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Phosphate Treatment of Compressed Air Pipes (Internal)

I have a customer who is proposing to treat the internal surfaces of carbon steel compressed air pipes using a phosphating treatment for corrosion protection. The compressed air is at ambient temperature, is known to be wet, and may contain traces of oil vapour. There is no plans to apply a protective coating over the phosphate layer - the treated surface being left exposed to the air flow. These pipes are 10 inches ID. My customer has asked my opinion of the proposal - unfortunately I do not know enough about the subject.

My limited understanding of the treatment tells me that this would be a very temporary corrosion prevention coating, and that the phosphate layer would not last long owing to the moisture, and the fact that the layer is extremely thin.

The other option being considered is the use untreated stainless steel. If this were the case - should the internal surfaces be passivated ? Any comments and advice would be greatly appreciated.

Jaimie Shaw
- Brisbane, Queensland, Australia

We have phosphated air line for street cars which are then painted on the exterior. If the phosphate is oiled it will give fair corrosion protection.( 72 to 96 hrs. Saltspray) If your compressed air is oily, it will only help the phosphate. You don't say the length of pipe needing coating, there are certain "Teflon" like products which could be spray applied on the ID to enhance the corrosion protection.

Wayne McKee
- Mississauga, Ontario, Canada

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Jaime, it is possible to phosphate inside steel piping as per your requirments. This has been done very successfully over many years on tubing from 25 mm. up to 300 mm. However, a lot depends on the ability to "contact" the surface with the phosphate solution, at the req. temp., for the req. time. My advice is to seek info from one of several good Supply Houses, with offices in Brisbane, or contact me with a reply at this site. You are looking for a "heavy Zinc phosphate system". As mentioned in an earlier reply, the oil in the air will only assist the corrosion protection as long as the oil/condensate is not too acid or alkaline.

Good luck,

Patrick Wm. Mc Ilvena
Melbourne, Victoria, Australia