Rapid Corrosion Screening in Poorly Defined Systems by Electrochemical Impedance Technique

One strength of the electrochemical impedance technique is its usefulness for rapidly screening the corrosion resistance of alloys or corrosivity as a function of process conditions. Curve-fitting using appropriate circuit analogue models can allow polarization or charge transfer resistances to be extracted even when there is limited understanding of the system under study. Reasonable estimates of corrosion rates can be made from these parameters. This paper explores the strengths and weaknesses of using this approach for screening the effect of pH on corrosion of steel in a plant waste stream. Good agreement is found between corrosion rates estimated by the impedance technique and short-term mass loss even when the analogous models are not perfect. Effects on interpretation are explored for slow migration of the corrosion potential and imprecise agreement between model and frequency response. Use of analogous circuit models are shown to be practical as long as their limitations are clearly understood.