Electropolish, Passivate, or Chemical Polish 4140 & 4340 Steel?

A. Hi Sakthivel. Nothing wrong with circling back to see if anything has changed, but I don't think it has :-)
4340 has < 1% Cr & < 2% Ni -- it's not stainless steel :-)

Luck & Regards,

A. Hi Mehdi. I hope your colleague recovers from that bus incident :-)
Passivation is a treatment for stainless steel which dissolves the iron on the surface, leaving a chrome-rich oxide layer. Unfortunately 4140 is not stainless steel; and its very small chromium content does not allow for this passivation to occur.

To determine whether embrittlement may have occurred probably should involve a two-pronged approach consisting of a careful process audit (and at this point we don't even know yet what the passivation solution was: citric acid, nitric acid, or nitric plus chromic acid) plus notch testing.

In thread 11297, Brian Terry and Ray kremer seemed to agree (at least in 2015 -- check for updates) that passivation does not cause hydrogen embrittlement ... but this only reinforces the need for a careful process audit because the passivation step per se may not have been the only operation that was done on the parts.

If the components must resist corrosion, some possibilities for surfaces which would not be softer than the substrate include chrome plating, electroless nickel plating, or salt bath nitriding, but at this point I can't say whether your parts are recoverable -- "the moving finger having writ, moves on, ...", etc.

Luck & Regards,

A. The best way to approach this problem is with a "chemical polish". It is non-electrolytic and produces excellent results on mild steel. It is available from several sources.

A. Jim:

I don't believe chemical polishing can handle that job. Our company has electropolishing solutions for steel. We also have some experience with 4340.

Regards,

A. I know that a company in Texas has very good luck electropolishing the bores of rifle barrels (mostly 4140). There shouldn't be too much difference between 4140 and 4130. Good Luck.

A. Hi Hassan. I think the main problem here is that you are calling 4340 "stainless steel" whereas it is a low alloy steel, not a stainless steel :-)

I'm not the stainless sheriff, but various sites say that a stainless steel must have "greater than 10%" / "11% or more" / 12-15%" chromium. There may be some highly specialized chemical formulations and operating conditions which can to some extent or other passivate or electropolish 4340 steel, but you're surely right that it isn't easy. But even if you accomplish it, what shall be your basis for calling it passivated or electropolished? When those terms are used there needs to be some performance expectation, not a mere claim that you exposed the parts to certain chemicals.

If I were you I might move directly on to trying to electropolish rather than passivate because after you achieve whatever is your goal for finish and appearance, now you have the problem of hydrogen embrittlement to deal with. You probably will not have achieved substantial corrosion resistance by passivation, but I think it's possible that electropolishing might measurably improve corrosion resistance :-)

Regards and good luck,

A. I agree with Ted. Even-though we have some cool methods of passivating 400- ss (as we call it, or "surgical steel", if parts are stored not in the proper way, rust will appear sooner or later.

Electropolishing is the best way to create thick passivating layer on the surface of 400-series steel.

Barrel or basket electropolishing is the cheapest and fastest way to create strong anti corrosion layer.

A. Indeed, 4340 is not a stainless grade.

Electropolishing increases corrosion resistance by virtue of the smoother surface (so does mechanical polishing), NOT the same as the passive chromium oxide layer that you get on stainless steel by removing iron from the surface in a citric acid or nitric acid passivation bath.

A. Hi Martin. 4140 is not stainless steel and it cannot be passivated in the sense that stainless steel is passivated, or according to the specifications for it. The drawing is wrong. Read the previous responses on this thread we appended your question to ...

However, there is a bit of a semantics issue that is sometimes confusing to everyone, viz., the word 'passivation' is just a word, not a specification ... and people who are involved in the passivation of stainless steel can't stop other people in other fields from using that word :-)
People sometimes call the chromate conversion coating process used after zinc plating 'passivation' (thread 55350); people sometimes call the final chromate or other chemical dip after zinc phosphatizing 'passivation' (thread 53135); and people sometimes call the processes done in carbon steel boilers to deter rust 'passivation' (thread 41786).

Luck & Regards,

A. Hi Martin.
We conducted various tests on passivation 400 series stainless steel, and came to a conclusion that only electropolishing provides the reliable corrosion resistant layer. Back in 2012 we provided our UE to Striker in Ireland. They were very satisfied with the outcome for surgical instruments electropolishing for burrs removal and corrosion protection. You can talk to Alan Ronan if he still works there. Also Medical Optics from Florida used this electrolyte after re-sharpening medical instruments of 400 alloy type.
We do not deliver this electrolyte retail, but if you find the shop nearby who is willing to obtain the solution composition and provide the service, contact me directly, please.