Spatial distribution of ph at mild steel surfaces using an iridium oxide microelectrode

Abstract

The distribution of pH near a metal surface indicates the positions of anodic (low pH) and cathodic sites (high pH). A microsensor, small enough that the pH sensing tip is confined to the diffusion layer, can be used to monitor pH near metal surfaces. This paper describes the mapping of pH near water-immersed mild steel surfaces using miniaturized iridium/iridium oxide pH microelectrodes in conjunction with a computer controlled micropositioner and data acquisition system. Two systems were analyzed: (1) a bare mild steel coupon exposed to artificial sea water, and (2) a mild steel coupon, first partially covered with the biopolymer, calcium alginate, and then exposed to artificial seawater. After 8 h exposure to seawater both coupons exhibited localized corrosion. On the coupon partially covered with calcium alginate gel, corrosion was limited to the area covered by biopolymer. On the bare coupon, corrosion was widespread. pH mapping of the coupons showed that low pH regions were identified with the corroded areas, and high pH regions with the uncorroded areas. These observations demonstrate that, in the abiotic environment, anodic sites on a mild steel surface can be fixed by partially covering the metal with biopolymer.