As Per Bs 1802 Flat Section

Flat (Rectangular) and Square sections are the generally used types of spring washers. Square section spring washers are used for use with fasteners with cylindrical heads and Flat section spring washers are used for all other types of heads. Spring washers of square section are lighter, and thereby, cheaper than the ones with a rectangular section.

These washers are normally available in Black finish. Other finishes like zinc coated (with hydrogen de-embrittlement) in Blue or Yellow passivation, Cadmium Plated, Nickle Plated, Bright Chromium plated and Phosphated are also available.

Technical Specification
Carbon Steel, Alloy Steel, MS, Copper, Brass & Stainless Steel (SS 304,SS 302, SS 316)
White Zinc Plated, Yellow Zinc Plated, Auto Black Plated, Chrome Plated & other as per customer requirement