Electrochemical potentiokinetic

The electrochemical potentiokinetic reactivation (EPR) test was proposed by Cihal etal. and developed by Novak and others as a fast, quantitative and non-destructive technique for establishing the degree of sensitisation of austenitic stainless steels. 

Fig- 7.65 Schematic EPR (electrochemical potentiokinetic reactivation) curves for three amounts of sensitization of an austenitic stainless steel. Passive film formed at (1). Downscans pass through maximum attack at (2). Environment 1 N H2S04 + 0.01 M KSCN at 30 °C. Curve (3) is observed if passive film continues to form on downscan. Source Ref 93... 

M.K. Ahn, H.S. Kwon, J.H. Lee, Predicting susceptibility of AUoy 600 to intergranular stress corrosion cracking using a modified electrochemical potentiokinetic reactivation test. Corrosion 51 (1995) 441-449. 

The authors found that the in situ electrochemical potentiokinetic reactivation (EPR) data obtained for the monocation I" " showed that the two Mn centers had identical spin densities, and hence the cation showed delocalized mixed valence the near-infrared spectrum supported this postulate. If such a delocalized system existed on the infrared timescale, then two bands would be expected in the carbonyl region of the infrared spectrum of 1+ at frequencies intermediate between those of the Mn(I)/Mn(I) neutral complex, (1861 and 1934 cm ), and those of the fully oxidized Mn(II)/Mn(II) species, which were predicted to be ca. 1966 and 2048 cm on the basis of the bands observed for CpMn(II)(CO)2PPh3 +, (Cp = cyclopentadienyl). In contrast, four intense CO absorptions were observed near 1888, 1931, 1952, and 2003 cm , which were typical of the trapped valent Mn(II)/Mn(I) species. The variation in the response obtained on the EPR timescale, ca. 10 s, to that observed on the infrared timescale, ca. 10 s, was taken by the authors as implying a time-dependent localization process. 

The failures by SCC of welded Type 304 piping exposed to water at 289 C with 0.2-8 ppm of oxygen on boiling water nuclear power plants created a demand for a nondestructive method for detecting quantitatively the degrees of sensitization in these pipes. A nondestructive method was needed so that measurements could be made directly on the heat-affected zones of pipes on the plants. For this purpose, Clarke et al. [20] selected and applied the electrochemical potentiokinetic reactivation (EPR) technique. A cell was designed that could be attached to pipes. 

Kearns, J. R., draft of new Standard Test Methods for Single and Double Electrochemical Potentiokinetic Reactivation (EPR) for Detecting Sensitization of AISI Type 304 and 304L Stainless Steels, ASTM Subcommittee G01.08, September 1992. 

H2SO4 solution containing 0.01 M KSCN by electrochemical potentiokinetic reactivation and a SVET. An attempt was made to relate the electrochemical behavior to the chromium concentration in the iron-rich region formed by ageing, which was determined by a laser atom probe. 

As mentioned before, austenitic stainless steels are susceptible to IGC due to sensitization caused by exposure to high temperatures (450-850 C). The IGC of austenitic stainless steel can also be characterized by normalized classical tests ASTM G28, ASTM A262-86, SEP 1877, AFNOR A05-159 and AFNOR A05-160, currently known as the Strauss, Huey and Streicher tests [54-57]. These methods however are destructive, difficult to perform on site and require sampling that can be harmful to the integrity of materials during service. For this reason, the electrochemical, non-destructive tests commonly known as EPR (electrochemical potentiokinetic reactivation) and DL-EPR (double loop electrochemical potentiokinetic reactivation) were developed to measure the sensitivity of austenitic stainless steels to IGC [58-66]. However, EPR and DL-EPR are based on measurements of characteristic potentials and currents of passive/active zones on potentiody-namic curves in an aqueous solution (linear voltammetry curve from oxygen to hydrogen evolution in the... 

As discussed previously, n is fundamentally related to the crack tip environment (pH, potential, anionic activity) and material (chromium denudation at grain boundaries) conditions. For practical use, n has been reformulated [1] in terms of measurable system parameters such as anionic activity (or solution conductivity, k), corrosion potential (( )j., which, in turn, is a fimction of the dissolved oxygen, hydrogen peroxide, and dissolved hydrogen concentrations), and the electrochemical potentiokinetic repassivation (EPR) parameter (which is related to the chromium denudation in the grain boundary). The formulation is of the form (Fig. 12)... 

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