The standard WA series washers are manufactured from 304 grade stainless steel. This material is incredibly corrosion resistant in normal environments and will give years of service with no significant level of degradation.
If you are mounting the WA series washer in an area that is considered highly environmentally corrosive then you can consider 316 grade stainless steel. This is more commonly known as marine grade stainless steel and if you want to learn a bit more you can find out here.
Regardless of what grade of stainless steel you chose it can still discolour depending on the environmental conditions. Often customers can mistake a contaminant for rust when it is actually ferric particles that have landed on the stainless steel and discoloured. A simple rub with wire wool or even a cloth can often remove this.