Materials Performance

DEC 2014

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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70 DECEMBER 2014 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 53, NO. 12 MATERIALS SELECTION & DESIGN tain locations, such as scratches in the pipe surface, areas of sediment accumulations, and at seemingly random locations. Accord ingly, the biofilms present in pig returns may be derived from relatively few loca tions, so microbial concentrations detected in composited samples may underestimate the actual concentration of microbial cells in biofilms attached to pipe surfaces. For this reason, even moderate concentrations of corrosionassociated bacteria detected in pigging solids can indicate a potentially high risk of MIC at specific locations within a pipeline. Microbiological Testing of Old Pipe Segments If pipe segments have been removed from ser vice and fresh microbiological samples were not taken at that time, then microbial growth tests can't be performed. Genetic testing, however, may still be able to detect DNA from corrosionassociated microorganisms because genetic testing can detect DNA from both living and dead bacteria. 25 DNA from dead bacteria will eventually degrade and become undetect able, so an interesting question is how long a pipe segment can be removed from ser vice and still yield useful samples for genetic testing. In one case, a pipe segment that had been removed from service sat under atmo spheric conditions for several months prior to testing. Biofilm swab samples from the pipe segment were collected, both from an area that did not have obvious corrosion as well as from corroded areas. Every effort was taken to obtain material from under neath corrosion products, as this offers the greatest chance of recovering intact DNA if bacterial cells were present. These samples were analyzed using genetic qPCR tests to quantify total bacteria, SRB, nitratereduc ing bacteria, and archaeabacteria (metha nogens and thermophiles). No DNA/bacte ria of any kind were detected in any of the samples at concentrations above the detec tion limit of the assay. Dry pipe segments that have been out of ser vice for weeks or months prior to sampling generally do not yield detectable FIGURE 5 Example of pig returns for a crude oil pipeline. FIGURE 4 Variability of microbiological samples from the same pig returns.

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