Corrosion requires both water and oxygen to occur, and in the case of underground corrosion, soil supplies these reactants for the reactions to proceed. It follows that the ability of the soil to store and supply water and oxygen will to some extent, govern the rate of corrosion of metal buried in soil. It is known that there is a critical level of moisture at which corrosion is soil is maximized. Too much water will stifle corrosion because the supply of oxygen to metal is severely limited under waterlogged conditions.
As part of my research, we investigated these mechanisms in detail. Buried infrastructure such as pipelines and storage tanks fail due to corrosion, causing large economic losses and environmental damage due to leaks and bursts, exposing contents such as oil and gas to the surrounding environments. An understanding of underground corrosion and its prevention helps increase longevity of such buried assets and is a step towards sustainable development. While corrosion and its prevention are handled by electrochemical practitioners, the installation of buried infrastructure and backfilling is under the purview of geotechnical engineers. Given that the problem of underground corrosion overlaps both these fields, its study should be an interdisciplinary effort.
Our experiments showed that the critical level of moisture for corrosion is related to a relationship known as the soil water retention curve, or the soil water characteristic curve. Combining results from several electrochemical experiments and soil tests we were able to identify the behavior of water in soil that governs corrosion. We saw that in different soil types, the continuity of air and water phases change differently with the degree of water saturation, and that the transition point at which the air phase becomes occluded coincides with the critical degree of saturation for corrosion (where corrosion is maximized) for each soil type. What was more interesting was that this critical water content for corrosion is the same as the optimum water content for soil compaction. It has been shown that the same mechanisms for air entrapment occurs at the optimum water content during soil compaction.
This finding is important because, it is usual practice among geotechnical engineers to compact soil to its optimum water content to maximize its strength. In the installation of buried infrastructure and the subsequent compaction of backfill, if this usual practice is followed, we will be inadvertently creating the most conducive conditions for corrosion of the buried metallic asset. So, the question is whether we increase our soil strength, of the rate of deterioration of the buried metal. One possible solution is to compact soil in the drier side of the optimum water content. But there may be other factors at play that need to be considered. What is more important is to identify that this problem is interdisciplinary in nature and needs to be solved that way. Corrosion engineers need to be aware of this in planning prevention techniques such as cathodic protection, and geotechnical engineers need to know the effect of soil compaction on the corrosion of buried metallic infrastructure.
Like many of our modern problems, underground corrosion needs to be viewed and solved by taking a multidisciplinary approach. Confining our engineering efforts to the traditionally isolated fields is likely to worsen the problem rather than solving it, and our knowledge and forces need to combine to achieve sustainable progress.
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| The point of maximum corrosion was identified as the degree of saturation at the inflection point of the water retention curve also coincident with the degree of saturation at the optimum moisture content (OMC) in the compaction curve |
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