17-4PH Passivation Problems

A. Chris,

citric acid passivation, also in ASTM A967, can be used without fear of adverse reactions from having both 303 and 17-4 SS.

adv.
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A. Tony,

Passivation of stainless steel requires the removal of free iron from the surface and then the generation of the oxide film that gives the stainless its rust resistant properties. I would check for the following.

1) The surface presented to the passivation chemical is clean enough for the chemistry to wet out and react with the steel.A poorly degreased surface may prevent a proper reaction. There are available passivation chemicals that can clean as well as passivate.
2) The passivation chemical has done its job. citric acid based materials may not penetrate as well as Nitric based systems.
3) Rinsing after each stage is thorough. Any highly alkaline chemical left on the surface will neutralise the citric acid and any acid left in the pores of the metal could promote corrosion sites. Poor rinsing is a common cause of surface staining.

A. Hi, Tony:

Investment castings of 17-4PH are not easy to get passivated correctly with either nitric or citric formulations, but a process can be developed to do it well. We have had great success with most customers with the citric formulations -- producing excellent passive parts. Sometimes it is a matter of good cleaning first, as John says.

The citric passivation actually goes DEEPER than the nitric, as born out by University studies, and there is no problem with residual citric left in pores due to poor rinsing (like there is with nitric). There are new formulations in development with citric that should do a better job on this kind of application.

adv.

Great success today with new formula using citric acid on 17-4PH investment castings. Still working on exact process, but this should be an answer to your problem. Excellent corrosion resistance and no staining.

A. The reason that whole surface is not bleeding pink is because that this surface is covered by oxide film. If you hold passivation liquid longer on the surface, the whole area will bleed pink.
Passivation with proper liquid will save the laser marking and protect the surface without discoloring.

adv.
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A. Hi Oleg,

We have developed our own passivation solution for martensitic stainless steel. It is not based on Nitric or citric acid s.

adv.
The solution (and gels) were successfully implemented at Bama Technologies in Italy. Contact me for more information.

A. Oleg,
Much of the problems and failings of passivating castings result from the very bad surfaces that some castings have. The greater the roughness and porosity, the harder it is to get good results. Treatments such as sandblasting, electropolishing, and vacuum impregnation can help.

I could try to hazard a guess at what you can do to improve things if you describe your current process.

Tony,
I would ask which product of ours you are using, perhaps you should contact me directly.

It's somewhat likely that the issue is a result of it being a casting in the first place, the surface of a casting is generally not very consistent or friendly. Are you doing any surface prep work like bead blasting?

I would go ahead and test a few parts in a pickle-then-passivate process to see if it helps any, before making any decisions about whether that's the direction you want to go.

I can recommend local electropolishing - only at the area of laser mark. It will remove all alloy alternations that are caused by laser heat.

adv.
Contact me for more information.

A. Patrick,
Laser marks are tricky regardless of the grade. It usually takes some experimentation to nail down the best laser settings and passivation process.

(adv. Please contact me if you would like some tips on that.)

I only rarely see a laser mark that can pass the copper sulphate test. Clearly there is a lot of iron in laser mark, which is also why the passivation acid ruins it when you leave it in too long. I typically skip copper sulphate on laser marks and use a water immersion test instead (usually the boiling version).