Internal corrosion sometimes occurs under deposits of solid particles on the bottom of transmission pipelines. The solids trap water with soluble products and other nutrients which can support the development of microbial communities and may lead to Microbiologically Influenced Corrosion (MIC). Corrosion processes associated with the metabolic activities of specific bacteria have been discussed elsewhere, but the simple presence of large microbial populations may increase the risk of internal corrosion owing to the ability of biofilms to extract and concentrate water at the pipe floor.
As a method to monitor the internal corrosion threat in transmission pipelines and recommend mitigating activities for corrosion management, reliable microbial content and corrosion activity correlations are desired. Sludge samples have been obtained from cleaning pigs at the pipe trap and analyzed using Biological Activity Reaction Test (BART™) (or serial dilution test), Dean-Stark analysis, XRD and EDX. These tests provide information about certain bacterial populations, water / solid / hydrocarbon content, and crystalline/elemental composition of these solids, respectively. Despite best efforts, bacterial population/activity of pipeline sludge samples exhibit high variability and are difficult to correlate to actual internal corrosion in a pipeline.
Considering that bacterial populations in pipeline sludge may be a meaningful representation of the internal corrosion threat to a transmission pipeline, a more rigorous approach on the sludge sampling procedure is necessary to improve the accuracy and reliability of the bacterial assays. It is also important to control such variables as storage temperature of the samples, exposure to air, and storage duration prior to enumeration — as these may affect the viability of the sample and enumeration results. This report presents historical pipeline sludge analysis data and suggests a method to evaluate data containing high variability. Practical recommendations to reduce data variability through handling and storage of sludge samples are also discussed.