Contamination in metal GTAW techniques?

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Hello, yesterday i was welding some pipes, I cleand the pipes off with a wire wheel, and finishing disk.  The pipe was mild steel, while i was welding everthing turned out as i wanted but at some  spots it did not want to seal, a air bubble our pocket kept on bursting out, like a tiny pinhole. All these times this happend to me i ussaly just sit there and use throttle control until it burns out.What technique should be used for this? Also with aluminum what method should be used in the simmilar situation were contamination is in the weld?I know its my job as the welder to make the metals clean but in some situations you cant get the metal as clean as you want. this info would be very useful.
Reply:wen that happens to me i grind it out then fill it againChuckASME Pressure Vessel welder
Reply:were the pipes you were welding welded together so it was completely sealed? there needs to be some sort of vent or the heated gas in the pipe will try its darndest to escape somehow (like making a small hole in your puddle). if this is the case maybe try drilling a small hole in the soon to be sealed tube, weld, weld the small hole shut.