Screws and bolts treated by the hot dipped galvanized method are specifically designed for outdoor use. Zinc is used as the coating in both methods and acts as a barrier against the tannic acid in the wood. Fasteners made from iron with chrome and nickel added are known as stainless steel. They are the most rust resistant screws and bolts available. And, they are the most expensive.
Stainless fasteners are hard and ideally suited for outdoor furniture construction.
The high price can sometimes be prohibitive but using these screws will add years of life to your furniture. In that regard, they are a small investment which will pay dividends in the long run. Commonly, green or gray screws, and sometimes a bronze colored screw, can be found in the deck building section of many hardware stores. Often these screws are ceramic coated or plated with a rust preventative to delay the metal reacting with wood acids. I've used these screws on a few outdoor projects and they seem to stand up well.