When you don't know the correct torque for a fastener does the torque required to loosen it equal the correct torque to tighten it to? Presuming it's not rusted on, that is. Or, if you can't use it directly is there a rule of thumb?
No, absolutely not. During tightening the torque you’re putting in gets “used” in three ways:
Overcoming friction between the head of the bolt and the joint surface
Overcoming friction between the threads
Stretching the bolt/compressing the joint via the “ramp” action of the threads. This is the actual “tightening” action, though in typical bolted joints only about 10% of the torque you’re putting in goes to this factor; the rest is used up in the friction described above.
During loosening, you still have the two friction components to overcome, but obviously you’re not working to stretch the bolt—in fact the ramp effect kind of helps to loosen the joint.
You can sometimes establish a relationship between tightening torque and loosening torque for a given joint, but it’s not very precise.
Sometimes people talk about “restarting torque”, ie the torque required to tighten a bolt just a little more than it has previously been tightened. In this case, the physics is more similar to the initial tightening, but restarting torque is still not equal to tightening torque, because typically “static” friction (what you’re working against to get the bolt moving from a stop) is greater than “dynamic” friction (keeping the bolt moving during tightening).
In theory yes, but there are other factors apart from corrosion that you mentioned : such as temperature : different metals expand at different rates so if the part was torqued at 12 Deg C then the ambient T rose to 25 Deg C it could increase the clamping force...
Edit : "in theory" means that there is no dirt, corrosion (mentioned by the OP) and the correct lubricant (if any) is used as specified.
Temperature, mechanical welding, corrosion, solvents or lubricants involved, as well as many other variables will change the force required to loosen a fastener.
There are many torque charts such as this one, one here, and even for SAE here, as well as calculators out there that will give you a general idea of the proper torque specifications for tightening, though keep in mind these assume like materials. If you have a steel fastener going into a brass rivnut or aluminum casting, you're going to want to use substantially lower values. Most non-critical fasteners can live with "good and tight", whereas most large fasteners are fine with one or two ugga-duggas.