By John Aru, Corrpro Canada Inc.
Once it has been constructed, the greatest expense for a steel water storage tank owner is ongoing maintenance. Inspections and coatings maintenance are critical to maintain the exterior appearance of the storage tank and to prevent corrosion on the interior dry surface areas. Interior wet surface areas are a different matter and prudent owners apply cathodic protection in conjunction with appropriate coatings to prevent corrosion and extend the life of the coating.
The submerged surface areas endure the most corrosive environment in a tank. Coatings used for total immersion must withstand a number of harsh conditions such as constant water permeation to the substrate and ice damage in the northern climates. If the tank is left unprotected, these and other factors often lead to costly premature coating failure and serious corrosion damage.
Cathodic protection for water storage tanks has been recommended by the steel plate fabricators, independent tank inspection companies and owners throughout the country. Cathodic protection has been proven to double, triple or even quadruple the life of the interior submerged coating, thereby saving the owners tens or even hundreds of thousands of dollars over the life of the tank.
Two examples of the value of using cathodic protection in conjunction with coatings, are the 500,000 US gallon elevated tank built in 1962 and the 200,000 US gallon elevated tank built in 1964 for a municipality in Wisconsin.
Tank inspections performed in 1999 revealed the submerged surfaces of both tanks to be in excellent condition with no corrosion. The original coating in the 200,000 gallon tank was intact and no immediate recommendation to re-coat was necessary. The original coating in the 500,000 gallon tank had failed in some areas; however, where the cathodically protected steel was exposed, no corrosion was present.
This case study reveals that the use of cathodic protection systems has prevented corrosion while preserving the coatings on the submerged areas of the steel tanks for more than 36 years. In each case, cathodic protection has eliminated the need for as many as three and possibly four interior wet surface area re-coatings. The estimated cost savings to the owner are more than $250,000.00.
This is only one of countless examples where the application of cathodic protection to steel water storage tanks has proven to be a cost-effective means for long-term corrosion protection and interior coating life extension.
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