Applications of Linear Polarization Resistance Method to Corrosion Monitoring of Mild Steel in Cooling Water Systems

Abstract

Laboratory and field tests of corrosion rates of mild steel in cooling water for heat exchangers and reactors were made by the linear polarization resistance methods. In the laboratory tests, intermittent measurements of polarization resistances (R_p) of rod specimens were made with 0.1Hz galvanostatic pulses for solutions with zinc-phosphate inhibitors and an organic inhibitor. Gravimetrical corrosion rates (v_w) were also calculated from weight loss of the specimens at the end of each polarization experiment. The value of K in the equation of corrosion rate (v), v=K/R_p, for each specimen was determined with the measured v_w and the mean value of 1/R_p over the whole period of the polarization test. The average K obtained from values of K for the solutions with 0-200ppm zinc-phosphate inhibitors was used for the estimation of corrosion rates (v_p) in field tests from measured R_p. Mean values of v_p were in a good agreement with v_w within 2mdd for cooling water treated with zinc-phosphate inhibitors and without inhibitor. It was found that solution resistance should be corrected for R_p with comparable solution resistance.