Clean, Pickle & Passivate 304 SS - Problems

[Mike31416]

I've been working on a problem with passivating 304 SS tubes for some number of weeks. I've combed through ASTM A967 and A380 several times. Our process, at face value, meets the specification. We use an alkaline cleaner (tripolyphosphate based), followed by 20% nitric acid & 1% ammonium bifluoride pickle then 25-45% nitric acid for passivation.

The net result is a black carbon smut that is not visible, until wiped with a lint free wipe dampened with acetone or even water, which produces a smudge on the wipe. The carbon content has been confirmed by Auger analysis and our Cr:Fe ratio have measured as low as 0.3 : 1.

We've recently had success using caustic potash to remove the smut and ultimately achieved passive layer through citric processes by an outside consultant. Same material, same heat, even previously cleaned through our processes yielded 1.6:1 ratio.

My question now is whether one of the cleaning agents jumps out at someone like the tripolyphosphate as an agent that may induce carbon formation. Or whether this sounds more like a case of sensitized stainless as in the carbon may have been baked in the surface from improper cleaning or a dirty oven or defective cooling cycle.

Notes:
We use a branson ultrasonics for cleaning and DI rinse.
Carbon content of 4% at a depth of 450 angstroms.

Thank you in advance,

Mike

 

[EdStainless]

I am concerned about your pickle. We are using nitric/HF pickle so it is hard for me to comment.
I doubt that the smut is carbon. It is metal, mostly Fe with some Ni. The particles are sub-micron in size making it look black. Take smears with synthetic wipes and have them dissolved in acid for ICP analysis.

How hot is your pickle? Are the tubes clean out of pickle? They need to be. What is the metal content of the pickle bath?

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Plymouth Tube

 

[unclesyd]

What type of oxide/stains are you trying to remove with the pickling step?

Can you give some times and temperatures?

How many cycles on your pickling bath?

 

[Mike31416]

Ammonium bifluoride appears to be a common substitute for straight HF, I guess because the AB is a solid and subsequently easier to store, ship and handle.

The pickle bath never exceeds 120F and may sometimes be performed at room temperature. The samples which were sent for analysis were at 120F.

Order of operations: 130F alkaline ultrasonic bath 10minutes, DI ultrasonic rinse 5 minutes, pickle @ 120F Max 10minutes, DI spray rinse, DI ultrasonic rinse 5 minutes, nitric passivation (75F typical), DI cascade rinse 5 minutes each triple baths, blow dry with compressed air.

These are real typical processes, the only thing I've thought we missed was a neutralization step after acid processing. I'm accustomed to sodium bicarbonate following acid.

Pickling is used for a few different oxide removals, but the primary use is for black/grey oxide from electrolytic saw cuts. Our customer recognizes it as a pretreatment to precede nitric or citric passivation.

We had smears analyzed from another product and found Fe, Cr and Manganese present in the ID.

We also do see a high carbon content with auger analysis.

I'm certainly more of a tool designer than a chemist and have had a real crash course in metallurgy lately. Would the consensus lead us to believe that this is really bad material or poor cleaning processes?

Thanks,

Mike

 

[EdStainless]

So you may have carbon from the solutions used in the cutting operations. You need to find a way to remove those.
Pickle hotter.
Is your spray rinse a pressure blast or just a garden hose flow?

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Plymouth Tube

 

[unclesyd]

Smut is common problem when pickling SS especially #04. A you have witnessed the smut is quite tenacious. When we pickled a lot 304/304L pipe we would essentially rinse with saturated steam. I've had more problems when using ammonium bifluoride than when using HF, especially on delicate parts.

Things to check:

Does your alkaline bath have a rinse aid incorporated in it.

Don't let the part dry down before it is completely rinsed.

You need to use top shelf ingredients.

Is your present procedure doing the job other than the smut?

 

[Mike31416]

Spray rinse is garden hose.

Pickling removes the oxides with relative ease. The parts appear clean after both alkaline cleaning and pickling, yet smut still remains.

More insight, we also have electropolish in-house. I ran a fairly heavy ep and even that didn't completely remove the smut. That part failed Auger with Cr:Fe @ .9:1.

I've been in this industry using the same suppliers and manufacturing processes for nearly two decades now and have never encountered such problems, particularly with smut.

Kind of back to my original question, anyone see this condition occur from poor annealing processes?

Otherwise, I'm confined to changing my cleaning process only, which is fine if that is true, but if there is a chance that I could eliminated it through raw material specification, that would be even better.

Ed, I've been talking with Andy and left you a voice mail yesterday. Seems this conversation would go much quicker via phone. Sometimes email is just too many words.

Thanks again,

Mike Doyle

 

[metman]

Mike31416 ,

You mentioned sensitzation. This can be clearly revealed metallographically with a very small sample.