Abstract
In a previous investigation, AC corrosion rate data generated from weight loss experiments was compared with the results from a model for AC corrosion that was developed using a modified Butler-Volmer approach. The model considered the anodic and cathodic Tafel slopes, diffusion limited oxygen transport, interfacial capacitance and solution resistance. Both experimental and model results highlighted the importance of the interfacial capacitance on the rate of AC corrosion, especially at a frequency of 60Hz. In the present work, an extension of this finding is presented to investigate the influence of scale formation on AC corrosion rates. Scale formation at a holiday in a pipeline coating, such as calcium deposits, carbonate deposits or corrosion product, changes the interfacial capacitance of the steel. Thus, steels in soils which are prone to the formation of these scales may have significant difference in AC corrosion rates for the equivalent AC current and pipe-to-soil potential. To investigate the influence of scale formation on AC corrosion behavior, the corrosion rate of carbon steel API grade X65 pipeline steel in a soil simulant solution (NS4) with and without calcium carbonate and iron carbonate scales and under cathodic protection potentials were studied. Using the proposed model for AC corrosion and within the context of scale capacitance, polarization resistance and solution resistance (as measured for the samples via electrochemical impedance spectroscopy) data were analyzed. The results of this work have implications on industry standards and the evaluation of AC corrosion rates in different soil types.
