Stainless corrosion. Passivate or no? 2

[limey1]

I am a mechanical Engineer and Im trying to get a better understanding as to what happens to stainless tubes when you use an abrasive cutoff + deburr. If you dont flash pickle, is the resulting corrosion caused by iron/contamination deposits, or is it thermal oxides, preventing air passivation from taking place. FYI the cut-off wheels are bonded aluminum oxide, the deburr is stainless wire brush. I dont have a scanning electron microscope nearby so I am unable to verify what is happening, if anything, to the parent material.

 

[gr2vessels]

limey1,
Are you sure the discoloration at the end of cut is corrosion? For "iron" contamination you need somehow the presence of iron contaminants in the vicinity of you stainless steel tubes, which you don't have apparently.
I assume also that the stainless steel tubes you mentioned have more than say 13% Chromium, which is enough for the restoration of protective layer of chromium oxide. There are other corrosion activities to affect the stainless steel, but those are related to the "corrosive" environment and process fluids and are function of time (much longer than that required to cut the tube). If the appearance is the issue, pickling is your answer, otherwise try slowing down the cutting disc and/or ignore the discoloration.
cheers,
gr2vessels

 

[limey1]

As far as iron contamination, would it be possible that as the deburr wheel brush wears (Brush is S/S),it could transfer to the ends of the tube. I am looking at cold-saw technology, as this would give me a cleaner cut, with less heat being put into the tube. Having less to deburr would also mean I could use a less aggressive method to deburr the ends. Does this make sense? The goal is to not have to flash pickle.

 

[stanweld]

Water cooling (flooding) during abrasive cutting should abate the heat tinting observed.

 

[EdStainless]

You may be getting rusting from the fine particles of SS that have been left by the cutting. The discoloration and non-metallic residue sure don't help either.
Cold cut with a good coolant will reduce your issues, but after brush deburr a flash pickle or at least a good passivation is a good idea. Maybe a citric acid passivation would be enough?

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Still trying to help you stop corrosion.
formerly Trent Tube, now Plymouth Tube

http://www.plymouth.com

 

[limey1]

Cold-saw with coolant definately seems to be part of the answer. As far as deburring, ive seen some technology that uses a 'finger' belt sander which cleans up the I/D and O/D. Im hoping to get adequate passivation without pickling...period!! As far as defining the current process, I was thinking of performing a free iron/ iron oxide contamination test (ASTM A380, section 7.3.4). I believe its also known as the 'ferroxyl' test. At least this test should tell me wether I have an iron issue, and if not, could it be thermal oxidisation. Any thoughts/ opinions would be appreciated.

 

[rustbuster]

Copper sulphate solution would indicate the presence of Fe. Brushed on the area if Fe is present it will turn pink.

 

[EdStainless]

If you want to just test for Fe you can use white Head and Shoulders shampoo. Any free Fe will cause blue spots.
There are other problems besides Fe that can lead to rust or blushing on SS.

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Plymouth Tube

 

[limey1]

Thanks for your input Gents, I performed the Ferroxyl test which came up positive. Ed, thanks for the Head and Shoulders test. I will check with the lab, but I think they only stock Pantene :]

 

[strider6]

can this article help?

HEAT TINTS ON STAINLESS STEELS CAN CAUSE CORROSION PROBLEMS
The dark heat tint formed alongside welds during welding of stainless steel (SS) the heat-affected zone (HAZ) is a thicker chromium oxide scale with a mixture of iron, nickel, and other oxides. This thin layer is lower in chromium, the primary constituent that gives SS its good corrosion resistance. Corrosion that would not occur elsewhere can initiate in the HAZ unless the heat tint scale and the thin chromium-depleted layer just beneath are removed. Removal by rotating fiber brush, pickling, or electropolishing readily restores this area to base-metal resistance.

http://www.corrosionsource.com/technicallibrary/nidi/literature/pdf/14050.pdf

S.

http://www.corrosionist.com

 

[EdStainless]

The reason that H&S is blue in some parts of the country is that they make it in systems that have some CS piping and it picks up Fe. They dye it blue so that each lot comes out the same color. In full SS systems they can keep it white.
Smear it on and use DI water in a mister to keep it moist.
I love to shame fabricators.

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Plymouth Tube