Passivating stainless welds

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Alright from what I've gathered searching google, it's using an acid to clean the iron of the welds.I got an email from someone, didn't say anything about what he wanted me to weld but wanted to know my welding process if I could passivate the welds and do sanitary welds.From what I found following his website is it's probably home brew setup.  My guess is this is the same guy that wanted me to weld some bungs on some kegs for $20 a keg awhile ago.  So we'll see if this goes anywhere.What are you guys using to get gas to the backsides of the welds when you do sanitary welds on stainless?What kinds of acid are you guys using that I could use for this project.  I'm assuming I'd have to passivate the backside of the weld as well as the surface, so something that gets inside that I wouldn't have a problem cleaning out.  Would my LWS have something for this?  Thanks in advance for your help.
Reply:Passivate.. I learned a new word today. MM350P/Python/Q300MM175/Q300DialarcHFHTP MIG200PowCon300SMHypertherm380ThermalArc185Purox oaF350CrewCab4x4LoadNGo utilitybedBobcat250XMT304/Optima/SpoolmaticSuitcase12RC/Q300Suitcase8RC/Q400Passport/Q300Smith op
Reply:Passivating stainless...removes any foreign contaminants or imbedded metal particles which can compromise the corrosion resistance of the stainless steel.The process differs by the grade of stainless.  Here's a procedure you can use for 300 series stainless steel; which is probably what a home brew system is made from.1. clean welds with SS wire brush to remove and loose oxidation.  Must be a ss brush.2. clean welds and rest of components with a commercial degreaser and rinse with clean water.  Failure to do this may cause discoloration, as the passivation bath may preferentially attack contaminants on the surface3.  immerse parts in a 20% (by volume) solution of Nitric Acid.  The bath should be warmed to a temperature of 120-140°F.  Leave parts in the bath for 30 minutes. do not deviate from any of these parameters, or the acid solution may discolor the stainless steel.4.  Remove parts from bath and flush thoroughly with cool, clean running water.  You may choose to neutralize the acid by immersing the parts in an alkaline solution.  Baking soda mixed with water will work.  Then complete the process by rinsing with clean water.The passivation bath should be mixed using de-ionized water.  If there is residual chlorine in the water, it can have an adverse effect on the parts.If Nitric acid isn't available, you can substitute a 10% (by weight) solution of Citric Acid.  Raise the bath temperature to 150°F.Since acid is potentially very dangerous, you should not perform this operation unless you have the proper ventilation, PPE, and familiarity with the chemicals involved.  Once you're done, you also have hazardous waste that you have to properly dispose of.Realisticly, this is something that should be handled by a specialist.  If you have a chrome plating shop in your vicinity, they can probably handle this for a reasonable fee.  I'd subcontract this part of the job out, and charge accordingly.  Unless you're going to make a business out of this, it's probably not worth the trouble to do it once.Benson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:We use a Walter set up, basicaly all it is is a mild acidc soloution that "reverse electroplates" the carbon contamination from the SS welds. When you back purge, and propperly weld the joint it is not a concern.Disclaimer; "I am just an a$$hole welder, don't take it personally ."
Reply:Originally Posted by TozziWeldingWe use a Walter set up, basicaly all it is is a mild acidc soloution that "reverse electroplates" the carbon contamination from the SS welds. When you back purge, and propperly weld the joint it is not a concern.
Reply:I am not saying that, if you are in a "clean" shop with no carbon contamination, pickeling should not be required.Disclaimer; "I am just an a$$hole welder, don't take it personally ."
Reply:Passivation is the reformation of the chrome oxide layer."Pickling" removes heat tint, embedded iron etc to facilitate passivation."Passivity" is the ease of which a metal can form oxide layers, eg: copper and aluminium are very passive.Stainless steels will form a rich Chrome / oxide layer by simply being left in open air, however contamination, embedded iron, heat tint etc may restrict this from occuring, hence we remove these elements by pickling, brushing, polishing or blasting to allow passivation to take place.In reality, if you have no heat tint, embedded iron etc, stainless steels will passivate themselves. We merely assist this by pickling to remove contamination.Many products are sold as "passivation" chemicals, however are really only pickling pastes / fluids.10% nital will remove light heat tint and embedded iron, heavy heat tints may require vigourous brushing or polishing. Be very careful with pastes containing hydroflouric acid.Hope this helps.
Reply:That's about the best explanation of it I've ever heard, chrispc66. Thanks!
Reply:Well I'm going to meet with him although I've got a bad feeling about dealing with him at all.  So say he brings over a keg like I suspect, this guy wants stuff welded for a home brew setup.  How would you do it?  My first inclination would be to plug up any holes and run a hose with some Argon flowing in there and let it fill with argon, then weld keeping pressure flowing into the keg.  I'm not gonna monkey around with the passivation, but say I keep this clean treating it like I would aluminum, don't use anything I use for carbon steel, would he need to passivate the weld?
Reply:I doubt your weld would need passivating, if you used stainless brushes in the preparation.  What you have to watch out for, and what he's probably asking about passivation for, is incidental contact of the keg with other steel objects.  Say the keg spent it's life sliding in and out of a steel rack on a beer truck. or conveyor system at a brewery. or sliding on steel decking at the loading dock.  The plain carbon steel that transferred to the keg surface would corrode when exposed to the heat, water, and bleach used to sterilize and operate the equipment.  Stainless is vunerable to corrosion if stuff on the surface starts to corrode.  A small scratch filled with rusting steel will cause the surrounding stainless steel to rust too.  Add a source of chlorine, like from bleach, and you can have cracks form in the stainless steel.  It's called chlorine-induced stress corrosion cracking.Your welds aren't the likely problem here, if you follow good practice, back purge the weld area, and only use stainless brushes to prep the weld.The passivation is most likely to cure the surface contamination from normal wear and tear on the outside of the used keg.If you're not familiar with sanitary welds, the idea is to ensure that there are no crevices, cracks, or corrosion pits in the vicinity of the weld.  These become places that can trap bacteria and lead to food poisoning, or worse skunked beer.  Proper back purging of the welds, will help maintain the corrosion resistance of the surrounding metal in the heat affected zone.  You need to be alert to ensuring that the root of the weld doesn't have any undercut, or pin holes that can be a place that traps beer residue.Hope this helps...Benson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:Passivating steels only affects the surface (and is more important with plain steels); I think what can be a problem is deeper intergranular corrosion caused by carbide precipitation. This has to do with technique; keeping austenitic steels in the sensitive range (800-1400*F) for too long. You might want to look at section 7.1-8 in the Bible; maybe read the whole section.