How do they make them at the factory then, superglue?, of course it can be welded they either don't know how or are not confident enough in their own ability to offer warranty, take it to a fabrication or welding company, or somebody that mods cars for a living.
Aluminized steel welds more or less the same as mild and with normal mig ( I used to make new ens for a big firm
)
Im guessing the reason they have said it cant we welded is as it is liklely to fail soon after welding leaving an unhappy customer, coated heat cycled pipes are difficult to weld when there on a 3/8 wall, As said alluminised is simply a coating, under it would normaly be a very low carbon mild steal, I belleve higher contents of silicon are used in the filler materials when welding as new, also higher % of C02 are also used for Mig welding, Corrosion is only one issue, the thermic cycles make the job extremly tricky, these of course are exaclly the same as stressing and distress a joint (or stress reversal) also heat speeds up the effects of cracking within the material, when welding often the sudden very high input is enougth to destroy any integrety the material had..
I know little about exhaust welding, but have had a fair bit to do with high and low pressure steam, which cycles in a very simlar manner...
That said, A joint faluire is unlikly to kill anyone on an old exhaust, so if your prepered to have a go, you have very little to lose other than a few hours of time
