The Validity of Electrochemical Methods for Measuring Corrosion Rates of Copper-Nickel Alloys in Sea Water

This work was stimulated by reports that electrochemical methods for measuring the polarization resistance of copper, nickel, or copper-nickel alloys in aqueous systems may lead to appreciable errors in the estimates of the corrosion rate. In the present work, the polarization resistance (Rp) of two copper-nickel alloys (90:10 Cu:Ni and 70:30 Cu:Ni) was monitored as a function of time in flowing sea water that contained 0.045, 0.85, 6.6, or 26.3 mg/dm3 dissolved oxygen. Rp values were obtained using the linear polarization, potential step, and AC impedance techniques. The total weight loss during the test period was calculated from the area under the 1/Rp versus time curve and compared with the measured weight loss. The experimental data support the validity of electrochemical methods for measuring corrosion rates of copper-nickel alloys in sea water, provided that a sufficient number of Rp measurements are made during the first 24 hours, and provided that proper consideration is given to the capacitive elements of the metal/solution interfacial impedance. Previous reports that electrochemical measurements may lead to underestimates of the corrosion rates are thought to have resulted primarily from undetected (and very high) corrosion rates at short exposure times. Reported overestimates of the corrosion rates are thought to have resulted from the use of a potential sweep rate that was too high (linear polarization technique), a time for current decay that was too short (potential step technique), or excitation frequencies that were insufficiently low (AC impedance technique).