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Pickling & Passivation of Stainless steel


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I've been making cheese knives out of stainless steel shorts, so unknown grade stainless.

I recently saw a post on instagram where the guy was forging stainless jewellery and he said he was pickling and passivating the stainless in aluminium sulphate before polishing.

Googling explained what the pickling and passivating process is but I couldn't see any references to aluminium sulphate being used.

Also if the passivating process leaves a coating only microns thick wouldn't polishing wear that coating off?

 

Does Anybody have any experience with these processes?

 

 

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I'm in the transportation business hauling mostly bulk liquids in cargo tankers made from stainless.  After repairs we usually have the inside passivated with a weak nitric acid solution and then the repaired area polished.  The tanks are either 304 or 316 stainless steel.  My understanding is that stainless will generally get the chromium oxide layer which significantly retards corrosion anyway when exposed to oxygen, but passivation is much faster and provides a more uniform layer.

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Passivating removes any free iron on the surface of the stainless increasing the concentration of nickel and chromium. As you note, it is a very shallow surface effect, and grinding and heavy buffing will require you to redo the passivation. I also have not seen aluminum sulphate used for this but it may well work. In water, it hydrolyzes producing a mild sulphuric acid solution. 

I prefer to use 10% citric acid in water solution (ASTM A967). I use it heated to 140 F (60C). It should take about 10 minutes. The citric acid waste is biodegradable and the solution will in fact grow mold which is a down side if you are not replacing it regularly. This solution also works well for copper pickling although it is slightly slower than hot bisulfite or sulfuric acid pickles. Citric acid is way safer to use than traditional nitric acid which is a very strong acid.

I passivate to prevent any rust spots that might appear from iron that is picked up during forging using steel tools. If I grind or buff a piece, I don't bother passivating it. If an item will be near salt water, passivation helps a lot to prevent rust.  

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Second the call for citric acid as the passivation method.  We do large stainless fabrications and while most other solutions are nasty and require waste disposal, citric acid generally isn't a big issue.  We use the commercial product Citrisurf.  It comes in different levels of acidity and varying thicknesses from thin liquid to paste.  Not hard to make your own version from even something like "fruit fresh" at the grocery store.

For localized passivation and cleaning, nothing beats good old fashioned toxic and nasty "pickling paste".  We save that for something like cleaning an individual weld spot on stainless to remove discoloration of the HAZ.  Works great and fast for small spots where you wouldn't be dealing with much nasty stuff to clean off after.

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Citric acid is a relatively mild acid that is sold as a powder. It is used in cooling and also in baking products. It is used in middle eastern cooling as well as other styles of ethnic cooking. it also used in baking and candy making. Citric acid is the chemical that is found in sour gummies. (delish, my opinion). It can often be found in super markets, and more likely ethnic grocery stores. Also, stores that sell canning ingredients will have it.

If you cannot find any, one seller is ziyad brothers. www.ziyad.com, which lists where you can find it and also recipes. I read that information from the bottle label, from the spice rack. There are also other sellers

Citric acid should keep for a long time, if stored dry.

SLAG.

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Thats a good tip for using the citric acid thanks Up till now if I needed to pasivate SS I used a comercial paste and it is nitric and hydrofloric acid both of which are nasty nasty stuff. The hydrofloric acid if you get it on your skin can actually damage your bones and the fumes over a long period damage teeth to so Ive been told. Actually on the subject of Hydrofloric acid I have read somewhere that Viton rubber if over heated partly breaks down into hydrofloric acid and Viton is found in alot of things in automotive and engineering related items. Cheers Beaver

 

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Passivation leaves the surface looking pretty much like it did when you started. Dull surfaces remain dull and polished surfaces remain polished. The most obvious change would be that, if you had any rust stains, they would be removed. If the item was forged in a coal forge, some black marks from the fire may be cleaned off as well. Therefore, if I want the piece to look hand forged, I often do not passivate it. In my climate in central NC, forged items left outdoors continue to looked fresh forged long term even without passivation and (my favorite part) no finish.

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He mentions an important thing:  if you are thinking of using it for outdoor stuff in your part of the globe---forge a piece out and let it weather---start right now and get an idea over the years at how well it's gonna work!

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  • 3 years later...

If I were mixing up a passivation solution of water and citric acid powder from the canning supplies section at the grocery store, how much powder should I use per unit of water? And measured by weight, or volume?

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