Passivation Process for SS316L+304

Maybe Milo will see this and give his expert opinion.

Here's what I know. You should definately use citric acid at 4 to 10% by weight. It's used extensively in the beverage industry. It's safe and easy to procure. There's no particular hazard associated with its use, as with nitric acid, and you can passivate at room temperature. 304 and 316L are both of the same family so the same solution will work for both. What I don't know is how long to leave the parts in the solution. With citric, it's hard to over passivate and it won't etch the steel like nitric can so it's hard to over passivate.

Electropolishing requires equipment that most people don't have.

My vote is for citric acid.

What is it being used for?

Sugar cane press.

316l rollers for crushing sugar cane and 304 as a side pillars, both are new brand but are not shiny or have any plastic covering them

According to Boeing's specifications, to make 100 gallons of citric acid bath, you use 50 gallons of water and 75 pounds of citric acid. The balance of any shortfall can be water. They give the control for the citric acid as being 8.0 to 15.0 oz/gal (60 to 112 g/l). I'm not sure if this is they mean this as the concentration or not. The soak time will be 40 to 120 minutes at ambient temperature, and a water rinse afterward. The parts will need to be free of any scale and grease to be effective. This will work for any corrosion resistant steel.

Hope this helps.

I forgot to mention - the Boeing spec calls out Anhydrous Citric Acid.

Tropicana pulp-free might not work.

The company I work for builds nothing but 304/304L/316L products for the sanitary industries. We have used citric when required by specifications of the customer, but day in and day out on our filter housings and heat exchangers we use an ambient solution of 2 parts weak nitric acid (really about 67.2% acid despite the name) to 3 parts DI water. Time in the bath is at least 1/2 hour. Without any other acids, nitric does not etch ss very quickly, and polished parts left for hours are still fine when removed. (add 5% of hydrofluoric acid, and the bath would quickly etch the surface - if finish is not critical, this will get the iron out a lot quicker, but also be very dangerous - this is our pickling solution) With definite contamination as you have, I would at least go the nitric acid route if you do not want the danger of the pickling bath. Citric acid works fine too for us, but we are very sure we have very minimal contamination at the point of passivation, as we use special and segregated tools to avoid contamination. Therefore, I can't say we have truly tested citric acid under serious contamination situations.

The OP wants to do this "in home". Home is no place for nitric and certainly no place for hydrofluoric.

Oh - home means literally his abode - I was thinking of "in house" as at a small shop not experienced with chemicals.

I do believe he mentioned electro-polishing. That is definitely not a home done project, thus my confusion over the location of the operation.

Yes-I did not recommend the hydrofluoric, but it has been in homes in the past. Commercially sold wheel cleaners used to contain just about the same amount of HF as used for pickling. I know from a bad experience - I got some under my finger nails and the skin turned grey and burned for over a week. Now I know to put some Malox on it - but not then. Anyway ----

I am just not sure with bad contamination that citric acid will do the same job as the nitric.

Nitric when kept ambient is not as bad as most think. At this concentration you never smell it and the bath gives you about a half a minute to wash it off if splashed onto you. Most commercial drain cleaners are far more dangerous than nitric acid. Root remover for septic systems is usually a very strong sulfuric acid and can be used in the home. Now that's some bad acid. Accidentally mix the two previous and the commode comes off the floor. Sometimes a bit of danger is required to get a job done right. Just glove up, goggle up or face mask, and keep it in the garage not the house and in a covered container, preferably sealable when not in use. I have used it at my house for metal cleaning.

bob02 is building a one-off sugar cane press, or at least that's my impression. If one goes back and looks at the history of bob02 they will see what looks like a novice trying to build this machine. No offense bob02 and please correct me if I'm wrong. ^{It wouldn't be the first time.}

You are damn right Bro!!!

In fact I will be the first person of my country to build this machine for home use. No such thing has ever been made for home use but has been made for the road side kiosks which cannot get fit into any kitchen or room.

I will try to make its video as well and post it on the you-tube.

the people that I'm working with, although are veterans but still don't have idea about chemical treatment and other mathematical calculations that may be required to build this kinda machine which is that small.

I'm seeking help cause its always hard to build such thing that has never existed in the past.

sounds detail but confusing..o.k let me sort it out.

a) In plain words....how many grams or milliliters(ml) of citric acid for how many ml of water? what container to be used? plastic, metallic?

b) will the nitric acid leave its poisons on the rollers once the procedure is done? should it be plain water rinse and even after that rinse..will the N.acid residue be their? keep in mind that the product is intended to be used for sugar cane juice so no poisonous residue should be permitted.

c) the dangerous pickling procedure!! I'm ready to try it also at home. Let me know the appratus+chemicals to buy, what container to use? should a pair of gloves be used? what is it dangerous to ???eyes..fingers??

