Packed powder electrodes for characterizing the reactivity of granular iron in borate solutions

Electrochemical aspects of the corrosion of iron metal have been studied using polished disk, wire, and iron coupon electrodes, but these model systems do not represent many characteristics of the granular iron used in environmental remediation applications. To address this issue, we have modified a rotating disk electrode with a cavity that accommodates a wide range of iron powders. By comparison with conventional Fe⁰ and Pt⁰ disk electrodes, we found that our powder disk electrodes (PDEs) packed with unpretreated, < 147 μm granular iron give anodic polarization curves that are unaffected by the underlying disk material and are consistent with a large electroactive surface area of iron that is initially coated with an air-formed passive film. Thus, we believe this electrode design will allow us to begin electrochemical studies of the reduction of aqueous environmental contaminants by relevant iron powders. In preparation for this, we report here on some of the experimental factors that effect response of an iron PDE in pH 8.4 borate buffer. Cavity size and rotation rate have synergistic effects that suggest that most of the iron powder is electroactive, hydrogen evolution in the active region is kinetically limited, and iron dissolution in the active region is affected by mass transport of solutes out of the cavity pore space and by the formation of a passivating film.