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98560 MEASUREMENTS OF OHMIC DROP FREE POTENTIAL IN THE PRESENCE OF STRAY CURRENT

Picture for 98560 MEASUREMENTS OF OHMIC DROP FREE POTENTIAL
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Product Number: 51300-98560-SG
ISBN: 98560 1998 CP
Author: Luca Bertolini, Pietro Pedeferri, Bruno Bazzoni, Luciano Lazzari
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The paper reports and discusses field and laboratory results obtained with the application of the lateral ohmic drop extrapolation technique which allows the elimination of the ohmic drop contribution affecting the potential measurement of buried metallic structures (pipelines, concrete reinforcement) in the presence of stray current interference. The measurement is carried out, for a short period, by recording simultaneously the potential E and the lateral ohmic drop delta V. Two reference electrodes are used: one placed above the pipeline, the other one far enough away to measure the potential difference versus the former, at least higher than 20-30 mV. Afterwards potential values are plotted against the lateral ohmic drop. When the technique is applicable, a straight line is obtained and the true potential of the structure can be achieved by means of the graphical extrapolation to zero of the lateral ohmic drop delta V. The technique has been proposed and applied to cathodically protected pipelines in presence of stray currents. Recent surveys have shown that the method is also applicable to free corroding buried structures (for instance, the casing used for road crossing or rebar in reinforced concrete) and gives the true corrosion condition. The extrapolated potential value is regarded as the true free corrosion potential (or mixed potential) without the ohmic drop contribution induced by the stray current. In the case of structures in free corrosion conditions, the method can be related to the Stern-Geary linear polarisation resistance theory which is based on the linear relationship between potential and external current around the free corrosion potential. For the lateral ohmic drop technique a potential change is "spontaneously” induced by the external electric field. Therefore, instead of measuring the current exchanged by the structure, in this case not possible, the lateral ohmic drop is measured. The latter is however proportional to the unknown current exchanged by the pipeline. The straight-line obtained by plotting the E vs delta V confirms the linear relationship on which the technique is based.
The paper reports and discusses field and laboratory results obtained with the application of the lateral ohmic drop extrapolation technique which allows the elimination of the ohmic drop contribution affecting the potential measurement of buried metallic structures (pipelines, concrete reinforcement) in the presence of stray current interference. The measurement is carried out, for a short period, by recording simultaneously the potential E and the lateral ohmic drop delta V. Two reference electrodes are used: one placed above the pipeline, the other one far enough away to measure the potential difference versus the former, at least higher than 20-30 mV. Afterwards potential values are plotted against the lateral ohmic drop. When the technique is applicable, a straight line is obtained and the true potential of the structure can be achieved by means of the graphical extrapolation to zero of the lateral ohmic drop delta V. The technique has been proposed and applied to cathodically protected pipelines in presence of stray currents. Recent surveys have shown that the method is also applicable to free corroding buried structures (for instance, the casing used for road crossing or rebar in reinforced concrete) and gives the true corrosion condition. The extrapolated potential value is regarded as the true free corrosion potential (or mixed potential) without the ohmic drop contribution induced by the stray current. In the case of structures in free corrosion conditions, the method can be related to the Stern-Geary linear polarisation resistance theory which is based on the linear relationship between potential and external current around the free corrosion potential. For the lateral ohmic drop technique a potential change is "spontaneously” induced by the external electric field. Therefore, instead of measuring the current exchanged by the structure, in this case not possible, the lateral ohmic drop is measured. The latter is however proportional to the unknown current exchanged by the pipeline. The straight-line obtained by plotting the E vs delta V confirms the linear relationship on which the technique is based.
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