Abstract
Seasonal fluctuations in temperature and soil moisture content can affect the cathodic protection of underground pipelines. These effects are particularly pronounced in high-resistivity soils which can experience large fluctuations in soil resistivity because of episodic or seasonal changes in moisture content. Under some circumstances it can be difficult to demonstrate compliance with various CP criteria, even though the moisture content is so low that corrosion is unlikely to be a significant problem.
This paper describes the results of a study to investigate the effect of seasonal changes on the protection of pipelines in high-resistivity soil. In addition to establishing the nature and magnitude of the seasonal fluctuations, a second aim was to demonstrate that corrosion rates are low even when compliance with a given CP criterion cannot be demonstrated.
Environmental and CP measurements were made on operating pipelines in both Canada and the U.S. Daily monitoring of soil characteristics such as resistivity, pH, temperature and oxygen content was performed at the pipeline to soil interface using a novel pipe-depth probe and automated measurement system. Coupons were used to measure on-, off-, and native potentials, also on a daily basis, as well as native corrosion rates during periodic site visits. A method is proposed for predicting corrosion rates of polarized coupons to demonstrate that rates are low in high-resistivity soils even though compliance with a particular CP criterion may not be met.
