ANAEROBIC CORROSION OF GRANULAR IRON - MEASUREMENT AND INTERPRETATION OF HYDROGEN EVOLUTION RATES

Anaerobic corrosion of iron metal produces Fe2+, OH-, and H-2(g). Growing interest in the use of granular iron in groundwater remediation demands accurate corrosion rates to assess impacts on groundwater chemical composition. In this study, corrosion rates are measured by monitoring the hydrogen pressure increase in sealed cells containing iron granules and water. The principal interference is hydrogen entry and entrapment by the iron. The entry rate is described by Sievert's law (R = kP(H2)(0.5)), and the rate constant, k, is evaluated by reducing the cell pressure once during a test. For the 10-32 mesh iron used in this study, k initially was 0.015 but decreased to 0.009 mmol kg(-1) d(-1) kPa(-0.5) in 150 d. The corrosion rate in a saline groundwater was 0.7 +/- 0.05 mmol of Fe kg(-1) d(-1) at 25 degrees C-identical under water-saturated or fully-drained conditions. The rate decreased by 50% in 150 d due to alteration product buildup. The first 40-200 h of a corrosion test are characterized by progressively increasing rates of pressure increase. The time before steady-state rates develop depends on the solution composition. Data from this period should be discarded in calculating corrosion rates. Tests on pure sodium salt solutions at identical equivalent concentrations (0.02 equiv/L) show the following anion effect on corrosion rate: HCO3 > SO42- > Cl-. For NaCl solutions, corrosion rates decrease from 0.02 to 3.0 m.