Sorta OT - How important is passivation when welding stainless?

[belfert]

I have a commercial type all stainless grill with wheels that can be towed.  It has has a broken weld on a support post.

I need to take it somewhere to get it welded up.  How important is it that the place that welds it passivates the weld area after doing the welding?  My friend that works in a machine shop says passivation isn't that important and their shop generally doesn't do it unless they are doing something involving food.  He did say it might rust a little around the weld.

[chazwood]

I'm a firm believer in passivation

[kyle4501]

At work, we weld stainless all the time mostly 304 & 316 from 20 ga thru 1/2". The only time we passivate is for food grade or if the customer requires it for the chemicals being used. Usually, cleaning with a ss wire brush or media blasting is all that is needed.

But if you are set on passivation, here is a little more info:
http://www.thefabricator.com/TubePipeProduction/TubePipeProduction_Article.cfm?ID=888

[chazwood]

Thank you.. Now I know what it means.

[belfert]

I shouldn't have put "commercial type" as it really is a commercial grill.  So, it is food grade.  I know that none of the stainless welds so far have rusted even though the grill has been stored outside for a number of years.

I'm thinking I might have to call a machine shop that specializes in commercial kitchen fabrication to see if they can fix this.  There may be other welds that are starting to crack since this is a trailer that bounces around a bit on the road.

I bought this thing on Craigslist and have no idea who made it and it probably wasn't a local shop.

[kyle4501]

Passivation just improves corrosion resistance. On a grill, I doubt a little corrosion will make any difference. I also wonder if it was passivated when it was made?

There are many things that can cause the weld to crack. A good welder will be able to fix it right the first time.
The trouble is in finding a good welder. We have plenty of people applying as welders who don't know squat about welding. Seems that they think making an arc is all that matters. . . .

[Nick Badame Refrig/ACC]

Hi Brian,

We passivate but, like Kyle, only because of food servicve use.

I don't see any reason to do so in your bus..

Hay Kyle, my dad still likes to Heli arc... Lol  can't get him to change.

Nick-

[Stan]

Chaz: I don't understand your two answers to the question. Please explain.

[belfert]

Passivation just improves corrosion resistance. On a grill, I doubt a little corrosion will make any difference. I also wonder if it was passivated when it was made?

This is a custom made all stainless grill that is designed to be towed.  It is for commercial food service use.  It hasn't rusted after sitting outside for several years.  The weld would be on the outside side of the grill portion itself.

I just talked with a shop that fabricates stainless for commercial kitchens and the guy answering the phone said that they don't necessarily passivate everything.  He said a good stainless weld should not rust as long as the proper grade of stainless is used in the welding process.

Nick, this is not for my bus.  I don't believe my bus has any stainless at all.  Dina used galvanized steel for the frame and also undercoated the entire bus.   No rust really except right below the radiator and the wheels and hibs.

[boogiethecat]

I disagree totally with anyone who says don't passivate.  FWIW, I weld stainless almost daily...

Here's why:

Stainless steel is an alloy that consists of mainly nickel and iron, with some chromium and other things added in depending on the alloy series.  When stainless is manufactured it is passivated- meaning that all of the free iron on the surface is removed by an acid.  The reason Stainless IS stainless is because there's a layer of chromium dioxide left on the surface AFTER passivation that is corrosion and rust proof, and no iron.  If there's iron on the surface, it is NOT rust proof, and will behave just like common steel in an environment where there is moisture; RUST.

Now take that nice piece of stainless whatever it is, and weld on it.  What you've just done is melted a small area, and in doing so you've remixed all the ingredients in that area, allowing iron back on to the surface.  It is SO common to see a wonderfully made stainless steel tank with two little rust lines following the welds, one on each side.  This is because the iron has been redissolved on each side of the weld and not removed by passivation.  Those tanks will fail.
People who weld on stainless and don't passivate are plentiful, and it's probably simply because they think stainless is stainless no matter what you do to it.
It ain't!!!

Wire brushing or grinding won't help.  All it does is remove the surface garp, exposing fresh surface that is in the same condition as the surface you just removed, full of iron !  IF you don't believe me, weld some stainless, brush or grind it and make it pretty, and set it outside for a few days in a moist environment.  Here in San Diego all you need is a night or two with the dew that happens.  In the morning the nice surface you just cleaned will show that telltale brown rust color, and all it will do is get worse as the days go by.

Fortunately passivating is not hard to do.  In the "old days" passivation was done with hot nitric acid. Yuck.  But lately it's been discovered that citric acid works even better, and is environmentally and health-wise quite friendly (can you say "lemonade").
If you do a lot of work with stainless, you can set up a tank and soak it in a 120 degree bath of citric acid, or another very good (and practical) way is to use your welder set at about 10 amps, and with a "wand" and some  chemical (ammonium persulfate + citric acid) you can electrochemically passivate it in a hurry.  The "wand" is a stick of stainless rod a few feet long with fiberglass tape and a hunk of scotchbrite wrapped around the end.  You hook it to your Tig's handpiece terminal, and your work to the ground, dip the end into the goo and rub gently on your area.  Smoke and sulfur stink ensue but really quickly the area is passivated.

Here's the source for the citric acid and the other stuff, and a lot of good reading:
http://www.stellarsolutions.net/

For the "wand" I use CitriSurf 9002 and for a dip tank I'd use CitriSurf 2050

So there you have it... passivate!!!!!

Cheers
Boogie

[kyle4501]

For the record, I agree that passivisation is a good thing. I just don't believe it is a requirement for success in all applications.

Just for information, The machines we make out of stainless include steamers, ovens, wash boxes, chemical dip tanks. Mostly for the textile industry. Lots of humidity & 40+ years in the business & we haven't seen rust to be an issue & we don't passivate. We do wire brush welds (must be a clean stainless steel wire brush) & media blast (usually glass beads, but sometimes sand). We use separate areas for stainless welding & carbon steel welding. We also manage the air flow in the shop to keep carbon steel grinding dust out of the stainless welding areas.
We have a reputation of being the best in the industry & get almost twice what our competition gets for the same machine. 10 years after installation, our machines still look new when compared to others.

Recently we have gone into the food prep machines. Those bring a whole different set of rules but for different reasons.


For the grill at hand, I don't believe passivisation will make any difference at all if the weld is done properly with the correct prep, heat & filler material. I doubt the last weld failed due to lack of passivisation - likely caused by lack of penetration, over stressed or fatigue.

Now, if we were building holding tanks or pressure vessels . . .

[belfert]

I think I'll see if I can find a welding shop that can do the job properly with passivation.  The welding shop that did my hitch and some other welding certainly would not be the right choice.

I am sure the original weld broke because the grill trailer tends to bounce a bit when towing it on the interstate. 

Any part of the trailer you can see except the wheels, lights, tongue is stainless, even the fenders and the light support brackets are stainless.  No rust now and I want to keep it that way.