The sulphation of mild steel in 70 per cent sulphuric acid*

Abstract

Current-decay curves at constant potential taken on freshly prepared etched surfaces have been analysed to provide information on the development of the sulphate films formed at potentials below +0·60 V. The decay is divided into three sections. The first, which is rapid and culminates in an abrupt fall in current, is attributed to the formation near the metal of supersaturated solution, from which rapid crystallization of ferrous sulphate occurs to form a loosely adherent layer permeated by saturated solution. The second stage includes a step in which the current increases or decreases slowly before a more rapid decay begins, and is attributed to formation of a compact film under the looser film. This film develops by three-dimensional growth of nuclei that are approximately cylindrical in form and are nucleated simultaneously. When the compact film becomes sufficiently thick, parabolic decay begins, the current then being controlled by ionic conduction through the compact film, and a steady state is reached when the rate of film thickening due to the current passing through the film equals the rate at which the outer surface of the film dissolves. At potentials below +0·4 V the outer film formed initially is modified, probably by recrystallization; the properties of the final appear to be identical with those of the film formed initially at higher potentials.The picture of a thin compact film underlying a substantial loosely crystalline film is rather different from the classical picture of the sulphate films formed in dilute sulphuric acid. Those films were thought to be porous, and to provide purely mechanical protection by obscuring much of the surface.