There’s a vast amount of different materials, platings and coatings to choose from. This talks about many of these options, we’ll focus on some of the more common ones here. The material selection is influenced by required strength, temperature, resistance to corrorsion, joint materials, and cost.
- Zinc plating: Because of steel’s tendency to rust, you’ll never get bare steel fasteners. The most common covering is zinc, but this won’t stand up to outdoor conditions.
- Black Oxide: Most common on socket head cap screws and other machines screws. This provides very mild protection against corrosion and usually has an oil film added for additional protection.
- Hot-Dipped Galvanized: For outdoor use, this provides the best (common) protection next to stainless steel.
- Stainless Steel and aluminum: These materials are inherently resistant to corrosion because they form a tough oxide layer when exposed to oxygen. Note that the strength of stainless is much less than alloy steel, and even less so for aluminum.
- Galvanic Corrosion: When placed in contact along with an electrolite (like humid air), certain metals form little batteries that corrode each other. It happens when the metals are substantially electrochemically different from each other. For instance, brass and zinc plating wouldn’t be a good choice. More informaion.