Dear all,

Here comes the Pickle & Passivation discussion again. Hold on tight.

I have stainless steels castings that have already been annealed and pickled&passivated.
Then I machined them with steel tooling (clamps etc). The pieces are relatively big so there was a high clamping force and whatever that entails regarding smearing or impregnating Fe particles on the surface.

I think that I need to passivate them. Does anybody else here think I also need to pickle?

In the ASM Stainless Steels Handbook it recommends just Nitric Acid for cleaning passivation. Nitric and HF is for welds.

Many people tell me that Citric acid can also be used for cleaning / passivation.

Not enough information to respond to your question.

What is the end use of the final machined castings that would require another pickling and passivation step?

They will be used in offshore saltwater application. Low temp.

I just don't want any surprises like blooming etc..

Machining itself is enough reason to re-pickle if you want optimum corrosion resistance. Machining exposes surfaces which have some chromium depletion. I'm currently running a battery of tests to quantify this.

Michael McGuire
http://stainlesssteelforengineers.blogspot.com/

You didn't say what the alloy was, but it doesn't really matter.
I would pickle.
What worries me about machining is the risk of smeared or folded metal forming crevices on the surface.

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Plymouth Tube

Hercules28,

Machining also works-hardens a near surface layer so if you want optimal corrosion resistance that needs to go via pickling.

MH

http://www.linkedin.com/pub/luke-autry/1b/510/566

Alloy is Super Duplex CE3MN. So all in favor for Pickling!
Thanks

McQuire, why would you say that "machining exposes surfaces that have some chrome depletion?"

I would think that it exposes base metal which has 100% chrome levels.

Pitting initiates at inclusions because inclusion growth in the solid state causes chromium depletion next to their interface. This is the primary reason. Machining has the same effect as abrasive polishing which is demonstrably bad for corrosion resistance

Michael McGuire
http://stainlesssteelforengineers.blogspot.com/

Mc is correct with this. That is why when you test for intermetallics in duplex alloys you need to use a freshly ground surface. Such local defects will have been pickled out of the original surface.

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Plymouth Tube

mcguire (Materials)
26 Jun 12 18:18
Machining itself is enough reason to re-pickle if you want optimum corrosion resistance. Machining exposes surfaces which have some chromium depletion. I'm currently running a battery of tests to quantify this.

Mcquire did you ever finish your study?