18-8 Stainless Steel—The choice for wet and outdoor environments, these washers resist rust. However, they won't hold up to the harsh chemicals that 316 stainless steel can.
Black-Oxide 18-8 Stainless Steel—Typically chosen for their appearance, these washers have a dark, matte coating to create a finished look. They resist rust in wet and outdoor environments, but won't hold up to harsh chemicals as well as 316 stainless steel.
Highly-Corrosion-Resistant 316 Stainless Steel—Superior to 18-8 stainless steel, these washers won't degrade when exposed to harsh weather, salt water, and most chemicals.