Premature Failure of API 650 Oil Storage Tank Bottom Plates Due to Soil Side Corrosion

摘要

The above ground storage tanks (AST's) in the refinery is experiencing bottom plate underside (soil) corrosion at an extraordinarily high rate of 1 mm/year. This has resulted in the failure of 4 tanks within a period of 9 years since commissioning (2006-2015). The corrosion morphology is of severe localized corrosion with large deep pits. The design of the underside corrosion protection for the AST's is by providing an impressed current cathodic protection (ICCP) system (MMO grid anode system). Recent and past cathodic protection (CP) survey of the tanks clearly reveal a very low (electropositive) polarized potential across the tanks indicating the CP system has not functioned correctly since commissioning. The corrosion rates observed has been amplified by the ingress of water (through the gaps between annular plate and foundation), primarily from leaking fire water sprinklers, which bring in bacteria and corrosive anions like chloride and sulfate. With the CP system being ineffective and no other means of corrosion control, the tank floor was exposed to a severe form of bacterial and under deposit corrosion leading to perforation and loss of inventory within 8 years from commissioning. A short term stratagem has been employed whereby the corrosion rate was substantially slowed thus permitting distribution of the tank turnarounds across as many years as possible. Vapor phase inhibitors were used to temporarily extend the life of the tank bottom plates. For the long term, the refinery recommends a high resistivity bitumen sand mix foundation (without CP system) over the existing CP design based on trouble free track record of 20-30 years of the bitumen design that was experienced in tanks of other plants within the company. The article summarizes the observations, details of the CP survey, tests, results and forward path taken up by the refinery.