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Passivating Stainless Steel with Nitric or Citric Acid?

I'm building a 304 SS beer keg boiler, I'm having some SS ferrules and a drain fitting welded on. My question is, after I have it welded can I use a citric acid bath to passivate the stainless steel to prevent the welds from corroding? I read that a solution of 4% to 10% citric acid and water heated to 150 degrees F will remove any iron from the stainless steel surface and after rinsing with clean water it will promote a good chromium oxide layer to make the SS more corrosion resistant. Manufacturers of SS pots, cookware and other stainless steel products use a hot nitric acid bath to passivate the SS. I can get nitric acid but it's nasty stuff and it has to be used in a certain way or it can ruin your stainless, on the other hand citric acid is said to not attack the SS but only remove any iron from the surface and after rinsing and drying the SS is much more corrosion resistant.

Comments

  • You can, but I wouldn't worry about it so much. Certainly I wouldn't bother with nitric acid.

    It's nice to have some citric acid around for cleaning your copper column components, and it's easy to find in bulk food grade for pretty cheap. You'll get use out of however much you buy. You can do your initial cleaning run with a citric acid wash, smells a whole lot better than vinegar, cheaper too. You'll easily hit your temps.

    If you have corrosion of your stainless welds without passivation, you've got a welding problem, passivation isn't going to fix that.

    That said, the inside of a still boiler is pretty aggressive (mildly acidic and hot), I'm sure any minor surface corrosion due to weld contaminants wouldn't last long.

  • I have seen some SS weld joints turn yellow and brown where the wire was fed into it. What about pickling right after the weld is made? That pickling paste is some horrible corrosive fuming stuff but it will remove any scale or discolor from the weld. I'm not a welder but I did a lot of sandblasting and I always thought that if you didn't remove that black scale and discoloring from SS welds it will rust right away and I figured it would be a good thing to give it a acid cleaning after having it welded???

  • I just talked to a professional welder and he told me that when you weld 304 SS it breaks down the chromium oxide layer no mater what. The only way to not have that is to use 316 or better stainless steel.

  • Never heard that. 90% of ethanol plants here in the US use 304.

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  • Your welder will pickle it as a matter of course if he's any good. Certainly for the price your being charged you'd expect him too. They have an electrically charged machine to do it with.

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  • I believe the pickling is what brings the welds back to looking like clean stainless steel but when I get it back I'm still going to wash it good with the citric acid bath.

  • It won't hurt. Seems to me that backset is as acidic as a regular citric acid solution anyway, so it's certainly going to be something that your boiler deals with very regularly.

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  • edited July 2015

    Tricleanium is the ticket for pasivating stainless, it is TSP 1kg is about $15. Look near the paint section at Bunnings. Also very good for beer lines. Afterwards starsan or other sanitiser. Stainless is good, easy to use but the chemicals are different.

  • I never heard of tricleanium and There is no Bunnings where I live.

  • Oh, no wonder, your in Australia, I'm in the USA.

  • edited July 2015

    I know for certain 304 stainless does not like chlorides. Here is chloride corrosion of a 304 suction side flange on a brackish water pump, after just six months in service. I specified all 316, but they did not listen. I'm sending the flange back for enshrinement in their hall of shame... (I also changed out the flanges with victaulic connections in 316)

    Do not use chlorine bleach to clean/sanitize 304 stainless...

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    I'm more like I am now than I was before.

  • 316L for brine I think.
    A green courer and elbow grease works ok for me.

  • Shit, now I am wondering if the chlorine added to my cooling tank would cause problems.

  • @Kapea said: I know for certain 304 stainless does not like chlorides. Here is chloride corrosion of a 304 suction side flange on a brackish water pump, after just six months in service. I specified all 316, but they did not listen. I'm sending the flange back for enshrinement in their hall of shame... (I also changed out the flanges with victaulic connections in 316)

    Do not use chlorine bleach to clean/sanitize 304 stainless...

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    I've been reading up on the subject and I know you should never expose 304 Stainless steel to chlorides or any kind of chlorine bleach sanitizer cleaner, It will cause small pitting in the protective chromium oxide layer and once that happens the free iron underneath will start rusting and once it starts the SS has to be re pasivated if its not too late like in your photo. Even regular 316 SS will rust from chlorides exposure but it just takes much longer to happen. They have a 316 surgical alloy steel that has Molybdenum and Titanium in it that resist chlorides and salt water but it's very expensive. What kills 304 stainless steel is free iron. Stainless steel has Iron in it but it's protected from rusting by the chromium oxide layer on the surface of the steel, once that layer breaks down it's over with. Any time stainless steel gets machined or welded on or cut with a plasma cutter or a torch it has to be cleaned and then pasivated. If it has been cut that new surface can rust because it's raw metal and if it has been machined it can rust because the tools used to machine it have free iron in them and tiny pieces of the cutting tool have been imbedded in the SS, if that free iron is not removed by passivation the SS will turn yellow brown and start rusting. Another thing is that if you have stainless steel in direct contact with regular steel or a ferrous metal that has free iron in it, it will right away start rusting at the contact point especially if you have brackish or mineral water going through that union (Galvanic corrosion). You can put copper or brass in contact with SS and it will last a long time because copper and brass has very little or no free iron in it. I have silver soldered stainless steel to copper and had no problems with it. I use brass fittings for copper to copper connections on my distilling parts but the brass doesn't come in contact with any hot vapor or liquids because brass has lead in it and I don't let anything with lead in it to come it contact with my liquor. Here is a photo of a small piece of 304 stainless steel that has rusted because I drilled a hole in it and I grinded it with a bench grinder that I had before used to grind regular steel and that regular steel on the grinding stone contaminated the stainless steel, also photo of Stainless steel soldered to copper with no problems.

