Electrochemical testing of soils for long-term prediction of corrosion of ferrous pipes

Deterioration of buried ferrous assets is a significant concern to water utilities that collectively own thousands of kilometres of buried water assets. The prediction of pitting rates through electrochemical testing may substitute for more invasive and expensive in-situ direct measurement of corrosion. Previously, the LPR technique applied to soils has been considered a ‘black box’ technology that has been difficult to replicate. Based on this, a thorough examination of the LPR technique on soils and the applicability of the technique to long-term corrosion is justified. A new electrochemical cell was developed to measure the polarisation resistance of soil to various ferrous materials. The cell design and physical soil characteristics were tested to understand their effect on measured polarisation resistance in the short-term. Of these characteristics, the moisture content of the soil and the compaction of the cell were found to have the most profound effect on polarisation resistance. In addition, a series of two-week experiments were carried out to measure the corrosion rate over this period. Ultimately, the variability in the results prevented any firm findings with respect to corrosion rates in soils. These tests did, however, indicate that the soil moisture content and initial compaction of soil into the system would likely dictate the test outcomes. The most aggressive conditions, from a corrosion perspective, were soils that were only partially saturated. This closely aligns with the data obtained during the field exhumations undertaken as part of this work. In order to compare the applicability of electrochemical testing of soils to pitting corrosion over the longer term a series of exhumations were made on cast iron pipes. Soil samples and measured corrosion (expressed as a pitting rate) were obtained to compare with short-term electrochemically measured polarisation resistance. Favorable correlations were obtained between buried pipes of a similar age and measured polarisation resistance. This has not been reported previously and is a significant finding. Field observations from the sampling program and the results of the electrochemical testing also revealed that environmental factors and burial practice could influence the corrosion observed. More sampling would be necessary to fully substantiate this result. Though favorable correlations were obtained it should be stated that electrochemical testing is a short-term measurement of the kinetic parameters of corrosion that is not necessarily a true representation of long-term corrosion that may relate to diffusion processes. Despite the limitations of short-term testing versus long-term corrosion processes, the electrochemical test developed as part of this research provides a simple and cost effective way to estimate the pitting rates of soils toward buried cast iron pipe.