Outdoor furniture construction requires a fastener with exceptional corrosion resistance, that's why epoxy-coated or stainless steel screws are generally used.
Building picnic tables and other furniture intended for outdoor use requires fasteners intended for the outdoors as well!
For general exterior projects, 305-grade stainless steel is usually recommended. 305 passivated stainless steel screws provide corrosion-resistance and prevent staining in premium materials like Redwood.
Marine or seaside environments require more corrosion-resistance than 305 can provide. In those cases, 316-grade stainless steel is recommended.