters of the slip-dissolution model for stress-corrosion cracking. It has only been possible due to exact analysis of stress-vs.-strain curves and the determination of supposed value for fracture-strain of oxide film at crack tip, f. (Values of rate constant for the bare surface reaction at the crack tip, k and current decay exponent at the crack tip, m estimated for various partial pressures H2S (ppH2S) made possible the evaluation of changing electrochemical conditions under which cracks occur in various DSS during mentioned SSRT experiments