Anodic Behaviour of Mild Steel in Deaerated Carboxylic Acid Solutions containing NO 2 −,Cl − and NO 3 − Ions

Abstract

The anodic behaviour of mild steel in deaerated solutions of acetic, oxalic, tartaric, malic and citric acids, in the presence and absence of NO₂ ⁻, Cl⁻ and NO₃ ⁻ ions, was studied by the potentiostatic technique. Except for acetic acid containing Cl⁻ or NO₃ ⁻ ions, the corrosion potential in all the acids was shifted in the positive direction in the presence of the anions. The passivating film in pure oxalic acid is Fe₃O₄, in tartaric acid probably the duplex oxide Fe₃O₄/Fe₂O₃, and in the remainder a non-stoichiometric higher iron oxide.

NO₂ ⁻ ions in acetic acid tended to promote passivation, but at 10⁻⁵ – 10⁻³ M they accelerated the dissolution of steel in the other four acids; at 10⁻² M dissolution was inhibited in all acids, and passivation was more readily achieved. 10⁻⁵ and 10⁻⁴ M Cl⁻ ions inhibited the dissolution of steel in acetic, oxalic and tartaric acids, but greatly enhanced it in malic and citric acids. At higher concentrations the passivating film was generally attacked in all acids. Similar behaviour was found in the presence of NO₃ ⁻ ions.