Originally Posted by SpikeBrewing
A "sanitary weld" is a vague phrase. It can mean that the backside of the weld was purged of Oxygen by using Argon or Helium. But in regards to home brewing it pretty much means the back side weld is aesthetically pleasing. That kettle was not back purged and it most definitely would not be considered 'sanitary'.
Time to put on my dairy plant hat again.
I don't have our exact corporate standards for a weld, but they do need to meet or exceed 3A sanitary standards. For a tank ferrule, that would generally mean TIG welding with a gas blanket on the back side to prevent sugaring. The interior surfaces would need to be within certain tolerances for smoothness and be free of pitting.
It gets tough when you weld SS tubes, because you can't go in and buff out the back side if you screw up and blow through.
Our ace welder at the plant can do a tube so well that it's hard to find the weld.
For home brewing purposes, I'd consider a weld that's been ground/polished to the same finish as the surrounding base metal inside and out to be sufficient. Even if the rest of your system isn't to sanitary design standards (threaded fittings, etc), you at least know your weld won't harbor any residues that would allow something to grow. If you can get that with a MIG, more power to you. A silver soldered or brazed joint that's been finished to similar standards would also be sufficient, albeit not as strong mechanically as a TIG weld.