Corrosion of mild steel: A new interpretation of the AC impedance spectra of corroding mild steel

The use of ac impedance for the study of corrosion has been around for a long time. The method of interpreting the data was based on equivalent circuits that mimic the experimental ac impedance spectrum. The models developed to date all try to use the strict logic of electronic circuitry. The reaction chemistry occurring at the electrode surface has been ignored. This has caused problems when the information at frequencies greater than 10kHz are evaluated.; The effects of a biofilm on ac impedance spectra have never been evaluated. The nature of the bacteria in the biofilm and the nature of the extracellular polymer does have an effect on the spectra.; It was proposed that the information above 10kHz was a result of the formation and propagation of a pit on the surface of the 1018 mild steel coupons tested. A technique to control the location and size of a pit was developed to test this hypothesis. With this controlled pitted coupon, the effects of different conditions were examined.; A model for the evaluation of the spectra collected under various conditions was developed. This model was applied to spectra from highly controlled to uncontrolled experiments. It was found to accurately model the experimental spectra in all cases.; The model assumed that the passivated surface and pitting region could be treated as two separate electrodes with their spectra superimposed onto each other. This was a new approach because it allowed the separation of the chemical process effects for the first time.