Stainless Welds And Care

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citizensnips

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Did a bit of a search and couldn't really get any definitive answers. Basically I've got a fresh weld on the inside of my stainless brew keggle and I'm wondering how to tackle it. In other words should I grind it down a little, then sand it then passivate it? at least that's what I've been reading....should I use pickling paste, bar keepers friend or even 'pbw' which I couldn't figure out what it meant. Can someone shed some advice, maybe give me a specific product I can get in Australia that would work? I feel I've read to much and am getting too confused about what to do with a bloody weld. any advice appreciated
Cheers
 

QldKev

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First it must be cleaned it up with a flap wheel or your favorite toy. I personally wouldn't touch it with a grinding wheel they are too course. But ensure you clean up the chrome depleted layer, but you don't have to grind the pot away to nothing. Also remember to use flap wheels/grinding disks/fav toy that only ever see stainless, don't ever use a disk that has touched anything else. Then 100% yes it will need to be pacified. I would recommend getting the proper pickling paste as they help clean up any traces of an oxidized layer (ie chrome depleted) and then will pacify it. Rather than buying a full tub if you clean it up then a sheet-metal/boiler-maker shop will normally whack some paste on cheap. As a minimum clean it up and throw some straight starsan on it for a few hours. PBW isn't what you want as it's alkaline, you really want acid.

QldKev
 

bigfridge

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First it must be cleaned it up with a flap wheel or your favorite toy. I personally wouldn't touch it with a grinding wheel they are too course. But ensure you clean up the chrome depleted layer, but you don't have to grind the pot away to nothing. Also remember to use flap wheels/grinding disks/fav toy that only ever see stainless, don't ever use a disk that has touched anything else. Then 100% yes it will need to be pacified. I would recommend getting the proper pickling paste as they help clean up any traces of an oxidized layer (ie chrome depleted) and then will pacify it. Rather than buying a full tub if you clean it up then a sheet-metal/boiler-maker shop will normally whack some paste on cheap. As a minimum clean it up and throw some straight starsan on it for a few hours. PBW isn't what you want as it's alkaline, you really want acid.

QldKev
Wow - that's some strong advice there Kev, but it is not strictly correct.

As a university qualified metallurgist with 6 years experience in a stainless steel manufacturing plant, I can say that while Kev's advice is common among Internet folklore - it is an overkill.

Firstly we need to understand what happens when the keg is welded. The applied heat forms a melted pool of base metal that flows to fill the gap and is replenished by the filler metal. The weld pool solidifies quickly and has the same properties as the base metal.

The problem area is the Heat Affected Zone (HAZ) - which is that region of the base metal that was heated by the welding process, and this heat may have promoted undesirable changes. Kev is correct that you can get Chrome depletion, but this is only on the extremely small area where the material was hot enough to be liquid and Chrome Carbides are formed from the Carbon atoms present in the base metal.

Stainless steel obtains its corrosion and heat resistance from the layer of Chrome Oxide that readily forms whenever the bare metal is exposed to the atmosphere. It is this chrome oxide layer that makes stainless impossible to solder without using specialised flux that 'eats' the protective layer away. The protective layer will naturally reform provided that there is Crhromium available to oxidise.

Where the Chromium has combined with carbon there will be microscopic areas where the protective layer cannot reform hence tiny rust spots can result. However, items designed to be welded are usually made from a low-carbon version of the 304 grade stainless (called 304L) so that there is little chance of the Chrome carbides forming.

Pickling paste 'passivates' the HAZ by dissolving away the surface layer so that fresh cromium is exposed to the air to allow the passive oxide layer to form. You can acheive the same result by some grinding to remove the surface layer of the weld area.

Because the chrome carbides can only form in the liquid metal pool we ar eonly talking about the outside of the vessel. The discolouration on the inside is only cosmetic - the steel will still hav ethe same corrosion resistance as it is the actual chrome oxide layer that has discoloured. You can just give this a quick grind/polish to make it look pretty again.

Kev is correct that you should not use any abrasives that have been contaminated by use on non-stainless steel. There will be particles of iron/steel embedded in the grinding media and thse can be come embedded in the stainless - giving an area without the chrome protection. Just make sure you buy some new gringing wheels etc.

You can buy 'low iron' versions at a considerably higher cost designed for gringing stainless, but unless you are working on a part for a nuclear power station or jumbo-jet engine mount - it is not really necessary.

HTH,

David
B. Sc. (Metallurgy)
 

QldKev

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My knowledge isn't just Internet folklore, I'm a sheetmetal worked by trade and it's what I was taught. About the main difference in what I was taught (late 80's) was to always use the pickling paste, but you say no need to if you have cleaned up the area properly. Maybe our's was just to ensure the job went out safely.

