Although I have not been welding pipe, I have in fact been welding AL-6XN plate in both ¼ inch and ½ inch thicknesses. I have been using ERNiCrMo-10 in lieu of ERNiCrMo-3 as well. The big thing you must remember when welding AL-6XN is that you must use a filler metal that is overalloyed in Mo to maintain the best corrosion resistance. When using a filler metal that is not overalloyed in Mo, microsegregation of Cr and Mo in the weld will occur and reduce the corrosion resistance of the weld. Even thought the dash 3 filler is considered overalloyed in Mo (9% Mo) when compared to the AL-6XN, dilution must be considered. For that reason I chose the dash 10 alloy (13.5% Mo) and it has worked well.
Also, according to documentation from Rolled Alloys the heat input should be kept as low as possible. I used a heat input of about 33 kJ/in. Rolled alloys suggest 40 to 50 kJ/in but no higher that 50 kJ/in to help prevent hot cracking.
I have not used the GTAW process due to it's slow production rates. Instead, for the ½ inch thick plate I used the GMAW-P (Pulse) process with the ERNiCrMo-10 filler and a single-V groove using a 60 degree bevel no root opening with a backgouge. This process worked very well and we achieved travel speeds of 8 ipm and a deposition rate of about 5 lb/hr. Mechanical and RT test were very good as well as bead appearance. For the ¼ in thick plate I opted to look at PAW using the keyhole mode with the addition of the ERNiCrMo filler. I used 100% Ar for both the plasma and shielding gas. The weld was completed in a single pass using a square butt joint, no root opening, and a backing gas, also 100% Ar. This process worked extremely well and we achieved travel speeds of 7 ipm. Mechanical and RT test were excellent, not to mention a sweet looking weld on the back side of the joint.
For welding pipe with a wall thickness of 5/16 inch or less I would highly recommend looking at the PAW process. No joint prep, and welds completed in a single pass! Doesn't get much better than that.