Changed gas, something is wrong

[Tatey]

I recently changed from straight CO2 to argon mix (my workmate has given me his virtually full bottle + regulator). When I was welding with CO2 I had no issues, I would get a slight brown residue around welds but I had read that this was ok when welding with CO2. However since changing to argon mix my welds have a massive brown patch around them, I've never seen it happen before so I am wondering what might have gone wrong. Here are some pictures:





I have tried playing with the gas rates, ranging from barely any to way to much, but nothing seems to solve it. The welding wire is the same stuff i've been using with CO2 and its always been fine, it is still bright and has no rust. The shroud is clean so isnt restricting gas flow and the welds look fine other than for the brown mess left around them.

Does anyone have any ideas of why this is happening and how to stop it?

I thought it could be the gas is not actually argon mix but my workmate has welded with it and said it has always been fine for him.