Welding Tips and Tricks

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We had a 316 food grade job recently.
As we were cleaning the job with the pacifator, there were black spots that wouldn't pacifate out.
Anybody else have this problem?
Anybody know what causes it?
What did you do to fix it?

Is your gas certified "high purity?" Has the bottle used for the purge (for internal fault) or for the weld (external fault) been analyzed?

Was this on the internal inspection? Possibly contaminated purge gas or a leaking purge line. You could try to wire-brush this with a .008 ss wire "ball" at pipe ID + a 1/4" on a long shaft, to prove/disprove a surface condition and repeat passivation. To prove the result (if it works), purge the pipe to 100% O2 for maybe an hour, and re-inspect.

(Run all that by the engineer--- I'm not sure any of that's "approved".)

For an external fault, all the above applies, as well as these possibilities... The tungsten became contaminated on a grind (someone used your stone for something other than tungsten?), there's a small leak in the torch somewhere between the flowmeter ball and the cup, or the torch sat unused a while and picked up moisture in the line.

The next possibility is a fail in the prep (a residue left while dressing the parts), which may show internaly, externally, or both. Contaminated cleaning solution (BlueGold?), inadequate rinse, contamination from the tooling that cut/squared the pipe.

Some of this may not apply at all. I often have to work to "oxygen clean" standards, but have very little food grade experience.

You've got me curious, so please share what you find.

Steve S

Your guess is better than mine! It could be contaminated tungsten. The part wasn't a pipe, just a few parts of 10ga, bent with a flange to weld to another piece of 10ga. I think it was supposed to be some kind of food grade conveyor prototype. Anyways, the parts each had a bend in them, which made for two lap-jointed seams and two flare-bevel joints. simple enough, but the first time we did the part, nobody knew what "sanitary weld" meant. we (myself and my engineer) thought it meant "clean it up real pretty." We welded all but one side and pacifated it (is it spelled "passivate?"). The acid from the process got inside the pocket and stayed there. When the part came back, we found out what they meant by "sanitary" and tried to make it that way. The acid made the previously pretty weld look like tar, no joke. The next time we made the part, we did it right, but there were still the little black spots. I guess it passed their inspection, as we never got the part back and I haven't heard anything.

Yes, for some reason, in welding, it's "passivation" or "passivating".

I guess "pacifying" isn't a "manly" enough word...



Steve S