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How can we prevent rust inside tubing

 

Sir ,

I would like to tell you our problem. Problem is that I am using iron phosphate. My job is very complicated.

I have followed process advice under

1 derusting
2 water rinse
3 water rinse
4 iron phosphating

Problem is that after process I get bad result on inside of the job (it's hollow). I get fine result on the outer surface but after a few minutes the inside turns rusty. Please tell me a good suggestion immediately.

Thanks,

Shaikh Rashid Mehboob
- Pakistan

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Hi Shaikh,

Are you processing pieces of tubing or is this a continuous process? By your description, I am guessing that it's individual pieces.

Your problem lies in your process sequence. The reason the tubes are rusting is that they are left with an acidic surface following phosphating. If it is possible, I would rearrange the sequence to be: derust, water rinse, iron phosphate, water rinse. If your set-up cannot be rearranged, then I suggest to add a rinse step after phosphating. You can possibly add a rust inhibitor chemical to that final rinse to help retard the onset of corrosion.

You don't mention anything about a dry-off step after phosphating. If the tubes are left to dry naturally in air, then you increase the chance of corroding. An oven dry-off is preferred.

George Gorecki
- Naperville, Illinois

I ran into this problem when working with hose couplings. You trap air inside the tube which means that some of the original rust doesn't get removed and additonal rust can develop in the tube. I got much better results when I put a in-tank filter in the acid/rust remover with the exit of the filter system going through sparge bars under the parts and moving the acid through the parts. I agree with the previous answer with a water rinse after phosphate and a final rinse with a rust preventative in it. But first you have to make sure you have all the old rust removed.

Jeanne Kitazaki
airline - San Francisco, California

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Phosphates give little corrosion protect with out another finish on top.

One answer might be to use a tank of water displacing oil after the final rinse after phosphate.

You would have to have a conical bottom to the tank with a drain tap to drain off the water.

Also it would be an idea to measure the pH of the water drained off to see if the oil is getting contaminated by the phosphate (use simple pH papers).

Martin Trigg-Hogarth
surface treatment shop - Stroud, Glos, England