Oil & Gas

Case study 1: Consultancy (2ndopinion) for a corrosion case in a gas storage tank

Customer:

Oil and Gas Company

Research:

A second opinion was requested. Evaluation of available data and reports on a corrosion case in a gas storage tank were done to determine if MIC is the present corrosion mechanism or if other corrosion mechanisms are involved.

The results and conclusions:

Corrosion products were scratched from the tank bottom and analyzed with SEM-EDX and for presence of MIC relevant microorganisms. Sulfur was detected and microbial analysis indicated presence of sulfur oxidizing bacteria (SOB) with a DNA based technique, qPCR. It was concluded that MIC is the active corrosion mechanism and that SOB were transported during hydro-testing into the system. It was recommended to clean the tank and the whole piping system with a biocide to stop microbial activity.

However, some puzzle pieces were not fitting. On the one hand, only uniform corrosion was present in the tanks. On the other hand, the tank was flushed with nitrogen, therefore no oxygen was present anymore. It is possible that detected SOB were transported with water in the system during hydro-testing. But SOB need oxygen for their microbial activities and localized corrosion would be expected. Furthermore, detected sulfur is also part of the stored butane gas and therefore probably not from microbial origin.

    Recommendation

    No further treatment of the tank is necessary because the corrosion process was probably influenced by water that was used for hydro-testing and temperature differences due to the installation process of the tanks. After the present oxygen is used up uniform corrosion will stop.

    It is recommended to inspect the tank again the next time while it is opened and cleaned. Additionally, corrosion coupons can be installed in the tanks to monitor the evaluation of the corrosion process. Corrosion coupons can be used for a combined approach of failure and microbial analysis to define the corrosion mechanism and the present corrosion rate. With this method it can also be tested if corrosion relevant microorganisms are active/ inactive inside the tank.