Fasteners and other moving metal parts are the most vulnerable. Once scratched, or worn deeper, surfaces grow active. Electrical potentials increase. A bonanza for corrosion, fatigue, and failure. Now that we're past the doom and gloom, let's see how to fix it.
After all, what is corrosion, really? But a chemical reaction at the metal surface. A place where unstable, non-adherent products can form. And at an accelerated rate, in the presence of oxygen, dissolved salts, changes in surface pH, mechanical roughness, or other factors.
So, how do we stabilize metal surfaces from corrosion? Whether it's protection from pitting, filiform, galvanic, exfoliation, stress or fatigue corrosion, first ask yourself: Is my service static or dynamic? Are my metal surfaces contacting and rubbing?
Dynamic situations carry wear, so you'll need to also consider increased hardness, lubrication, or both. Polymers based on polytetrafluoroethylene (PTFE), or dry film lubricants, can be a smart place to start.
Is there direct sunlight?
For static environments, too, coating coverage should be continuous. Non-rubbing surfaces must be free of voids or pin holes. The parent metal should not be exposed. Of course, thicker coatings like urethane, epoxy, and phenolic can improve permeation strength. Just be sure your mechanical requirements are understood.
What are your temperatures of service? Micro porosity of organic-based coatings will increase at higher temperatures, allowing possible penetration and corrosion initiation. So, don't rule out ceramic or sacrificial metallic coatings for compatibility, unless toughness is essential, too.
Do you know the corrosives and how they form on your metal surface? Have you found your ideal coating barrier?