I use the Muratic Acid (diluted HCL) to remove the old zinc plating from parts before I replate them. If you are careful, use it in a VERY well ventilated area, and don't leave the part in there too long (if it's cold, it works slower) it works wonders. The parts come out like new. Works to get ALL the rust out of exhaust manifolds that have been found in JY. The HCL reacts slower to the cast iron and doesn't do as much damage to the structure. On steel parts, it will eat thru it pretty fast if you are not careful. I've used it for years now with great results. Just have to be very careful, and I mean very careful. When I get it it's diluted about 25:1 and I dilute it further so the reaction doens't happen too fast or too violently. If it's not diluted further, it will bubble up quite seriously, and release a lot of nasty fumes. There are safer removers of rust and plating now available that are reasonably priced. Eastwood has good products as well as some vendors on ebay. I've tried a lot of them with good results. I still use Muratic Acid becasue I have a lot in stock. Once it's gone, I will be changing over to something different. It does an awsome job of cleaning your driveway as well as cleaning the garage floor before painting.
$.02 from the coatings salesperson...
BE VERY CAREFUL with muriatic acid. Very. As said above, ventilation and dilution are extremely important to watch. Ignoring this can result in tragedy.
That said, the stuff is very effective in a lot of applications.