QldKev
 

pk.sax

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I love this discussion.

In fact, when I got my vessels welded, I did pretty much this, flap wheel, starsan, leave it be.

Re, what's better, pickling or grinding? Well, Kev, pickling takes no time at all. Grinding does. Ever try and pay for a sheetie to work on stuff and you start authorising time saving things over paying for them to spend more time on it. In a home environment, time isn't always king, so what is done commercial isn't always optimal for a hobbyist.

As an engineer, I hold this in perspective when trying to plan a job with a sheetie, never give them charge of the basic design. Do that yourself and then let them make it easier to manufacture, controlling that they don't inadvertently break your basic design. Just gets the best out of both theory and experience.
 

woodwormm

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ok slightly off topic...

if i'm drilling or cutting slots into a stainless member (false bottom or crab cooker style basket for a BIAB rig) should i just drill/cut my slots/holes and let it be for a few weeks in the open air... or should i spray with neat Starsan? Is some other acid usable? citric perhaps? Starsan aint the cheapest thing to go using straight on a hundred holes...
 

bigfridge

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ok slightly off topic...

if i'm drilling or cutting slots into a stainless member (false bottom or crab cooker style basket for a BIAB rig) should i just drill/cut my slots/holes and let it be for a few weeks in the open air... or should i spray with neat Starsan? Is some other acid usable? citric perhaps? Starsan aint the cheapest thing to go using straight on a hundred holes...

Few weeks !!!!!!!

A few minutes is all it takes. By the time that it is washed and dried the oxide layer would have started to re-form.

If you find this hard to believe think about this for a minute - if the oxide layer didn't reform so quickly you could just give 2 stainless pieces a quick grind and solder it the same way as any other piece of mild steel. Instead you need to use agressive flux to 'eat' away the flux so that the solder can adhere to the base metal.

Dave
 

bigfridge

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My knowledge isn't just Internet folklore, I'm a sheetmetal worked by trade and it's what I was taught. About the main difference in what I was taught (late 80's) was to always use the pickling paste, but you say no need to if you have cleaned up the area properly. Maybe our's was just to ensure the job went out safely.

QldKev
Back in the late 80's I was the guy writing welding procedures and telling people to 'always pickle' - it was my job. ;-)

A properly executed weld will last a few million years - probably longer.

Taking a few shortcuts buy not having to buy some expensive paste (around $50 for a small pot) and exposing an untrained user to dangerous chemicals (not to mention the environment) will definately lead to a reduction in life for the welded piece.

You will probably only get a 1,000 years out of it or maybe as little as 100 years ;-)

As a real-life datapoint, I welded my first boiler about 20 years ago with a stailess electrode in an Arc welder (strictly a no-no due to the heat input) and it is still in regular use as a grant on my current system - and it does not have a single spec of rust despite being ground with standard (but un-used) grinding wheels.

RDQHAHB
 

bigfridge

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Come on John Palmer, the game is up, stop calling yourself David...
:lol:
Actually John and I have discussed this very subject about 10 years ago - he was advocating that it took a few weeks for passivation to occur naturally. He agreed with me that it actually happened very quickly.

At the time he was working in an aircraft factory so they probably took this sort of thing fairly seriously.
 

katzke

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Who welded it? If they are any good follow their advice, or they should have already done it.

Did a bit of a search and couldn't really get any definitive answers. Basically I've got a fresh weld on the inside of my stainless brew keggle and I'm wondering how to tackle it. In other words should I grind it down a little, then sand it then passivate it? at least that's what I've been reading....should I use pickling paste, bar keepers friend or even 'pbw' which I couldn't figure out what it meant. Can someone shed some advice, maybe give me a specific product I can get in Australia that would work? I feel I've read to much and am getting too confused about what to do with a bloody weld. any advice appreciated
Cheers
 

pcmfisher

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Probably all you will need is a polish with a green scotchbrite scourer :)
 

citizensnips

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Wow thanks a lot for the replies all, especially qldkev and bigfridge....cheers for the well informed knowledge. Just to clarify, am I best to get a new flap disk that's suitable for stainless, clean the weld and surrounding area and then spray with some starsan? I kind of got lost in amongst all the science with what the actual solution was......sorry.
 

pk.sax

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Use a flap disk that has not seen use yet.

Spray/rub in some starsan or whatever other acid you can. Pickling paste is acidic too and purpose made.

Leave it alone for a day or whatever long to passivate.

At the end of this, going by all above comments, you've done overkill and the pot/keg will be good for centuries if not millenia.
 

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