Passivation of Stainless Steel

[Samurai Dave]

I'm taking a welding class at the local community college, I love TIG welding and have started a small project in stainless. My welds turn blue/black and the immediate area around the weld discolors. Any suggestions for removing this discoloration?

I've ordered some pickling paste (made my Harris) from the local welding shop. Runs about $45.

I've looked on-line and there seem to be several machines offered that will provide an electric current to some paddles/wands that when applied to a stainless steel weld soaked in their "solution" will magically remove bluing, scale and discoloration; without any abrasion or scratching. I've talked to a few welders that seen it done, so it appears to work. However the machines look pretty simple, and they are PRICEY ($1,000 to $3500). Is it just a transformer generating about 50 volts AC?

Thank you in advance for lending me the benefit of your experience.

[dabar39]

stainless

All the stainless work that I have done was in commercial applications and we weren't that concerned about having to completely get the bluing out of it. But we did clean it with a stainless toothbrush immediately after the weld was made and still hot, then just went over it with a drum type emery cloth flap wheel on a drill. It was good enough for commercial applications. Dave

[phaxtris]

those machines have a little pump inside to pump the fluid onto the material aswell as using the electricity, they do work, i have used them, but much like the paste if you dont rinse the stuff off it will make the stainless a differant shade

[Anti-GMAW]

A battery charger with sponges soaked in muratic acid. Way cheaper but clean it off when done. But I suggest using a stainless or brass wire brush. Scotch brite pads also work great. But if you need a completely scratch free surface I suggest just cleaning it of with Nitric acid. Also passivation is not the blueing you see, thats oxidation occuring from hot metal coming in contact with the air. Passivation is accually the formation of the chromium oxide layer on the surface of the SS (thats what alows SS to be so corrosion resistant). It usualy takes about 24 hours to reform after welding but can be sped up by swabing the weld with Nitric acid. The acid forces the stainless to protect itself by forming the oxide layer. If you can rig up a simple purge over the weld then you wont get any oxidation at all to begin with. If the part is small enough you can put it in a container and purge it with argon. Try taking a Rubbermaid bin or whatever works and hooking a gas line to the bottom, put a candle in a spot higher than the weld itself. Light the candle and turn on your purge, when the candle goes out then that means it's full of argon. Weld your part and let it stay in there until it's cool to the touch, leave the purge on the entire time. You don't need that much of a flow rate on the purge because all it's doing is taking the place of the air. Just put a Y conector on your flow meter and one hose goes to the machine and the other to your purge. If your doing a larger part I'd be happy to suggest methods for purging the weld zone only. What is it that your doing and why is it so critical not to have discoloration?

[fun4now]

best solution is to keep it coverd in argon, maybe just turn up uor flow rate or purge the back side if thats the problem area. if its the front where you are welding thats the problem , turn up the gas flow or add a trailing gas line. several trailing systems available. also arc-zone caries a large cup to help with coverage of larger area's.
Anti-GMAW prity well coverd it , its all about the argon coverage to keep it clean to star with. good flow in the front and a back purge on thiner stuff usualy take car of it.

[sjmiller]

Aweful answers - geez

It doesn't matter what type of metal you heat - it changes colors depending on how hot it gets. It's called the HAZ - if you are using a mller 200, or 300DX - you can tweak the welding parameters to minimize the HAZ - wire brush needed for clean-up....

Steve

[fun4now]

yes it will discolor usualy gold, red, blue tones if properly coverd with gas but if he is getting black he is light on the gas coverage.

[Anti-GMAW]

If it gets 100% gas coverage until its cool then it soudn't change color at all, but that means it needs to be coverd the whole time wich isn't a common practice in most aplications. Fun4Now is right, a gold to blue color is the apropriate color range to know you have good gas coverage. It can still turn black though if he gets to hot and hits the temp range neccesary for carbide precipitation, also known as sensitisation.

[Anti-GMAW]

Hey SJMILLER, Why did you title your post: "Aweful answers - geez"?