304 stainless pitting issue - looking for source of chlorides

[kelcgza]

Helping out on a problem in a 304L part that sporadically has pitting after deep draw process.
I honestly don't know all the details yet of the exact processing steps other than what I can glean from the failed part:
-Deep draw in at least 2 steps (there are remnants of a shockline on in the ID from the first hit)
-post-forming anneal (annealed microstructure, fairly mixed grain structure, but definitely not worked)
My guess is insufficient rinsing and drying based on the pitting pattern on the parts - pitting worst in the bottom ID radius of the cup as if liquid remained in the bottom.
Typically not caught until the pits perforate to the OD.
The pitting is almost certainly caused by chlorides from an unknown source.
- lubricant, unlikely, but I'm not sure
- cleaner, probably alkaline, maybe it has an inhibitor preventing passivation?
- rinse water, will suggest analysis to be sure.
anything else I'm missing?

 

[mfgenggear]

OP
Inthe first plant I worked they used deionized water to rinse.
If the are using city water, there are chemicals.
Sample the raw stock to eliminate contamination from the raw manufacture.
Then ask the sheet metal forming shop for a router. View and analyze the steps. Then audit the manufacturing steps. Its the best way. Take notes,
Just don't interfere or bother the operators just monitor. I

 

[kelcgza]

I knew I posted too soon...
-lubricant is a chlorinated parafin (sigh)
-rinse is city water

I wanted to be prepared to suggest all the places to look, but I didn't have to look hard.
Ultimately, I suspect it is a poor rinsing issue.
  

 

[EdStainless]

What a combo.
Their last rinse should be hot DI.
And make sure that the parts are able to drain while they dry.
I'll add that we had a mandatory time limit of 4hr between process and final rinse.
Production just had to plan around it.

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P.E. Metallurgy, consulting work welcomed

 

[mfgenggear]

Op
Don't stop there analyze the entire process, good luck.

 

[mfgenggear]

Yes in addition we use to use packing material that was neutral. No chemicals. Only approved pickling, neutralize , and rinsing techniques.

 

[kelcgza]

even worse...no agitation during the alkaline cleaning prior to passivation. (extra large sigh)
Pitting only occurs sporadically, so 99% of the time the process has been fine--slapdash, but fine.
They are going to switch to non-chlorinated products and DI water.

Also by the way, I've been a lurker for years and relied on the collective 'you' for helpful insights. So thanks all!

 

[mfgenggear]

OP
I observed parts in stores started corroding , as ED says once it starts
Hard to remove. Make sure it gets a good clean and rinse. May be Ed can advise more. So my point is visually it looks good, but as time passes parts start visually corrosion.

 

[EdStainless]

What are they using for passivation?
Draw, clean, rinse, dry, anneal, passivate, rinse, dry, or something like that.
Getting things in the correct order is as important as doing them correctly.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[kelcgza]

Citric passivation.
Draw, clean, rinse*, passivate, vacuum anneal, restrike for final part shape, clean, rinse, citric passivate, vacuum anneal. Double the chances for pitting!

*it wasn't clear to me how or if they dried the parts, I suspect they clean-rinse-dunk directly into the citric in the baskets used for cleaning.
I also didn't get timing on how long the parts lay around with draw lubricant on them before cleaning.
They are switching out the lubricant and the rinse water, but there are many other places to clean up the procedures.

 

[dik]

hot DI?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[EdStainless]

I hate to say it but there is no real reason to passivate prior to anneal unless they are hoping to clean the parts after a poor cleaning.
Citric passivation, hot? How do they monitor bath chemistry?
Citric passivation is slow. At least compared to warm 40% Nitric.
Our rinse was two baths.
The last one was fresh DI with continuous (while we were operating) makeup.
We had conductivity meter and alarm on this tank.
This tank overflowed into the first rinse tank.
If they change lube they may need to change cleaning chemistry to get good results.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed