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51317--9809-The Influence of Scale Formation on the AC Corrosion of API Grade X65 Pipeline Steel Under Cathodic Protection

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In a previous investigation, AC corrosion rate data from weight loss experiments was compared with the results from a model for AC corrosion developed using a modified Butler-Volmer approach. In the present work, an extension of this is presented to investigate the influence of scale formation on AC corrosion rates.

Product Number: 51317--9809-SG
ISBN: 9809 2017 CP
Author: Elmira Ghanbari
Publication Date: 2017
Industry: Energy Generation
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In a previous investigation we compared AC corrosion rate data generated from weight loss experiments 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 we extend this finding 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 change the interfacial capacitance of the steel. Thus steels in soils which are prone to the formation of these scales may have greatly different AC corrosion rates for equivalent AC current and pipe-to-soil potential. To investigate the effect of scale formation on AC corrosion we present corrosion rates for carbon steel API grade X65 pipeline steel in a soil simulant with and without calcium carbonate and Iron carbonate scales and under cathodic protection. Data are analyzed using our 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). The results of this work have implications on industry standards and the evaluation of AC corrosion rates in different soil types.Key words: AC interference on pipelines corrosion carbon steel scale formation interfacial capacitance.

Keywords: AC interference on pipelines, corrosion, carbon steel, scale formation, interfacial capacitance

In a previous investigation we compared AC corrosion rate data generated from weight loss experiments 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 we extend this finding 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 change the interfacial capacitance of the steel. Thus steels in soils which are prone to the formation of these scales may have greatly different AC corrosion rates for equivalent AC current and pipe-to-soil potential. To investigate the effect of scale formation on AC corrosion we present corrosion rates for carbon steel API grade X65 pipeline steel in a soil simulant with and without calcium carbonate and Iron carbonate scales and under cathodic protection. Data are analyzed using our 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). The results of this work have implications on industry standards and the evaluation of AC corrosion rates in different soil types.Key words: AC interference on pipelines corrosion carbon steel scale formation interfacial capacitance.

Keywords: AC interference on pipelines, corrosion, carbon steel, scale formation, interfacial capacitance

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Case histories where throttling down the cathodic protection was evaluated to determine the impact on reducing the AC corrosion threat. Includes the use of fast-response electrical resistance corrosion rate probe monitoring technology.
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ISBN: 9461 2017 CP
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Publication Date: 2017
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Challenges associated with coordinating the modelling, design, and installation of an alternating current interference and mitigation systems. The project consisted of a 65 kilometer long double circuit 500 kilovolt (kV) overhead transmission in a heavily congested right-of-way corridor with more than 80 pipelines.
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