Development of Integrated Mechanistically-Based Degradation-Mode Models for Performance Assessment of High-Level Waste C
- PDF / 527,760 Bytes
- 8 Pages / 386.64 x 620.1 pts Page_size
- 36 Downloads / 201 Views
(and other anions) into crevice must occur to balance cationic charge associated with H* ions [1-3]. The exacerbated conditions inside the crevice set the stage for subsequent attack of the corrosion resistant material (CRM) by passive corrosion, pitting (initiation & propagation), stress corrosion cracking (initiation & propagation), or other mechanisms. Clearly, the development of an adequate crevice corrosion model for determination of the exact nature of the local environment is prudent. Alloy 22 has several desirable attributes that make it an attractive candidate for fabrication of high-level waste containers. It is less susceptible to localized corrosion (LC) in environments that contain Cl than Alloys 825 and 625, that were leading candidates in earlier designs [4]. The unusual LC resistance of Alloy 22 is believed to be due to the additions of both Mo and W, which should stabilize the passive film at very low pH [5]. In regard to the addition of tungsten to Alloy 22, note that W0 3 appears to be less soluble at low pH than MoO 2 and MoO 3. This material therefore exhibits a very high repassivation potential, approaching the potential required for 02 evolution [6]. The repassivation potential is believed to be the threshold for initiation of LC [6]. In experiments with simulated crevice solutions (10 wt. % FeCI3 ), very low (passive) corrosion rates are observed [7,8]. Finally, no significant localized attack of Alloy 22 was observed in crevices exposed to simulated acidified water (SAW) for one year. In essence, the SAW medium
855 Mat. Res. Soc. Symp. Proc. Vol. 556 © 1999 Materials Research Society
is about one-thousand times more concentrated than the ground water at Yucca Mountain (J-13 well water) and has been acidified to a pH of approximately 2.7. These tests were conducted in the Long Term Corrosion Test Facility (LTCTF) at Lawrence Livermore National Laboratory (LLNL) [9]. However, we have now observed the crevice corrosion of Alloy 22 during anodic polarization at very high potentials (1.2 V vs. SCE) and ambient temperature in electrolytes saturated with chloride salts, as reported here. A variety of research is being conducted at LLNL, directed towards possible degradation modes of Alloy 22. Corrosion modeling for Total System Performance Assessment (TSPA) is a key component of this work. Models include simple correlations of experimental data [9], as well as detailed mechanistic models necessary for believable long-term predictions [4-5]. Several interactive modes of corrosion are possible and have made it necessary to develop several models that ultimately should be integrated. These process-level models include those to account for the inhibition of corrosion by protective ceramic coatings, pH decreases and CI increases in crevices, pit initiation and propagation, as well as stress corrosion and hydrogen-induced cracking. Ceramic coatings are now being considered for both corrosion allowance and corrosion resistant materials. This publication addresses the development of models to account f
Data Loading...
