system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how does WD40 dissolve rust on tools and metal?
gpt
It doesn’t.  WD-40 is a very light oil/solvent, so it just creates a slippery layer that allows loose corrosion to wipe off onto a rag.  The same thing will happen if you use some soapy water or any other light oil.  

Originally WD-40 was primarily engineered to *displace water* (WD-40 stands for Water Displacement formula #40).  It works alright for light lubrication and short-term corrosion prevention but it will wash off over time, and it really only cleans and removes stuff because it contains mineral spirits.  It is not technically formulated as a penetrating oil.  It does not break up or penetrate corrosion as well as some other dedicated products do (Kroil, PB B’laster or Seafoam Deep Creep) but will work in a pinch just because it’s such a thin, light oil.

Those products have a very low surface tension so they “penetrate” microscopic cracks and pores in corroded metals and reduce the friction between parts so they can break free and move.  Once the part is moving, the corrosion breaks up into tiny pieces and the oil helps the broken-up oxides slip past each other and out of the tight spots to keep things from binding up.  None of them technically dissolve rust they just provide a means for the rusty oxide powder to slide away from where it was built up.