First of all
1) penetrants wick in between crystals , grains & flakes to provide a pair of surfaces with a lower co-effent of friction that is all that they do
Or yo put it another way they simply wet the surface.
2) rust removers do just that remove Fe2OH ( red rust ) . and some can remove FeO ( mill scale )
They do that because they are acids that react with the oxides ( all metal oxides are alkaline ) forming a salt that is soluable in the rust removing solution.
3) Rust converters usually just replace the OH bits with a more stable compound POH for instance .
Now for the fun bit rust is physically bigger than steel so it occupies more space which is one reason why it bubbles up from under the paint
Note the bubble is not because of the H2 gas generated as H2 molecule is so small it passes through paint like it is not there .
One of the big reasons why we do not drive Hydrogen cars right now is the difficulty in keeping hydrogen inside the fuel tank .
Anyway the empty space between the threads on a nut & bolt for instance is finite so if the volume of rust produced is greater than the volume of the empty space the pressure on the threads becomes massive & you can not undo them so you get the same effect as those phone book joining experiments people like Myths Busters are so fond of , or the drop of water on two peices of glass that makes them impossible to pull apart .
Fe2(OH) is very uncommon as it usually has a few water molecules as well so we have Fe2(OH)6(H20) which is the loose flakey rust through to Fe2(OH)3(H20)
Then on top of that you get FeOHOFe happening where 2 irons bond to a single water and that is really tough to break free
There is more to it than most think and the important thing is to identify which route to take as once you have put a penetrant on the surface you can not use a deruster unless that deruster can break the bond between the penetrant and the rust
Any acid will react with rust the only difference is the speed and weather the acid is a reducing or oxidizing acid
Why ?
Reducing acids usually do not attack metals because they are the reduction product of an oxide ( usually ) so they tend not to consume the parent steel ( much )
Oxadising acids will do that, oxadize the reduced metal so eat into what you are trying to remove which can make a bolt hole way too large .
Then there are the chleating agents, of which Evaporut is the one most will be familiar with .
These have to be slightly acidic to work on rust
They work by dissolving the rust then precipitating it , EDTA & DMG are the most common of these and often found on the ingredients list
The A in EDTA stands for Acid & the G in DMG is gloxamine which also acts as an acid .
In my workshop it is citric acid because it is very very cheap
Vinegar ( Acetic acid ) if I run out of citric acid
Molassis if it is tools that I want to have a protective coating when finished .
Commercial mixtures are generally better than all of the home brews and remember a commercial mix has to be SAFE both to use & dispose of .
Home brews do not have to be either
