A Case Study on Mixed Metal Oxide (MMO) Anode Failure of an Above Ground Storage Tank Bottom Plate

摘要

A case study on the failure of a cathodic protection (CP) system and consequent corrosion on the external surface of an above ground storage tank bottom plate is reported in this article. This tank was set on asphalt pad foundation. Also, it includes a none-metallic membrane one meter beneath the bottom plate, and a ring wall constructed around the tank perimeter. The space between the ring wall and bottom plate was filled with washed sand. An impressed current cathodic protection system was designed to prevent corrosion on ground contacting surface of the tank bottom. The Mixed Metal Oxide (MMO) anode system had been located in the fill area between the tank bottom and the membrane. After 8 years of service, a severe leakage on tank bottom was noticed and it has been decided to lift up the tank for close visual inspection. Visual inspection of anode system showed the failure of MMO anode wire loop. Further investigations by Scanning Electron Microscope (SEM) revealed that the MMO film on the titanium substrate is afflicted with localized loss because of hot spots in high current density or break-down voltage, which finally lead to interruption. Therefore, the CP system design calculations are compared to filed experience and preventive measures suggested.