Role of the Anion in relation to Metallic Corrosion and Inhibition

Abstract

WHEN metals, for example, mild steel, corrode in aqueous solution it is commonly accepted that certain electrolytes, notably chlorides, sulphates, etc., are corrosive, whereas other electrolytes, such as chromates, silicates, benzoates, are inhibitive. In this communication the claim is made that electrolytes cannot thus be rigidly classified, and that (with the exception of certain reducing salts such as formates) ‘corrosive’ or ‘inhibitive’ properties are a function of the concentration of the solution. All electrolytes are corrosive in solutions of sufficiently great dilution, and in conditions of high aeration corrosion-rates and corroding potentials obey a common relationship, as will be described here. On the other hand, at high concentrations the normally aggressive nitrates become inhibitive, while chlorides and possibly even sulphates show a tendency towards inhibition.