Should be instant fail, lot of weight and stresses on that arm to say the least. Normal procedure is to remove arm, and get it blasted so you can actually see whats solid then go from there. Would NOT advise patching that hole in situ for two reasons:
1) By the book, you're not allowed to patch it.
2) Patching it won't reveal the extent of the damage which is why its frowned on.
Outside the book (and I'm musing here, not advising), with it properly blasted back to solid steel a competent welder could cut out the bad and replace with good and make it seamless but this is not by the book. It will be simpler and wiser to get another that isn't rotten and fit that. Some get them galvanized for that reason.
As an aside, I kept my old ones which I'll rebuild when I have the time. Don't feel despondent - we all end up tackling this one.
