Citric Acid Pickling 3

[rnd2]

I would like to share this information.
About 400 small cast 316L parts were pickled and passivated before receiving. The parts require additional work by grinding. Rather than send them back to the foundry (not their problem anyway)it was decided to passivate the parts in-house. It was decided to try citric acid rather than nitric acid.
After a little research it seems about 15% strength at 80 deg. for 1 hour is the recommended treatment and that periods of longer immersion risk surface speckling.
My own observation was that the ground surface edge appearance changed very little, in fact there appeared to be neglible change. Very unimpressive. We decided to increase the temp and immersion time but also add a detergent to remove possible oil contaminants.
The strength was left at 15%, the temperature was raised to about 98 deg C (simmer), about 1/2% dishwashing detergent added and the parts left to simmer for 24 hours in an aluminium container, but resting on a sheet of G10.
The appearance, at least colorwise, of the ground parts now resemble the original P&P.

 

[rnd2]

Oops, the thread title should read Citric Acid Passivation. Sorry about that.

 

[EdStainless]

1. There should not be change in apperance unless there was something wrong with the original surface.
2. Al containers are a bad idea. You don't need any galvanic issues.
3. Wash first, then passivate.
4. There are comercial Citric preperations available that have many aditives to 'help' the process.

They are probably plenty good. Citric doesn't form the best passive layer, but it is a lot better than nothing.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[unclesyd]

Agree with EdStainless you have gone on step too far. Pickling can and normally will alter the surface, especially if you have any amount of scale present. Passivation should ever alter the surface and if it does generally there is no passivation.

Always try to use the lowest temperature possible in any passivation step.

One other point is before any pickling and especially passivation it is best to remove all soils on the surface of the part in a separate step.

 

[kenvlach]

Hi,
EdStainless and unclesyd are correct. You need to do a some more R & D. Before you congratulate yourself too much, or anybody else follows the described procedure, I can state that this is not a proper passivation. Passivated 316L should display no visible change in appearance. Etching, pitting or frosting is cause for rejection (per QQ-P-35C, Paragraph 3.6). A slight discoloration is permitted, but in my experience, this is an allowance for free-machining alloys (which contain lead or sulfide or selenide) and a few alloys high in carbides (e.g., 440C).

Either the alloy has been etched or foreign atoms have deposited on the surface. I don't know what G10 is (galvanized?), but Fe is more noble than Al or Zn which can deposit upon the surface in a displacement reaction.

Process specifications usually prohibit the use of any material other than 316 SS, titanium or polypropylene with passivation solutions without the consent of QC. I know, I was QC & wrote the specs for 2 metal finishing companies.

AMS 2700B Passivation of Corrosion Resistant Steelsis available from SAE for $49.

http://www.sae.org/servlets/productDetail?PROD_TYP=STD&PROD_CD=AMS2700B

ASTM A380 Cleaning, Descaling...and A967 Chemical Passivation Treatments... are available from ASTM for $39 and $33, respectively.

http://www.astm.org/cgi-bin/SoftCar..._PAGES/A380.htm?L+mystore+lyqv5728+1130365781

http://www.astm.org/cgibin/SoftCart.exe/DATABASE.CART/PAGES/A967.htm?L+mystore+lyqv5728

QQ-P-35C PASSIVATION TREATMENTS FOR CORROSION-RESISTANT STEEL, the older but still much used passivation standard (nitric acid solutions, no citric) is available for free at ASSIST:

http://assist1.daps.dla.mil/quicksearch/

Stellar Solutions, a supplier of CitriSurf(TM) formulations, is a good source of citric acid passivation information.

http://www.stellarsolutions.net/

 

[rnd2]

Thank you all for your replies. A bit more information. Well, our aim was to try and match the surface appearance of the original P&P which was done by the foundry, using nitric acid & HF.
A fine taper edge had to be ground on each part and that caused slight straw color heat bands to form right near the edges. 1 hour with citric appeared to do very little, but 24 hours and simmering in the the acid liquor did remove the straw color and restored the surface appearance.
Aluminium container was used because it was convenient and according to my copy of Philip A Schweiter's Corrosion Tables, citric acid at 15% and temperature 93 deg. C. has a corrosion rate of <20 mils /year.
kenvlach: G10 is a class of epoxy glass electrical insulation material. I doubt there was a galvanic reaction because the castings were always isolated from each other as well as the aluminium container.
I do not recommend this as a treatment for others to use.