Modes of Scale Growth on Grey Cast Iron,in Oxygenated Water Circulation Systems

Abstract

A characteristic mode of scale growth on cast iron, in flowing water has been reproduced. in short-term studies (72 h) under controlled, oxygenated conditions at 50°C. Despite the formation, after a short time, of an apparently protective ‘crusted’ scale, precipitation of Fe₃O₄ and a Green Rust (GR) within the scale continues to the end of the periods studied. This is explained in terms of an established corrosion modelfor iron. The iron Surface becomes pitted, while accumulation of precipitate in the region of pits leads to the growth of ‘chimney’ features aut of the scale. The internal structures of chimneys, both from controlled experiments and from corroded water pumps, have been studied by SEM. Three different forms of ferric oxide were found, in mixtures of varying proportions, on different water pumps. Certain scales, containing CaCO₃, are particularly adherent but do not seem to be protective against further pitting of the iron substrate.