Subsequent failure due to corrosion, particularly on storage tanks bottom plates for molten sulphur tanks, is one of the main factors upsetting any production facilities. Maintaining adequate level of corrosion protection for the bottom plates soil side is always challenging due to complex corrosion phenomena. Failure of storage tanks during production stage due to such corrosion phenomena can be catastrophic which would result in serious consequences subject to health, safety, environmental, cost, economic impact, damage to assets and company reputation.
As per current industrial practice, above ground storage tanks’ bottom plates are designed to be protected externally with ICCP system. However, the CP effectiveness often becomes challenging due to presence of air gap, improper selection of sand, improper CP design and formation of acidic environment. Such factors eventually lead to ineffectiveness of CP system due to restricted current flow. The situation aggravates in the tanks which is handling molten sulphur whenever there is moisture ingress occurs through annular area of the concrete foundation and settles underneath the bottom plate surfaces.
This paper focuses on sulphur tank failure due to multiple corrosion damage mechanism acting on this tank from soil side that had resulted in heavy loss of thickness and deep pitting.
During Tank T&I activity heavy metal loss of thickness was noticed from bottom plate of sulfur tank with help of MFL examination, considering this fact further UTT was carried out to validate the MFL results. Considering the heavy loss of thickness on bottom plates of this tank, the applicable sample was taken to perform further analysis.
The failure characteristics observed for the tank bottom plate were consistent with microbiologically induced corrosion, with a significant contribution of sulfuric acid, produced by the bacteria (such as sulphur oxidizing bacteria or iron oxidizing bacteria) with the effect of water ingress from the hydrolysis of the corrosion product between iron and sulphur (which consequently forms sulfuric acid). The affected areas are found with aggressive corrosion with heavy loss of thickness and deep pitting.
