Modelling and testing of optimum soil moisture levels in the corrosion of underground pipelines

Corrosion is one of the major factors leading to the failure of buried pipelines. Soil properties such as aeration, moisture content and level of compaction are known to cause variations in the level of corrosion of buried metallic structures. It is known that, at a particular soil moisture content, the corrosion rate reaches a maximum value. While this phenomenon is generally understood, an explanation from a soil mechanics perspective with mechanisms for soil water continuity and mass transport processes is currently lacking. This work fills this void by modelling the moisture-controlled diffusion transport and electrical conductivity in soil coupled to the electrochemical activity on the buried metal surface. Variations in the electrical conductivity and oxygen diffusion in sand at different degrees of saturation were determined experimentally. The results were used as input parameters in a finite element model. Results from the coupled finite element model were compared with experimental results from electrochemical corrosion tests. The tests were conducted on cast iron specimen buried in sand and the corrosion behaviour under various aeration regimes were studied. The presence of an optimum moisture/aeration regime, where the corrosion rate becomes a maximum was demonstrated and the mechanisms behind this phenomenological behaviour are discussed in this paper. The modelling and experimental results are expected to be useful in developing non-intrusive testing methods for underground corrosion.