Surface treatment is about more than just avoiding corrosion. A screw that is only meant to support an iron pipe out on a pole is no problem. But suppose the screw is under the engine and gets splashed with rainwater while an adjoining pipe emits a wave of heat? Suddenly it’s not so easy.
Besides, some surface treatments can wear off when the screw is set in place, or from simple chafing. How durable should the treatment be? If you add several coatings, it will affect the very dimensions of the screw. And believe it or not, some screws are supposed to be pleasing to the eye. The visible ones need to blend effortlessly with their environment. There was a time when all surface treatments were based on solvents. Today, most are water based.
There are roughly 100 kinds of treatment to choose from. Unless there are special circumstances, electrolytic treatment is usually the way to go. The fasteners are submerged in a liquid containing the desired surface treatment agent. When the electricity is switched on, the liquid reacts with the metal and forms a smooth coating.
For fasteners with higher tensile than 8.8 in the more exposed areas of the car, organic treatment is often preferred. The protection is both better and more complete, since the screws are placed in small containers that are centrifuged in a dye.