Repassivation Potentials for Long-Term Life Prediction of Localized Corrosion
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REPASSIVATION POTENTIALS FOR LONG-TERM LIFE PREDICTION OF LOCALIZED CORROSION NARASI SRIDHAR AND GUSTAVO CRAGNOLINO Center for Nuclear Waste Regulatory Analyses, 6220 Culebra Road San Antonio, TX 78228 ABSTRACT The effect of pit growth on repassivation potentials (Ep) of type 316L stainless steel (SS) and alloy 825 is investigated using a decreasing potential staircase technique. The EP decreases initially with increasing pit depth and then attains a value which is relatively independent of pit depth. The ET also decreases with increasing potential scan rate because of the decreasing time for repassivation with decreasing potential. The EP,is explained in terms of the effect of applied potential on changes in solution composition inside growing pits and its use is recommended as a bounding parameter for long-term prediction of localized corrosion. INTRODUCTION The effects of environmental factors - anionic species present in the groundwater and temperature - on localized corrosion of type 316L SS and alloy 825 have been examined using short-term electrochemical tests to measure the critical pitting potential (E,) and the repassivation potential Er [1,2]. Analogous parameters have been used to characterize the crevice corrosion behavior of these alloys [3]. The importance of these critical potentials is the assumption that localized corrosion can initiate and propagate if the corrosion potential of an alloy in a natural environment exceeds E, and E,, respectively. However, if the corrosion potential is lower than the E,• for an alloy, the alloy will remain passive in that environment. A critical issue in the measurement and use of E1, as a bounding parameter is its dependence on the extent of prior
pit/crevice corrosion.
EXPERIMENTAL PROCEDURES The alloys examined were type 316LSS (17%Cr-10%Ni-2.1 %Mo-Balance Fe), alloy 825 (31 %Fe-42 %Ni-22%Cr-3.2 %Mo- 1.8%Cu),andalloyC-22 (3.8%Fe-21.4%Cr-13.6%Mo-3%WBalance Ni). The solution consisted of 85 ppm HCO3 , 1000 ppm Cl, 20 ppm SO42 , 10 ppm NO 3-, and 2 ppm F, all added as Na salts, and it was deaerated with Nitrogen [1]. The initial pH at room temperature was approximately 8, while the pH after testing was approximately 9. Electrochemical measurements were conducted using a decreasing potential staircase technique at 95 ± 2°C. Cylindrical specimens were partially immersed in the solution to avoid crevice corrosion in the specimen-holder interface. The specimens were maintained at the opencircuit potential for 1 hour, then polarized to a potential above the expected pit nucleation potential, EP, for different times during which pits grew to varying extent as monitored by the charge passed. The potential was then reduced in successive stages, using a scan rate of either 0.05 mV/sec (10mV every 3 minutes) or approximately 700 mV/sec, until the current density reached a low value (10' - 106 A/cm2 ). A typical potential and current density versus time plot is shown in Figure 1.
Mat. Res. Soc. Symp. Proc. Vol. 294. ©1993 Materials Research Society
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All specimens were
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