I'm also supposing your student is referring to the "cathodic etching" or cleaning action produced on the reverse polarity (DCEP) side of an AC weld cycle with the GTA process. With a traditional powersource your current is switching polarities 60 times per second right?
On the Straight polarity side (DCEN) the electrodes flow from the electrode to the workpiece and gas ions flow from the workpiece to the electrode, 70% of the heat is on the work and 30% of the heat is on the electrode.
On the Reverse polarity side of the half cycle (DCEP) you have the electrons flowing from the workpiece to the electrode and the gas ions comming down to the workpiece. 70% of the heat is on the electrode and 30% on the work. When the gas ions hit the workpiece they blast off the oxides which will not readily melt because their melting point is over 3,000 degrees F compared to 1,200 for the aluminum. This cleaning/etching/cathodic action will be produced via argon, helium or a combination of the two.
Now the discussion of manipulating and controling that cathodic etch, (balence control, inverters, asymmetric AC power) would take many more paragraphs, and besides others have covered it with excellence.
Here are a couple of articles from Mike Sammons, who I think communicates these "theories" (nobody has actually ever seen an electron) better than anybody else.
http://www.millerwelds.com/pdf/Aluminations_Ad_Package.pdf
http://www.thefabricator.com/xp/Fabricator/Articles/Welding/Article33/Article33_p1.xml
If you do a Google search for "cathodic cleaning of argon" you should get all sorts of information describing several ways to use argon and how the plasma ions blast clean the surfaces. I don't fully understand all of the science involved or I would try and help more. Maybe other forum users will help describe this cleaning action further for you.
Forum
