My neighbor got me a bottle of the toilet bowl cleaner that has Hydrocloric Clorine in it and the stuff takes rust and corrosion off just about anything. It is a little acidic and should be used outside and washed off when done.
I used it on my custom stainless steel tailpipe and the metal looks new again.
The cool part is you just wipe it on or spray and the rust or discoloration melts off!! I have also used it on other metals incluning cast iron, but you need to clean off the residue afterwards and apply a rust inhibitor!!!!
I guess the give away free samples on this site if you fill out a request.
I know......but it for toilets!!!!
But the stuff works and makes crap like CLR look like fruit juice as far as removing rust!!!!

Its the HUSKY 303 super toilet bowl cleaner that is 23%HCL.

http://www.canberracorp.com/index_flash2.htm

 

It sounds interesting so I went to the site however you need to be a distributor before they send out free samples. I'm sure you might be able to get those samples at a plumbing supply house.

 

You don't need to be a supplier, there is a fillout form that shows you are buying a sample at no cost.