An oil and gas company
A second opinion was requested. An evaluation of the available data and reports of a corrosion case in a gas storage tank was performed to determine if MIC was the corrosion mechanism present or if other corrosion mechanisms were involved.
In the previous study, corrosion products were sampled from the bottom of a tank and analyzed with SEM-EDS and tested for the presence of MIC relevant microorganisms. Sulfur was detected with the SEM-EDS and microbiological analysis showed the presence of sulfur oxidizing bacteria (SOB) with qPCR, a molecular technique. It was concluded that MIC was the active corrosion mechanism and that SOB were introduced into the system during hydro-testing. The recommendation was to clean the tank and the entire pipe system with a biocide to stop the microbial activity.
However, some pieces of the puzzle didn’t fit. First, only uniform corrosion was present in the tank. In addition, the tank was flushed with nitrogen, so that there was no longer any oxygen present. It is possible that the detected sulfur oxidizing bacteria (SOB) were transported with the water into the system, but SOB require oxygen for their microbial activities. In addition, localized corrosion would be expected at MIC. Furthermore, sulfur was also part of the butane gas stored in the tank and probably not of microbial origin.
No further treatment of the tank is necessary, as the corrosion process was likely affected by the water used for hydro-testing and the temperature differences during the tank installation process. After the oxygen present has been consumed, the uniform corrosion will stop.
It is recommended to inspect the tank the next time it is opened and cleaned. In addition, corrosion coupons can be installed in the tank to monitor the development of the corrosion process. Corrosion coupons can be used for a combination of failure analysis and microbial analysis to determine the corrosion mechanisms and current corrosion rate. With this method it can be determined whether corrosion relevant microorganisms in the tank are active or inactive.