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PRCI PR-101-9221
- Time-Dependent Polarization Behavior as a Function of Soil Type
- Report / Survey by Pipeline Research Council International, 06/01/1995
- Publisher: PRCI
$425.00$849.00
L51735e
University of Florida
Need: Cathodic protection requirements for buried structures have traditionally been predicted by use of anode resistance equations which take into account the influence of anode geometry on the current distribution at the anode and the influence of soil resistivity on the driving force required to pass current to the structure to be protected. The conventional anode resistance formulas used for CP design were developed for bare pipes protected by remote anodes. Under these conditions, the current density at the anode is so much larger than that on the pipe that the current and potential distribution around the pipe can be ignored in resistance calculations. The current and potential distribution on the pipeline must, however, be considered for coated pipes with coating holidays or when anodes are placed in close proximity to the pipe.
Benefit: The objectives of this work were: 1. to develop and apply new experimental techniques for identifying the time-dependent polarization behavior of pipeline grade steel as a function of the ionic composition of typical soil leachate and to develop and apply methods to interpret the results in terms of fundamental physical processes; 2. to measure the time-dependent polarization of FBE-coated coupons with and without machined coating holidays that expose bare steel; and 3. to establish an approach for predicting the polarization behavior of coated coupons with coating holidays that expose bare steel. by comparing the predictions for the polarization of coupons with holidays with experimental values.
Result: An experimental procedure was developed for conducting bench-top measurements of the time-dependent polarization behavior of bare steel and coated steel with holidays that expose bare steel. A model, based on a description of the influence of calcareous films on the electrochemical reactions, was used successfully to describe the time-dependent values of instant-off potentials. A fixed set of parameter values was found to apply for all measurements made at a given electrolyte composition and oxygen content. The success of the model is evident in that a fit to within less than 20 mV was obtained with parameters that were not functions of time or potential. The model could be used, in most cases, to predict the time-dependent potential for coated coupons with machined holidays to within less than 50 mV. The agreement was not as good at applied potentials for which the hydrogen evolution reaction dominated, suggesting that more work is needed to explore the polarization behavior at very cathodic applied potentials. By showing that the time-dependent polarization behavior of bare steel can be described in terms of physical processes and by showing that independent measurement of the behavior of bare and coated steel can be used to predict the behavior of coated steel with holidays that expose bare steel, this work provides a foundation for the development of methods for predicting the CP requirements for coated pipelines.
University of Florida
Need: Cathodic protection requirements for buried structures have traditionally been predicted by use of anode resistance equations which take into account the influence of anode geometry on the current distribution at the anode and the influence of soil resistivity on the driving force required to pass current to the structure to be protected. The conventional anode resistance formulas used for CP design were developed for bare pipes protected by remote anodes. Under these conditions, the current density at the anode is so much larger than that on the pipe that the current and potential distribution around the pipe can be ignored in resistance calculations. The current and potential distribution on the pipeline must, however, be considered for coated pipes with coating holidays or when anodes are placed in close proximity to the pipe.
Benefit: The objectives of this work were: 1. to develop and apply new experimental techniques for identifying the time-dependent polarization behavior of pipeline grade steel as a function of the ionic composition of typical soil leachate and to develop and apply methods to interpret the results in terms of fundamental physical processes; 2. to measure the time-dependent polarization of FBE-coated coupons with and without machined coating holidays that expose bare steel; and 3. to establish an approach for predicting the polarization behavior of coated coupons with coating holidays that expose bare steel. by comparing the predictions for the polarization of coupons with holidays with experimental values.
Result: An experimental procedure was developed for conducting bench-top measurements of the time-dependent polarization behavior of bare steel and coated steel with holidays that expose bare steel. A model, based on a description of the influence of calcareous films on the electrochemical reactions, was used successfully to describe the time-dependent values of instant-off potentials. A fixed set of parameter values was found to apply for all measurements made at a given electrolyte composition and oxygen content. The success of the model is evident in that a fit to within less than 20 mV was obtained with parameters that were not functions of time or potential. The model could be used, in most cases, to predict the time-dependent potential for coated coupons with machined holidays to within less than 50 mV. The agreement was not as good at applied potentials for which the hydrogen evolution reaction dominated, suggesting that more work is needed to explore the polarization behavior at very cathodic applied potentials. By showing that the time-dependent polarization behavior of bare steel can be described in terms of physical processes and by showing that independent measurement of the behavior of bare and coated steel can be used to predict the behavior of coated steel with holidays that expose bare steel, this work provides a foundation for the development of methods for predicting the CP requirements for coated pipelines.
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