In the past I have "passivated" welds in S/S by brushing them with concentrated nitric acid. Bad stuff in that it is so corrosive to many things AND if one should get it in one's eyes....well I don't want to think about it.
I heard about using milder acids using a power supply and so tried it to clean the discoloration from and around the welds caused by the heat from the welding process. Not only does it work I discovered that I can use this method to clean up rusty stainless as well.
You need a dc power supply with an output of 25 to 30 Volts and some method of current
limiting the output during short circuits. You need an output of about 1/4 Amp but 1/2A is better. I used a "bench" supply obtained from the internet
ads for $75 which happens to have variably voltage and current
with a voltmeter and ammeter.
You can use the juice from a freshly squeezed lemon or lime yet FSR (Fiberglass Stain Remover) from Davis works better. The lemons have citric acid and the FSR has oxalic acid, both relatively mild acids which are enviornmentally safe to use. oxalic acid occurs naturally in some plants such as rhubarb.
Buy a couple of "alligator" clips, the larger ones with about 1/2 inch flat bills work well to clip to thin guage S/S. You can use an ordinary spring clamp to clamp a wire to handrails. Wire the positive from the supply to the workpiece. Clip the negative wire to an ordinary sponge. The sponge only needs to be about 3" by 1". Dip the sponge in the juice or FSR gel, attach the negative clip to a corner of the sponge and slowly wipe the sponge around the affected area of stainless. You may notice a small wisp of white smoke rising from the area. The FSR gel will foam a little. Wipe clean and inspect. Wipe off with a clean damp sponge.
Use latex or similar gloves to isolate yourself from the supply when handling the S/S and acid because even 30V can give you a tingle without them and you might drop or bump into things from such a reaction. If you inadvertently touch the two clamps you will get sparks before the supply current limits and the voltage goes to zero, no problem though.
Some discolorations caused by surface chemistry different than what this method can remove will have to be dealt with using normal polishing techniques. What IS neat, though, is that you can slowly remove rust without scratching the stainless with abrasives.
Check it out!