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)
