Cast stainless passivation issue? 3

[Hercules28]

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

 

[DesignerGuy16]

Shockingly similar to an issue we're dealing with on a similar SS cast part.

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

 

[Hercules28]

Interesting Jspisich,

Is it a periodic issue you are dealing with?
Any other parts that you take through the same passivation bath?

Got any pics?

 

[arunmrao]

Have you tried passivating a known piece of 316 material. In case,it does not exhibit the rust color ,the bath is cleared,and you need to look at the part. Perhaps some thing to do with shot blasting or cleaning operation.

_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[gafoorkti]

Please verify the composition of Cu by PMI. If Cu is more, then galvanic corrosion/discoloration can be anticipated.

 

[Ron]

What was used for the passivation? I've seen similar on polished metallurgical samples when bad nital solution used for etching.

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.

 

[unclesyd]

Your part looks strangely like the clamps for a Graylok type (4 bolt0 flange of which we have a couple of hundred. If my memory serves me correctly I have seen any two of the same color.
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.

 

[swall]

With ss castings, you need to do a nitric-hydrofluoric acid pickle first. This will get rid of surface crud from casting, including any burned in sand, investment shell residue, etc.

 

[EdStainless]

An alternate is to abrasive blast, make sure that the abrasive is new (one use only) and does not contain any Fe (blast furnace slag and metal shot are out).
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.

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Plymouth Tube

 

[DesignerGuy16]

Just heard our QC guy checked out the vendor and they had all sorts of iron contamination. Probably using the same sand for cast iron and stainless, not to mention passivation baths, mild steel tables, iron dust everywhere.

Time to find a new vendor!

James Spisich
Design Engineer, CSWP

 

[arunmrao]

Jspisich,

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