Odd Passivity Test Results 2

[seb135]

While testing a batch of stainless steel tubing, it was noticed that there was a small amount of what appeared to be rust on the ID of a tube. The tube was cut open through the center axis so the ID was exposed, and it did appear there was rusting. We decided to test the passivity of the material, but the results were odd. Both ends of the tube tested as non passive, but the center of the tubing tested as passive.

None of us can understand why we would see these results. Does anyone have an past experience with this happening or have some ideas as to what is going on?

 

[EdStainless]

A few things could be going on here.
Remember when you do a pasivation test you are looking for free iron on (or in) the surface.
I have seen cases where the ID near the ends got contaminated when the ends were cut and deburred, and then not well cleaned. The dust from an abrasive cutoff saw will rust, and fail a passivation test.
I have also seen tubes with a little bit of surface oxidation from the anneal process, this is often worse at the ends. If the oxide was not completely removed chemically, the residual material might be Cr depleted enough to fail a passivation test and to rust.

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P.E. Metallurgy, Plymouth Tube

 

[seb135]

Something that I forgot to mention is that we tested the tubing at the site of rusting (rust only appeared on a small stretch of tube), and that section tested as passive. The ends, which were non passive, didn't have any rusting that I could see. We used an open circuit potential tester, so passivity was tested at certain points along the tube. It may be that the rust occurred due to a problem separate from the passive layer.
The reason tubes have been inspected so closely is because we have had problems with pitting. I'm wondering if all of these factors could be related to problems cleaning out the pipes.

 

[EdStainless]

So were these real electrochemical tests that you did or a portable pseudo-pasivation tester?
I believe that cleaning (or lack thereof) is at the root of this issue.

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P.E. Metallurgy, Plymouth Tube

 

[seb135]

The passivity tester we use is the Walter Surfox passivity tester. Do you or anyone else have any information regarding the reliability of this tester? It often seems inconsistent to me, which makes finding the root of our issues all the more challenging.

 

[EdStainless]

There have been papers published, none of the quick test devices consistently match the real test values.
If you have access search NACE paper archives.

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P.E. Metallurgy, Plymouth Tube