When I was about 12 years old and just getting interested in engines I
heard my uncle swear that he got a stuck piston out of a motorcycle he had
by using bottle of Coke. It had set out in the yard all winter and had rusted
up inside. But we all know pop is sticky, water based and wouldn't itmake
a worse mess? After all, Mom always said "don't spill your pop, it
will make everything sticky".
As I grew up and got interested in cars and motorcycles myself I started to hear of other such "urban legends" and decided, hey, maybe there is something to this. The people that said it worked were quite insistant, and there sure was no proof that it did NOT work. So I decided to investigate. (amazing what little it takes to entertain me)
So here is what I've found - many carbonated beverages will remove rust. This is because the gas used, carbon dioxide when mixed with water, makes carbonic acid. To make rust, the iron oxidizes - it combines with oxygen. This is why rust is also called iron oxide. The carbonic acid reverses this reaction - this reversal is called "reduction." Here's a better reason - take a look at your Coke can - it has phosphoric acid as an ingredient. Phosphoric acid is the basis of Naval Jelly, a commercial product used for rust removal. Phosphoric acid dissolves iron oxide very quickly while etching metallic iron very slowly so you can leave metal in phosphoric acid with little damage.
The downside is that all acids contribute some hydrogen to the metal structure,
weakening the steel by hydrogen embrittlement - so always use only as much
time as is absolutely necessary to remove the rust. An advantage of phosphoric
acid is that it leaves a fine protective coating of iron phosphate. Because
this coating is not thick or durable some protection is still required.
Years ago supposedly Volkswagon use a process of phosphating metal prior
to painting as it provided a chemical protection against rust under the
So, spilling your Coke into your old engine wouldn't really be a bad thing if you were trying to remove some rust.