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Corrosion of mild steel phosphate tank

We are a tube manufacturing firm that uses a series of tanks that we dip our seam welded tube in and then apply a zinc stearate to the outside and inside of the tube.

One of the tanks in the line applies a zinc phosphate coating to the tubes. The density of the coating is approx. 10 to 12 g/m^2. The tank is heated to 70° C using electrical heating elements. After a period of 7 months this mild steel tank has corroded very badly, but only all along vertical sides at the bottom of the tank (tank 8.2 m x 0.6 m x 0.75 m). The rest of the tank is in perfect condition. It must also be said that the steel spout that supplies refill water to the tank also corroded badly.

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This sounds very like galvanic action, why however only at the tank bottom sides?

Any help would be appreciated.

Marcel Maree
Tubecon - Pretoria, South Africa

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