Niekie
My "opinion" was baised on about a thousand tests :)... I just diddn't blather about it before because it wasn't relevant to the Original Posters question.. But now I'll say a word or two in order to avoid confusion and purchases of expensive equipment that will live their service lives running on settings that can be achieved by equipment 1/4 the cost.
I did this type of testing at United Airlines with scrap engine exhaust and hot section componants for GE CF6-6 and PW JT8D turban engines.
The thought was that sometimes when field repairs are made on exotics/superalloys like Inco 718, Hastelloy X, Waspalloy, and good ol 347 austinetic stainless steel, that from time to time it was simply impossible to remove the oxides from these hot section parts as well as it can be done at the depot level.
We had settings something like:
Inco 718 .060-.090 thickness
DCEP 200-250 amps
DCEN 60-90 amps
Balence 96-99% DCEN
AC Freq 200 Max.. No benefit above this.
A cathodic etch was clearly visible even on completely non-prepped surfaces. It did have a positive effect on oxide removal. Superior wetting at the toes compared to samples welded DCEN with no surface prep.
The catch is that those oxides, especially on the tail section of a jet, often have other hydrocarbons included from fuel, hydrolic fluid etc. These hydrocarbons will *NOT* be etched away.. They will be drawn into the soluable base metal in the form of Hydrogen, which is not generally a happy conclusion.
AC current (in any form) will do less cleaning than abrasives, chemical cleaning or a wire brush.. The reasons AC are used to break down aluminum oxide are not valid for stainless..
The "traditional" forms of surface prep and welding current selection for stainless is there for a reason... Some people also claim that high frequency pulsed DCEN current can "stirr" the weld puddle and produce improved results... My experience is that this will only benefit semi-automatic GTAW where travel speed is strictly governed and fitup and fixturing is more consistant.
If you have time to waste... You may enjoy the excercise as there is much to learn about harnessing the full capabilities of an Aysemmetric AC welding power supply.
using 11 gage 304 with out any material prep, setting a Miller Dynasty at 50 amps, AC starting at 99% time at negative working to 95% negative, 120 hertz comparing DCEN at 50 amps. AC provided; a more base metal color match with less blues and darker straw color, a wider weld melt and quite good fusion, good wetting action without overheating, penetration was not attempted/measured with lap and tee joints. I'll try the square groove for penetration.
The new inverters with the 4 ac waveforms should be fun to experiment with also.