Don't forget to check the belt driven clutch fan. Of course that wouldn't be the source of your corrosion. I would agree with pulling the radiator and flushing that our real well. But I would do a real good job flushing out the block. I've seen blocks that were really clogged up with rusty deposits, for whatever reason. I could be the source of the water that was used to dilute the anti-freeze, or just very old coolant. Either way, until you get nice clean water running through the system, I wouldn't expect improvement in the cooling ability.
Your experience doesn't say very much about the abilities of your mechanic.