Waters concentrated in the solutes present in the J-13 well were used to simulate the effects of boiling and evaporation in the vadose water surrounding waste packages in the proposed Yucca Mountain geologic nuclear waste storage site. The corrosion performance of AISI 1016 low carbon steel (LCS) in these solutions was evaluated by DC polarization techniques. LCS showed low general corrosion rates with no localized corrosion in deaerated 10X, 100X and 1000X J-13 waters at 25°C. Higher water temperature up to 90°C and 100X and 1000X concentrations favored protective silicate precipitate layers which further decreased corrosion rates and significantly increased apparent pitting potentials. Corrosion rate increased significantly in aerated 10X J-13 water at 25°C but had lesser effects in 100X and 1000X. Aeration and chloride additions also significantly degraded the protectiveness of the inhibitive precipitate surface films. Addition of soluble Fes+ in aerated 10X and 100X J-13 waters, increased general corrosion at 25°C and 10X concentration and initiated pitting at higher concentrations and temperatures.
Carbon steel has been seriously considered as the corrosion allowance outer barrier material in recent multi-barrier waste package designs for the Yucca Mountain (YM) high level nuclear waste (HLNW) geologic repository northwest of Las Vegas, Nevada. However, most studies have been devoted to the corrosion-resistant inner-barrier alloys, such as stainless steels, nickel-chromium, and nickel-copper alloys.l,2,3,4 No results have been reported on the corrosion behavior of outer-barrier low carbon steel (LCS) in any simulated repository environment.
The purpose of this investigation was to evaluate the effects of accumulated solute salts and corrosion-product Fes+ ions in simulated YM vadose water, which is expected to reach nuclear waste containers during the repository period. Currently, water from the J-13 well at a location several miles adjacent to the proposed repository has been chosen as the reference ground water for initial YM studies.5 The chemical composition of J-13 water (Table 1) at a neutral pH indicates low corrosivity. However, nuclear decay processes in the wastes are expected to maintain container surfaces and nearby surrounding rock well above water boiling temperature for several centuries. The concentration of dissolved solutes during evaporative boiling may substantially increase the corrosivity of the vadose water. Ferric ion, Fes+, is also of interest, because it may have an effect on water corrosivity when formed by normal chemical aqueous oxidation of ferrous Fe 2+ ions by dissolved oxygen, or by the metabolic activities of iron-oxidizing bacteria. Fes+ may act as a potent oxidizer to accelerate general or localized pitting corrosion of carbon steel.
EXPERIMENTAL PROCEDURES
Specimen Preparation
AISI 1016 low carbon steel (LCS) was received in the form of 6.3 mm (l/4 inch) diameter steel rod from USX Corp., Monroeville, PA. The chemical composition of the LCS is given in Table 2. Cylindrical specimens 12.5 mm (l/2 inch) long were cut from the rod stock. When mounted conventionally at the end of a specimen holder677 with a water-tight PTFE compression gasket, a surface area of 2.8 cm2 was left exposed.
