Building a deck? It's not as simple as going to the hardware store and buying a box of regular nails anymore. If you're using ACQ or AC2 treated lumber, the use of bright or uncoated fasteners can have disastrous consequences.
The copper used in the aforementioned wood treatments corrodes standard fasteners—how fast the corrosion occurs is not a given, but there is no question as to whether the treatment is corrosive.
To illustrate this, consider the following pictures. Click on each image to enlarge!
This is a 10d common nail after two weeks in untreated lumber. It looks just like it did when it came out of the box. Shiny. Pretty. What's not to like?
This is a 10d common nail after two weeks in AC2 treated lumber. Notice the corrosion on the surface. Also note that this nail wasn't driven into freshly treated lumber—which usually has an extremely high moisture content—but rather that it was driven into a dry piece of treated lumber which had been sitting outside for a year. And while we're making notes, also note that his nail wasn't left out in the rain in order to make the picture more impressive—that type of rust is more orange in color.
Multiply the corrosion over months or years, and what is the result? More corrosion, less nail, and a deck or porch that may well end up on the ground.
To sum up:
The top nail is a 10d common nail. The middle nail is a 10d common nail after two weeks in AC2 treated lumber. The bottom nail? He's your friend—a 10d hot dipped galvanized nail. He won't mind if you use him, and you'll be glad you did. Of course, stainless nails are also a good option.