The correlation between molecular structure and tendency to maintain or to destroy iron passivity in aqueous solution—II. Activation effectiveness of anions present together with a corrosion inhibitor and/or an oxidant

Abstract

The relative tendency of 11 anions to destroy iron passivity has been assessed from open circuit potential measurements and/or from corrosion tests with 11 corrosion inhibitors and 4 oxidants. Sulphide, EDTA (sodium salt at pH 7) and sulphate were usually the most effective, and trifluoroacetate the least effective activators with dissolved air as oxidant. Chloride was the most effective in systems, giving higher redox potentials than that produced by dissolved air. Monatomic anions gave an approximate correlation between charge density and activation effectiveness. That sodium EDTA is an activator and strongly bound metal-EDTA complexes are inhibitors appears to be in accordance with the electrostatic hypothesis. There is some support for the electrostatic hypothesis of passivity stabilisation, but complicating factors intervene. The results indicate that these include hydrogen bonding, and the attraction between activator anions and other solution species, both of which tend to prevent adsorption of the anion at the metal-oxide interface.