Most tanks are fabricated from sheets of flat or rolled mild carbon steel, and are subject to various forms of corrosion.  Generally, an internal coating is applied to isolate the underlying steel, although like most forms of protection, failure of the coating at some future date will occur.  Since the predictive assessment of tanks is rarely performed, the first indication of a problem is typically when a leak or failure occurs.  Leaks at the pipe to or from a water storage tank also provides a good indication to a potential problem, given that the same level of corrosion activity would impact all such metal surfaces.  Generally, the type of failure and its location is greatly dependent upon the type of tank or pressure vessel involved.  Dedicated fire sprinkler water storage tanks experience less corrosion activity than domestic cold water storage or those providing combination fire/domestic service.

Old riveted pressurized fire storage tanks from the 1920s typically first show their problem at the rivets, and at the front half of the tank where water movement is greater.  Compressed air tanks and oil storage tanks show their greatest wear at the bottom where water collects.  Domestic cold water storage tanks more heavily deteriorate between the high and low water marks, and then show a lesser amount of deterioration below the low water line.  Expansion tanks, are similar, and exhibit the greatest deterioration where water level rises and falls.

The consequences of failure vary widely.  A leak at a rivet is likely to take years to advance whereas corroded through internal supports can bow out the side of a 20,000 gallon tank to a point approaching catastrophic failure.

Given their large size and the fact that most seem to have had the building constructed around them, addressing a corroded tank problem can be challenging.  Ultrasonic Testing performed in a standard grid will not only define its condition, but also provide a 3D visualization of its inside wall profile if sufficient thickness measurements are taken.  With its condition known and sufficient wall thickness remaining, the best remedial step is to remove any previous coating or rust and apply a more effective alternative.

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