These RoHS-compliant, metric size washers are made of stainless steel, European Standard 1.4305 (American Standard series 303). The cover disks are made of white technopolymer (Polyamide PA). The washers are used for shrouding screw joints when a visually appealing finish is desired. The plastic disks serve to avoid damage to finished surfaces.
You can see the other types of washers we offer in Section 11 of our online catalog here.