How To Weld Angle Iron?

by chris
(fargo, nd)

How does one TIG weld into the inside corner when welding two pieces of angle iron that have been joined at 90 degrees (each piece has a 45 degree angle). When I was welding two pieces of channel angle iron and going into the corner my arc was jumping all over the place and the metal went black quickly indicating lack of argon (I guess). I was welding at 120 amps which did a good job on the outside of the corner. I am a beginner so excuse my questions.