Getting rid of rust can cost a fortune in chemicals but if you have the time and can fully submerge the item to be de-rusted, here is a cheap way to do it. Submerge it in a black molasses and water solution. 1 part black molasses to 9 parts water. I have 40 litres of the stuff mixed up.
Where I used to work on Norfolk Island (in the South Pacific, halfway between Australia and Fiji - the place where the majority of the descendants of the mutineers from the Mutiny on the Bounty live), we had a 200 litre (44 Imperial gallons, 55 US gallons) drum mixed so we could dip big stuff. Like any small island, because of salt spray, rust is a real problem over there.

I know it sounds unlikely but these three pictures are of a bit of rusty steel I found and treated. The first is the steel as found, the second is with part of it cleaned up with a wire brush in an electric drill. The third photo shows the effect after a few days in molasses and water. Note that it is so clean after washing off the muck that it is already showing orange rust appearing after a few minutes exposure to the air.

Really severe rusting will need a week or so in the solution. I recommend removing the job after a few days and wire brushing to remove the loosened scale, a quick hose down and return to the pot for another go. It is essential that the steel is fully covered. Unprotected steel will corrode very badly at the air/solution interface where it come out of the solution. Note: Some alloys e.g., zinc will be severely corroded by this stuff so keep it for iron and steel only or test it first.

I bought 5 litres of blackstrap molasses (stuff they feed horses) from my local feed and produce store in Penrith (suburb of Sydney). It is quite cheap.