Hydrostatic testing of pipelines is an important step prior to commissioning. In this paper, we discuss two case studies wherein leaks were detected during hydrotesting of a newly constructed 12-inch pipeline at a client site. The first failure occurred in the body of a pipe segment coated with fusion bonded epoxy. The second failure occurred at a girth weld of a different pipe segment in the same line, which had been coated with an abrasion resistant outer wrap. Visual examination and stereomicroscopy revealed the presence of pits adjacent to the leaks. Scanning electron microscopy, energy dispersive x-ray spectroscopy, bacteria culture testing and metallography confirmed that the pitting occurred as a result of internal microbiologically induced corrosion.
Pre-commissioning hydrostatic testing of pipelines and the resulting corrosion (MIC) issues are often linked to test water quality, as well as post-test cleaning operations.1 In a 1998 study, it was reported that localized corrosion (pitting/crevice corrosion) accounted for 20% of failures in the chemical process industry with an estimated one half of those being MIC failures.2 Identification of MIC failures is not straightforward. Common characteristic features such as pit clustering, "tunneling" of pits, tuberculation, high microbiological counts, presence of sulfides (in the case of sulfate reducing bacteria (SRB)) and preferential weld attack have been used to anecdotally pinpoint field failures towards MIC.3,4 Some of these morphologies are similar to those found in other damage mechanisms, such as chloride pitting in stainless steels (SS), making it challenging to accurately diagnose MIC failures.3,5 However, corrosion rates for MIC could be too rapid to be explained by conventional pitting/crevice corrosion. Additionally, electrochemical studies for Type 304 stainless steel (SS) have shown that in the case of MIC damage, the corrosion potential within the pit is ennobled due to formation of microbes; while in the case of chloride-induced pitting corrosion, the pitting/breakdown potential becomes more negative due to local pit acidification.6 No evidence was found to support ennoblement of corrosion potential due to microbial acid production. In other studies, this ennoblement of corrosion potential (above pitting potential of austenitic SS) has been linked to the presence of manganese oxidizing bacteria (MOB), with manganese being observed in the corrosion products that were examined.7,8
