ANOXIC CORROSION OF STEEL AND LEAD IN NA-CL±MG-DOMINATED BRINES IN ATMOSPHERES CONTAINING CO2

The Waste Isolation Pilot Plant (WIPP) is a U.S. Department of Energy geologic repository for disposal of transuranic radioactive waste in bedded salt (Permian Salado Fm.). Postclosure conditions at WIPP will control the speciation and solubility of radionuclides. Microbial consumption cellulose, plastic and rubber in the waste might produce CO₂. Therefore, to prevent acidification of any brine present the DOE emplaces MgO in the repository to buffer ¦_CO2 and pH within ranges favoring lower actinide solubilities. Large quantities of steel and Pb in the WIPP may also consume CO₂.

We present results from experiments investigating the corrosion of steel and Pb under WIPP-relevant conditions. The objective is to determine the extent that these alloys consume CO₂ via the formation of carbonates or other phases. Steel and Pb coupons are immersed in brines under WIPP-relevant conditions using a continuous gas flow-through system. The system maintains the following conditions: pO₂ < 5 ppm; temperature of 26 °C; relative humidity at 78% ± 10%; and pCO₂ values of 0, 350, 1500 and 3500 ppm (balance N₂). Four high-ionic-strength brines are used: a Na-Mg-Cl-dominated brine; a predominately Na-Cl brine; and both these brines with organic ligands (EDTA, acetate, citrate, and oxalate).

Steel removed after 6 and 12 months shows formation of several phases dependent on the pCO₂. Lead coupons show no corrosion products at lower pCO₂ values but significant formation of a Pb-Ca carbonate phase at pCO₂ > 350 ppm. Mass loss data are used to calculate corrosion rates for each experiment. Steel corrosion rates are a strong function of pCO₂ for all brine types. Na-Cl brines appear to be more corrosive than Na-Mg-Cl brines. Steel corrosion rates vary from 0.13 ± 0.08 mm/yr at 0 ppm CO₂ to 1.06 ± 0.25 mm/yr at 3500 ppm CO₂. Pb corrosion rates show no consistent trend as a function of pCO₂ or brine type. Lead corrosion rates range from 0.32 ± 0.18 to 0.69 ± 0.61 mm/yr.

This research is funded by WIPP programs administered by the Office of Environmental Management (EM) of the U.S Department of Energy. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.