Effect of Gelatin on the Pitting Corrosion of Iron in Acid and Alkaline Solutions

Abstract

Potentiostatic polarisation curves for iron were recorded in dilute H₂SO₄ solution in the presence of increasing concentrations of Cl⁻ ion. The aggressive agent raised the active dissolution current density, and developed pits over the entire passive region. The passivation current density steadily increased with the increase of the Cl⁻ content, and finally reached the active dissolution value. Small additions of gelatin accelerated the active dissolution of the metal. This was explained by taking both the kinetics of dissolution of iron and the catalytic properties of gelatin in acid solutions into account. The same concentrations of additive greatly reduced the tendency of the metal to undergo pitting. Larger concentrations of additive produced reverse effects, namely, a decrease in current along the active region and an increase along the passive region. This was correlated with surface blocking and an increased ratio of cathodic! anodic areas on the surface of the metal.

Pitting corrosion of iron in alkaline solutions occurred at definite potentials dependent upon the Cl⁻ content of the medium. Similar to the behaviour in acid solutions, gelatin in small concentrations was effective in retarding or preventing pit formation. Larger concentrations of additive promoted localised attack, though to a lesser extent than in its absence.

Galvanostatic polarisation experiments with the iron electrode were similarly carried out in the two solutions, in the presence of varying concentrations of the aggressive ion. Pitting corrosion developed at definite potentials. These shifted noticeably in the noble direction on the addition of small amounts of gelatin. Larger concentrations of the additive were, however, less effective in reducing pitting attack.