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  • In your Chloride Corrosion Photo It looks like you threaded a piece of regular iron steel pipe into the Stainless steel flange, That caused both Galvanic corrosion and corrosion from the iron in the regular steel pipe contacting the SS. You can see where the rust is inside the regular steel pipe and where it continued into the stainless steel. That's a common mistake.

  • @Thomasedwin said: In your Chloride Corrosion Photo It looks like you threaded a piece of regular iron steel pipe into the Stainless steel flange, That caused both Galvanic corrosion and corrosion from the iron in the regular steel pipe contacting the SS. You can see where the rust is inside the regular steel pipe and where it continued into the stainless steel. That's a common mistake.

    X_X I have popcorn.

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  • @Smaug said: Never heard that. 90% of ethanol plants here in the US use 304.

    That's true but in manufacturing when they weld those big 304 SS tanks they remove that broken down outside layer with strong acids to get back to the clean stainless steel then they wash it in hot nitric acid and rinse it with clean water and let it dry and it forms a new chromium oxide layer protecting it from corrosion. According to what I have read if you have SS parts welded you can first clean away the heat bluing and black crust from the stainless with something called Pickling Paste (Or the welder does it in the shop right after welding) Pickling Paste is a super strong corrosive mix of hydrofluoric and sulfuric acid that removes the broken down layer from the welding and some of the good Stainless steel so it's a new clean surface then you can use citric acid instead of nitric acid to give it a final cleaning and after rinsing with clean water and drying it, it right away forms a new chromium oxide layer at home. Citric acid does the same job as nitric acid it just takes longer but it is much safer than nitric acid so you can do it at home Plus citric acid is easier to get for home use than nitric acid. They use hot 50% concentrate Nitric acid for their final wash and that stuff is dangerous and sometimes explosive without the right conditions and it lets off terrible bad fumes that will burn the hairs right out of your nose. That pickling paste is even a stronger acid but it's safer to use because you only need to use a little bit painted on the weld with a brush. I have seen photos of them applying pickling paste and they ware a protective suit with long protective gloves and a face shield and mask. They say you shouldn't even handle the closed container without protective gloves. You can buy Pickling paste but it cost like $50. something for just a small container. That Hydrofluoric acid in pickling paste is poisonous to the point that the fumes will kill you and it seeks out calcium. It attacks the calcium in your blood and if you get it on you skin it goes through and attacks your bones. I'm going to have the welder do it at the shop before I go to pick up my keg and I'm going to do a final citric acid wash at home. Citric acid is used to add tartness to soft drinks so it's safe to use at home, It has no fumes and you can leave your stainless steel parts soaking in it over night and it won't hurt them like nitric acid will if you leave it in there too long. You can have a metal plating shop do the whole process for you but it will cost you because they do mostly large orders not one piece orders. When I get done with my keg boiler I will expect it to last a life time.

  • edited August 2015

    Today I received my 5 LBS bag of citric acid in the mail so I decided to do a little experiment to see how well it would clean up rusted SS. I also tried a piece of brass and a piece of copper. I used a good heaping TS of the citric acid powder in about half a cup of hot tap water for each one and let it soak for about 40 mins. Here are before and after photos, It cleaned up the SS like new and it did a pretty good job on the copper but a not so good a job on the brass but this is just letting it soak in the solution, if I used a scrub brush or pad I believe it would have done a much better job. As you can see by the stainless steel I think it would do a bang up job on a beer keg just by filling it with hot water and acid powder and leave it soak over night. The acid water solution is supposed to be 4% to 10% by weight and heated to 150 degrees F but I didn't even do that.

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  • edited August 2015

    @Thomasedwin said: In your Chloride Corrosion Photo It looks like you threaded a piece of regular iron steel pipe into the Stainless steel flange, That caused both Galvanic corrosion and corrosion from the iron in the regular steel pipe contacting the SS. You can see where the rust is inside the regular steel pipe and where it continued into the stainless steel. That's a common mistake.

    Nope, that is a schedule 80 PVC nipple screwed into the flange. No galvanic corrosion, only chloride embrittlement. Tiny flecks of stainless steel (fractured by the force of the gasket sealing on the bearing surface of the flange) were caught on the surface of membranes downstream.

    In highly concentrated chloride locations I spec 904L stainless steel. Expensive up front, but the short payback period from its extended service life makes it an easy sell.

    I'm more like I am now than I was before.

  • Old thread but I thought I'd add a definitive answer from NASA!

    Citric Acid Passivation of Stainless Steel (PDF)

    Conclusions on page 49, TLDR; 4% citric acid at 140F for 120minutes takes longer but is potentially slightly better than standard techniques using nitric acid.

    more reading in standards ASTM A967 and AMS 2700 which both allow the use of nitric or citric acid.

  • edited June 18

    I can only say if you want to passivate stainless steel use an oxidiser, too do this the acid has to be relativ strong(nitric acid) if you manage with anything else ok. Might be clean after a threatment with whatever, but dont use any acid with chlorides in it. Hydroclorid acid is not the answer.

  • edited June 18

    A 15% hydrocloric solution (like Sno bowl) works just fine.

    What is your issue?

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  • when researching, I found the mix you can substitute when doing electric weld passivization is 'ice maker cleaner'..

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