Pitting Corrosion of Zinc in Aqueous Solutions: III. Effect of anode to cathode surface area ratio on pitting corrosion currents

Abstract

The technique of measuring pitting corrosion currents flowing under natural conditions has been extended to establish the effect of cathode and anode surface areas on the magnitude of the current. Zinc electrodes of various areas were short-circuited through a zero-resistance ammeter in a two-compartment cellfilled with 0·1 M K₂CrO₄. After the zero currents were measured, enough KCl was added to the anode compartment to initiate pitting attack. The variation of the pitting current with time was followed to the steady-state. The steady-state current changed linearly with cathode area, and the curve relating them to one another passed through the origin. This represents the first genuine experimental proof of the dependence of a partial current on the available area in an electrochemical rate equation. The variation of the steady-state corrosion current with anode area is also linear, but the curve does not pass through the origin. The curve represents the increased probability of attack as the anode area is enlarged.

The pitting attack on Zn can be suppressed by additions of K₂SO_4· or KNO_3· At equimolar concentrations, KNO₃ reduced the attack more than did K₂SO_4· Both additive anions are assumed to bespecifically adsorbed on the surface of the metal and to compete with Cl⁻ ionsfor active sites.

Measuring pitting corrosion currents in the manner described in the present investigation offers a simple and rapid method for evaluating and comparing pitting corrosion inhibitors.