Electrochemical and Weight Loss Measurements of Inhibition of the Acid Corrosion of Steel

Abstract

Stern-Geary and Tafel polarisation relationships have been obtained for steel in 0·5 M H₂SO₄ and 1 M HCl containing different concentrations of a wide range of n-alkylquaternary ammonium compounds and thiourea derivatives. The shape of the Stern-Geary polarisation relationship is found to depend on the difference between the anodic and cathodic Tafel slopes, b_a and b_c, in agreement with electrode kinetic theory.

For each solution the corrosion rate was derived from the electrochemical data in two different ways:

1. Substitution of the Stern–Geary gradient, ∆I/∆E, and the measured values of b_a and b_c in the Stern-Geary equation: II. Extrapolation of the Tafel lines to the point of intersection. The results from the two methods were in good agreement for all of the solutions tested.The corrosion inhibition values calculated from the Stern-Geary corrosion rates were also compared with those measured in separate experiments by weight loss. The agreement was good except for the n-alkylquaternary ammonium compounds in 0·5 M H₂SO₄, which gave considerable discrepancies at the lower levels of inhibition. This was shown to be due to differences in the pre-immersion treatment of the steel samples.