Degradation and failure susceptibility of carbon steels in simulated Yucca Mountain nuclear repository environments.

摘要

Environmental degradation and cracking of medium carbon steel (MCS) rock bolts and low carbon steel (LCS) I-beam have been investigated by experimental methods such as linear polarization, impedance spectroscopy, weight loss measurements, and electro-mechanical dynamic slow strain rate tensile (SSRT) tests, along with potentiostatic in-situ potential-current monitoring techniques. The experiments were conducted in concentrated aqueous environments of various temperatures, which simulated the conditions at the Yucca Mountain (YM) nuclear waste repository site, where the candidate structural materials introduced above, will be used for supporting the waste repository tunnels.; MCS corroded at medium general rates approximately around 40 μm/year to 200 μm/year in de-aerated simulated YM waters of various temperatures and concentrations. Increased temperatures increased the corrosion rates in the all de-aerated waters. Increased concentrations of overall species in the simulated waters also increased the corrosion rates, but only slightly. Impedance spectroscopy revealed similar trends for temperature and concentration effects on the rates in both aerated and deaerated environments. Aeration increased corrosion rates significantly in dilute (1X) and ten times concentrated (10X) waters at all temperatures. However, inhibitive precipitates on the specimens formed by oxygen-environment reactions at higher temperatures (up to 85°C) in hundred times concentrated (100X) waters decreased corrosion rates drastically, resulting some localized corrosion and pitting. The average rates were determined to be between approximately 100 μ/year and 1000 μ/year in the entire concentration and temperature range tested. Electrochemical results showed slightly higher rates compared to the other tests because of their much shorter testing period, therefore in general they should be taken as conservative upper bounds.; SSRT on LCS under various imposed metal-electrolyte interface potentials revealed strain aging embrittlement (SAE) and hydrogen induced cracking (HIC) at 85°C in simulated de-aerated 100X YM waters. The failures were brittle and of sharp cracks which always inclined in ∼45° to the load axis. Ductile failures along secondary trans-granular cracks around the main cracks occurred at room temperature (25°C) with some anodic and cathodic over-potentials around E_corr, revealing susceptibility of LCS to stress corrosion cracking (SCC) in the simulated repository environments.; SSRT combined with in-situ current and potential monitoring by potentiostatic methods revealed that Portevin LeChatelier (PLC) effect in LCS starts at ∼55°C, and showed precise correlations with open circuit potential E_corr and the corresponding current. Therefore, SSRT along the in-situ potential-current monitoring in aqueous environments becomes a powerful diagnostic research tool especially for investigating dynamic strain aging (Portevin-LeChatelier Effect) phenomena of metals in aqueous environments.