TP17-02: Can Rusted Surfaces be Effectively Passivated to Reduce Further Corrosion

The industry standard corrosion monitoring practice for cooling water systems involves inserting a clean, pre-weighed, metal specimen into a “coupon rack” and exposing it to the flowing water for a period of time, generally 30-90 days. Following exposure, the coupon is removed from the system, carefully cleaned, and reweighed. The weight loss is then related to an overall loss of metal thickness, which is typically expressed in terms of mils per year (mpy) or mm per year. Detailed procedures for conducting these tests are contained in CTI Std-149, ASTM D2688, and ASTM G1.

Although these coupon tests are very useful in evaluating the effectiveness of corrosion inhibitors on clean metal surfaces, they provide no direct information about the effectiveness of the corrosion inhibitor on metal surfaces that have developed a surface layer of rust and oxidation products after several months or years of service. The corrosion inhibitor must penetrate the oxidation layers and reach the underlying base metal in order to effectively arrest corrosion. The standard coupon weight loss method cannot provide insight into the passivation of rusted surfaces because it is not possible to determine the initial weight of the non-corroded base metal on a rusted coupon.

The ability to protect rusted surfaces is of critical importance because a corrosion inhibitor rarely encounters a truly clean metal surface, except perhaps immediately after a chemical cleaning. In the study presented in this paper, advanced electrochemical methods including electrochemical impedance spectroscopy (EIS), cyclic polarization (CP), and open circuit potential (OCP) were used to evaluate the effectiveness of corrosion inhibitors in passivating rusted surfaces. The relative performance of several traditional and advanced corrosion inhibitors is provided together with a mechanistic discussion.