Letter 16045

Corrosion of mild steel phosphate tank  

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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 8.2m x 0.6m x 0.75m). 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.

This sound 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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