Corrosion resistance of Yucca Mountain (YM) carbon steel rock bolt (0.44%C) and I-beam steel sets (0.08%C) towards electrochemical/general corrosion, stress corrosion cracking (SCC), and hydrogen embrittlement (HE) susceptibility has been determined using simulated YM waters in this Task 18 for YM site underground repository tunnel support. These findings show the corrosion behavior of these structural support members with and without applied stresses in simulated YM environments. The simulated YM waters were increased in nominal concentrations from Ix to lOOx (three different levels) for electrochemical and other corrosion studies. The effect of temperature, of significance to YM repository, on the corrosion behavior are presented using electrochemical as well as electro-mechanical studies. Conventional ASTM general corrosion tests, such as (ASTM G-85 YM water spray Fog tests), ASTM G-31 (Immersion Tests), ASTM G-85 (modified YM water Spray), ASTM G-60 (Cyclic Humidity) tests were performed. Electrochemical corrosion tests include potentiodynamic tests and impedance spectroscopy, stress corrosion cracking, hydrogen cracking. In addition, effect isolation of ions such as HCO"3, SiO" 2, that play critical role in affecting the corrosion rates of steels, have been determined. Microstructural and phase characterization of rock bolt and I-beam materials were performed by using scanning electron microscopy (SEM), xray photoelectron microscopy (XPS), and x-ray diffraction (XRD) analyses. In the present study, and we have measured higher corrosion rates for the (0.08%C) low carbon steel I-beam as compared to higher carbon (0.44%C) rock bolts; normally high carbon steels will undergo more corrosion than low carbon steels. It postulated that it may be due to: (a) Environmental effects due to the electrochemical reactions in this YM (electrolyte) in which dissolution as well as deposition of extrinsic ions are dynamic processes that make the corrosion process more complex, (b) Uniform distribution of carbides in rock bolts due increased Pearlite areas, as compared to the I-beams that have much smaller amounts of Pearlite. The variation of corrosion rates (CR) of rock bolts and I-beam as a function of temperature (between 25° and 85°C) is very interesting; these have been performed using de-aerated and aerated conditions showing lower and upper CR limits. A striking result is that the corrosion rate of rock bolts and I-beam increased with temperature from 25° to 45°C, thereafter there is a decrease in corrosion rate at higher temperatures for the lOOx YM waters (65° and 85°C). This CR maxima appears around 45°C for both these steels, in most cases, particularly, lOOx (nominal) YM waters. This can be explained taking into account the following competing factors: (1) temperature and ionic concentration generally increase the CR due to increased reaction rates, and on the other hand (2) dissolved oxygen effects decrease as the temperature is increased; it appears that the convergence occurs around 45°C after which the corrosion rates start to decrease. The decrease in CR at higher temperatures can be mainly attributed to the decrease in oxygen solubility and the adsorption of ionic species such as Mg and Si providing resistance to further corrosion, as determined by XPS analyses. The corrosion rates obtained for the rock bolts from polarization tests using deaerated and aerated electrolytes are plotted in Figure l(a). These plots show that in deaerated condition the corrosion rates are significantly low as compared to the oxygenated electrolytes; thus these deaerated conditions show lower corrosion limits. However, in the case of rock bolts, at Ix and lOx the corrosion rate increased with temperature up to 85°C (Figure (la)). In the case of I-beams, this trend in the decrease in corrosion rates was observed at all concentration levels [1, 10, and lOOx in Figure l(b)]. Data points, for example, CH-887 indicates cyclic humidity corrosion rates of 887 urn/year, and other general or immersion corrosion tests have been incorporated in Figures 1 (a and b) and show a reasonable agreement with the electrochemical results.
Carbon steel –Corrosion; Nevada – Yucca Mountain; Radioactive waste repositories; Rock bolts – Corrosion;
Materials Science and Engineering | Metallurgy | Structural Materials
Daemen, J. J.,
Smiecinski, A. J.
Corrosion research on rock bolts and steel sets for sub-surface reinforcement of the Yucca Mountain repository.
Available at: http://digitalscholarship.unlv.edu/yucca_mtn_pubs/73