Cast stainless passivation issue?
Cast stainless passivation issue?
(OP)
Hi,
I have a stainless piece from an outside supplier and before it was installed it was observed to have a discoloration all over it. Since the discoloration/corrosion is uniform I suspect that somehting happened during the passivating treatment. Maybe a contaminated passivating bath or something like that.
Anyone had similar issues? The alloy is CE8MN and you can see the difference with a normal piece in the picture.
Thx
Herc
I have a stainless piece from an outside supplier and before it was installed it was observed to have a discoloration all over it. Since the discoloration/corrosion is uniform I suspect that somehting happened during the passivating treatment. Maybe a contaminated passivating bath or something like that.
Anyone had similar issues? The alloy is CE8MN and you can see the difference with a normal piece in the picture.
Thx
Herc





RE: Cast stainless passivation issue?
We added passivation and now get a rust colored discoloration. Still haven't pinpointed the cause, but contaminated passivation bath is high on the list of possibilities.
James Spisich
Design Engineer, CSWP
RE: Cast stainless passivation issue?
Is it a periodic issue you are dealing with?
Any other parts that you take through the same passivation bath?
Got any pics?
RE: Cast stainless passivation issue?
_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year
RE: Cast stainless passivation issue?
RE: Cast stainless passivation issue?
Drop a little copper sulphate solution on the surface and see if it turns pink in a few minutes. If so, your passivation didn't work.
RE: Cast stainless passivation issue?
We don't passivate cast stainless steel parts and even if we did the results would very so variable as to be meaningless.
If the discoloration isn't allowed due to aesthetic purposed I would leave it alone.
Some thoughts on the discoloration.
Passivation will not remove oxide scales.
Dip the part in concentrated HNO3, Very Hazardous not
recommended.
Run the part through an alkaline rust stripper type bath.
Bead blast it.
For some reason was the part heated after passivisation.
What did the part look like prior to passivating it.
If passivation is absolutely required you may have to do a pickling step prior to the passivation.
If you need a pickling step adjust the bath to the bright side.
Surface chemistry of cast SS is so variable that it would almost impossible to get consistent results.
RE: Cast stainless passivation issue?
RE: Cast stainless passivation issue?
Then you should get warm 15% nitric to clean things up.
Or to be extra cautious you could blast and then pickle (warm 20%nitric + 5%HF), then pressure wash with clean water. At this point passivation is not really needed.
Passivation is a cleaning treatment meant to remove free Fe from the surface. Nothing more or less. By nature a clean SS surface will form a passive Cr oxide film.
= = = = = = = = = = = = = = = = = = = =
Plymouth Tube
RE: Cast stainless passivation issue?
Time to find a new vendor!
James Spisich
Design Engineer, CSWP
RE: Cast stainless passivation issue?
Unfortunately the trend is to manufacture all alloys under one roof,using the same facility. They make ductile irons and stainless steels ,ductile iron and high chrome irons,(Cr is a deleterious element for ductile iron),without batting an eyelid. Extra low carbon heats are produced in the same crucible. I fail to understand and appreciate. I would take extreme care while changing over from carbon steel to stainless steels. For extra low carbon heats,I would have a wash heat of stainless steel. Perhaps,this is one of the reasons, I lost a lot of money and eventually closed down.
I had doubt of iron contamination from the foundry,but thought that it was too elementary to highlight.
Glad that you could identify the fault and are on a correction course.
_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year