Zerovalent irons: styles of corrosion and inorganic control on hydrogen pressure buildup

Apparent corrosion rates have been measured for several commercially available zerovalent irons by monitoring hydrogen evolution in closed cells. Sievert-type rate constants (ks) were determined to account for hydrogen entering the iron lattice. Thus corrected corrosion rates (Rcorr) are provided for all irons tested in this study. Because the rate of hydrogen entering the iron lattice increases with PH2(1/2), and the rate of hydrogen production from corrosion, far from equilibrium conditions, is independent of PH2, at some time under closed system conditions the two rates become equal and a steady-state PH2 is attained. A relation describing this condition has been derived: PH2SS = [Rcorr/ ks]2 For the granular irons considered in this study, PH2SS values vary from less than one to eight bars, in contrast to the calculated thermodynamic equilibrium PH2 values for anaerobic corrosion, which range from 138 to 631 bar depending on the assumed product of corrosion. Because groundwater flow at an iron permeable reactive barrier removes hydrogen gas in the dissolved state, PH2SS values will be less than calculated using the relation above. A method is presented to calculate PH2 values along the flow direction in a PRB, and thus the maximum PH2 value that can possibly develop, assuming no bacterial utilization of the produced hydrogen.