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The nonlinear current-voltage response which is characteristic of a corroding metal/electrolyte interface prohibits the use of corrosion resistance measurements in conjunction with the Stern-Geary relationship, to provide an in situ determination of corrosion rates, at potentials removed from the free corrosion potential.
In this paper we describe the development of a technique for the electrochemical measurement of the anodic corrosion current (“corrosion rate”) under conditions of an applied cathodic protection potential. Our effort has been directed towards a simplified theoretical treatment of the harmonic current response at frequencies of 1f, 2f, 3f, 4f… to a 20 to 50-mV AC voltage perturbation at frequency f.
Experimental verification of the theoretical treatment was obtained by comparing the results of harmonic analysis with those obtained by mass loss determinations for copper : nickel alloy specimens of varying geometries, in oxygenated synthetic ocean water, at various levels of applied cathodic polarization.
AC impedance, cathodic protection (CP), waterbox corrosion, seawater corrosion
Program manager, Electrochemistry, Materials Research Laboratory, SRI International94025, Menlo Park, CA
Technical advisor, Electric Power Research Institute, Palo Alto, CA