Let me explain what we build again and passivate with nitric acid - pharmaceutical equipment. (tanks for medicines/filters for serums/manifolds for vaccines/etc.) Do you think the pharma industry would embrace the use of something that leaves a residue? We can't even use a grease based abrasive anymore. And, don't ever touch that metal surface with tap water. A good rinse with DI and the nitric is gone. In your case the tap water would be sufficient. The only "residue" left is a chromium enriched surface matrix for a few angstroms, which is the real reason for passivation. Chromium doesn't tend to corrode.

O.K got it. But let me know the three procedures in detail that I've asked for. I'll make its note and will print it out for future reference or procedure. It will be a bit hard to type procedures for all 3 methods that I've asked, but it will go long and even others will benefit from it.

A. I have never had to mix the citric acid (I have done the nitric bath many times in the "old days" when everyone did every job now and then). This is done at our finishing plant on the other side of town. Give me until tomorrow and I will contact them for the mixing directions. I know they also add EDTA as a preservative, but for a one time use, this is probably optional. The citric is on the order of 5% or so. Citric acid is usually mixed in a polyproylene container. (it is essentially vinegar or orange juice and similar handling techniques apply) This should be fairly easy to obtain as it is not an EHS.

B. Covered previously

C. Best to leave this one alone. The HF, although in small quantities, is very dangerous to calcium containing materials (bones). Once on the skin, it heads for the bones and can cause some real nasty effects. This would require full coverage of all flesh. If we get someone exposed to this, we pour a magnesium or calcium containing solution on them and head them quickly to the hospital for continued first aid. Not sure where you are located, but a purchase of the HF by an individual in the USA would probably raise the eyebrows of our Homeland Security and invite a visit from the same. This is no doubt considered a potential terrorist chemical. Nitric and hydrofluoric are both EHS chemicals, but the reporting limit on hydrofluoric is only 500 pounds, indicating that the US EPA does not care for the stuff. I mentioned this procedure before I realized this was truly a "home" undertaking you are involved in. I would not try this at home, even though I have worked it as a with a purchased mixed bath ready for pickling from Oakite.

The nitric should be handled with care appropriate for strong acids and gloves/apron/goggles are recommended as noted previously. Use in a well ventilated area, because some vapors will be present. We keep the bath in a 304 ss container we built 25 years ago. This is how I know nitric won't over etch ss without a second acid present.

I am thinking that for a small application such as this, that you could use a cut lemon and either rub it on or leave the cut lemon in contact with the SS for half an hour then rinse it off - lemons are cheap and easy to use and non-toxic. If you can't get lemons, buy lemon juice at your local supermarket.

Thank you for your detail reply. I've come to a conclusion that C.Acid should do the job cause I bought the product new. But they are using steel tools to craft it out and that is surely going to leave some iron particles in it.

Let me know the procedure, with all ingredients, once you get it.

What about electro-polishing? can that be done at home? I have not received any advice on that?

As promised - citric acid. It will be tough to duplicate. I was not aware that we now buy a commercial mixture called Citri-Surf that has only 35% citric acid and a slew of proprietary chemicals. (surfactants/preservatives)

The guys in finishing mix it 4 to 10% by weight with DI water. I know we initially bought straight citric acid, but that was in the last century (mid 90's) when I got involved in that sort of thing. I am now just what I am supposed to be - an EE.

Sorry I can't provide a straight citric mixing formula.

I guess they dont want to leak their secret formula

any ways..do you have your own recipe??

The reason for the surfactants is to insure a clean surface so the acid reaches all metal ions hidden in the surface matrix. The finishing department circulates the mixture over the ss surface, so the surfactants will clean as the citric acid passivates. The difference between the citric acid and the nitric bath, is the citric will remove the "free iron" but do little to the surface matrix of the ss. The nitric also chromium enriches the surface matrix to insure corrosion resistance. You are looking just for the iron removal, so the citric is fine. As I said previously, I know we used to use just the citric with EDTA as a preservative. Skip the EDTA for a one time use. Be very very sure the surface is free of greases and take the chemical concentration of citric acid in the Citri-Surf mixture of 35% concentration by 4 to 10% with water and you get 1.4% to 3.5% citric acid to water by weight. This should work for you. Time of immersion? Citric can't really etch ss, so let it sit for hours to be sure it has worked.

See Post #6 of CR4 Thread http://cr4.globalspec.com/thread/13272/SS-Weld-rust-stain-removal.

The post that you have sent is very lengthy and confusing..in a sense that it does not direct as to what to do first and then next and so on...step by step.

Can you make it simplified for; what should I do once the rollers are ready..step by step??

All what I have about passivation and pickling: http://www.4shared.com/file/zYNXdbYo/Pickling_and_Passivation_of_St.html
. Please try to read.

Thanks..