Drive screws that don't corrode
Lots of screws are labeled appropriate for exterior applications, but only a few hold up to the elements.
Among your most-common options are stainless steel (which comes in different grades), silicon bronze, and steel with one of several available coatings. Here, we look at each. Although aluminum and brass resist corrosion, we don't include them because they're too soft for most applications. Also, aluminum will corrode quickly in contact with the chemicals in treated lumber.
Silicon bronze. This material is the standard in marine fasteners because it resists corrosion and doesn't promote rot in the wood around the fastener. It costs about twice as much as 304-grade stainless steel, so reserve screws made of this material for boat-building.
Stainless steel. Screws made of this material may cost twice as much as coated steel fasteners. But stainless steel is well worth it if you don't want to see any rust or stain marks on your project. Whenever you put time or expensive lumber into a clear-finished project, it doesn't make sense to use anything but stainless steel.
Most widely available stainless steel fasteners are either 304 or 305 grade. Both are appropriate for general use, although 305-grade fasteners have slightly greater corrosion resistance. For maximum corrosion resistance in extreme environments, say one where salt spray is common, spend the extra money for 316-grade stainless.
The Golden Rule for outdoor fasteners
Use screws, not nails, to join your outdoor projects. The additional gripping strength provided by their threads better resists the inevitable and considerable wood movement caused by outdoor exposure.