Photo Gallery E2:  External Tank Corrosion

Similar to our comments regarding external corrosion and weathering at steel pipe, external corrosion to any tank is simply due to a lack of maintenance.  External corrosion is a far greater problem for cold water storage tanks over most other types.  Large 25,000 gallon domestic water storage tanks are typically not insulated.  With only a small turnover of water entering and leaving, colder inlet water sinks and often stratifies toward the bottom of the tank where it produces a colder tank surface favoring moisture condensation.  Colder temperatures in turn corrode the vertical I beam supports if external, and most importantly, the area of contact between the horizontal I beam supports and tank bottom.  For square and rectangular tanks, the bottom is rarely painted – thereby creating another surface vulnerable to corrosion attack.

For cold water storage tanks, it is important to remember that rust increases in volume and can impart a tremendous physical force as it increases.  Rust between the tank bottom and supporting I beams can actually raise up a 40,000 gallon house tank an inch or more; producing new stresses upon all inlet and outlet piping.

Outdoor located tanks are obviously at greater risk, and require the same level of high quality protection as any exposed pipe run.  Although almost all cylindrical tanks will be supported by a cradle, its still possible to find some examples resting in dirt – thereby presenting a high corrosion condition at its contact point as well as an impossibility to ultrasonically test.  While most tanks can be rehabilitated by the removal of the old coating and application of something more effective, ignoring the problem then produces a secondary outer corrosion condition along the downward pathway of the water leak.