Belleville Lock Washers for Socket Head Screws&wo-fold Locking Washers Double Fold Self-Locking Washer(Carbon Steel/Stainless Steel), Good Corrosion Resistance.

These washers have smaller outside diameters than standard spring lock washers to fit neatly under socket head cap screws. Also known as conical washers, these have a slightly cupped shape that gives them spring-like flexibility to grip tighter than split and tooth lock washers. They minimize loosening from wear, temperature changes, and vibration. Serrations on the top and bottom provide extra gripping power. Load-rated Belleville disc springs are also available.
Zinc-plated steel washers are corrosion resistant in wet environments.
18-8 stainless steel washers have good chemical resistance and may be mildly magnetic.    

The arched shape of these washers presses up against a bolt and down against a nut to prevent loosening from wear, vibration, or temperature changes. Similar to a spring, they flatten out as pressure is applied, helping to absorb shocks. They grip bolts more tightly than split and tooth lock washers. Unlike split lock washers, which can only be tightened in one direction, these washers have a continuous body that can rotate in either direction.
Conical and curved washers are functionally the same, and are often referenced using their DIN specifications. These DIN specifications refer to slight differences in OD and thickness. DIN 6796 is the standard for conical spring lock washers, which are also known as Belleville washers. DIN 137A is the standard for curved spring lock washers; although this is no longer an active standard, it’s still commonly referenced.
Steel washers are stronger and more wear resistant than zinc-plated steel and stainless steel. They are best for dry environments, since moisture will cause them to rust. Zinc-plated washers have an outer layer of zinc that protects the inner steel from rust and extends the life of the washer. Similar to zinc plating, washers with a black phosphate coating are protected from rust in wet environments. They have a black finish that blends in with dark surfaces.
Stainless steel washers are more corrosion resistant than steel, and almost as strong. 18-8 stainless steel washers will hold up in wet and humid environments, but will corrode if exposed to salt water or chemicals. 316 stainless steel washers are the most corrosion resistant, and will not corrode from water, salt, or chemicals.