Citric Acid Passivated 420SS 3

[NickE]

We have a part produced from mod420SS (420 + 1.0Mo) that was passivated using Citric Acid from Stellar Solutions. Is there any need to bake these parts? Should I be worried about hydrogen embrittlement?

thanks
nick

 

[mcguire]

If the hardness is over Rc22 they will be susceptible to hydrogen damage. There can be residual hydrogen from processing prior to your passivation, e.g annealing, pickling. You can't rule out hydrogen from these sources, even if the passivation didn't add any.

 

[EdStainless]

Passivating with citric or acitic acids is nice because you don't have to worry about H2. They are slow, but they work. Was a bake part of hte original HT process? If not it might be a good saftey precaution.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[NickE]

ED-Stainless- wow thats great I like the Citric cause its not too dangerous or toxic.

The steel is Sandvik 7C27Mo2 (420mod), we recieve it in the hardened and pre tempered form. (~53HRc) Stamp a flat part out of it and then finish the edges and surfaces to add residual compressive stresses (~120ksi). The problem we're having with one part is that its in a wet envronment and we see failure due to corrosion and pitting. I thought it might be due to the presence of Free Fe on the surface from the finishing process. (We process 1095 parts in the same machines sometimes.) Passivation was performed using solution #2250 mixed six to one, and hot water (~130-160F) was used to mix. Time was 30 min w/ agitation every 5.

We are systematically eliminating causes of early failure of this part. We need to keep the really high mechanincal fatigue life of the base material, but would like corrosion resistance of one of the softer grades.


nick

 

[BobUXL]

One thing that you may want to consider about citric acid. I have been told that citric acid is a good chrome and nickel getter. So reprocessing of the acids become very expensive and in some cases, reprocessors are dumping the acids due to these reasons. Has anyone else heard of this?

Bob

 

[mcguire]

Citric acid may clean and passivate well, but be careful not to assume it is pickling. It is not strong enough to remove the chromium depleted layer which comes from oxidation. It will still be there after the oxide is removed. This means that corrosion resistance will be compromised compared to strongly pickled surfaces as would come from the mill.

 

[EdStainless]

In general, when using citric acid you will not remove enough metal to have problems with Cr+6. That is the same reason that it is not a pickle, not aggressive enough for removal of the Cr depleted layer.
There are comercial additive packages that you put into waste acid in order to make sure that there is no Cr+6.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[israelkk]

I hope no one is trying to re-invent the wheel. Paasivation of stainless steels should be done according to common commercial, military or aerospace specfifications. QQ-P-35C "Passivation treatment for corrosion resistant steels" which is avaiable at ttp://assist.daps.dla.mil/quicksearch/ specifically dictate nitric acid. For AISI 420 steel sodium dichromate additive is needed. Cleaning and descaling before passivation should be done according to ASTM-A380 or MIL-S-5002. When electrochemical cleaning is required it sould be performed in accordance with ASTM-254.

Regarding hydrogen embrittlement, removal of the oxide layer after heat treatment using chemicals may cause hydrogen embrittlement. Therefore, it is best to use mechanical cleaning if possible. Otherwise, according to military standards steels treated above 36RC (180ksi Ultimate tensile strength) should be baked for hydrogen embrittlement relief.

 

[NickE]

ok some answers.

(first please read my previous posts for more info on the process and this part)

I used standard ASTM A967-01 for my passivation spec. We use a high energy mechanical (wet)abrasive method to reduce the stress concentrations and induce very high compressive residual surface stresses (~120ksi). Along with 7C27Mo2 (~SAEJ405 420SS (with 1wt%Mo added for pitting resistance)) we also use 20C (~SAEJ403 1095). Both of these steels are mechanically processed in the same machines and there is a chance that there is some free Fe embedded in the part after this process. Currently our prototypes are reaching >300hrs in test, the first goal is to reach 500hrs. Samples that have been sent back failed show extensive spotty rust/pitting. These rust spots seem to originate from a localized point on the surface of the steel.

We dont really want to add an outside process. We have a current extensive water handling system. We would like to avoid any concerns with highly active acids such as nitric. A google search pulled up:

http://www.stellarsolutions.net/2250data.htm

We requested a free sample of concentrate and mixed it with hot water from the coffe machine, then placed the parts into the bath. Agitation and movement were done by hand every 5 minites for 30 mins. The parts were then rinsed in water air dried and packed for shipment w/o contacting any skin.

The hope is that at this time with the addition of passivation we can hit the first targeted hour goal.

Any other comments?

Nick