Without exception, in the UK, every single bandit fitted with standard exhausts will, after a few years, have a rotted collector box. If you look through the bike trader, almost every bandit over 4 years old that's for sale, has either had the collector repaired, a stainless set of downpipes fitted, or has a hole in the collector. So common is it, that several companies offer a repair service specifically mentioning the bandit collector box in there adverts!!
I understand the original is mild steel. That and the lovely winter we get and the local council's eagernes to cover the roads in salt and grit at the merest hint of a frost.