A4 square washers DIN 436 are a specific type of washer conforming to the DIN 436 standard. The washer is made of A4 acid-resistant stainless steel, which has excellent corrosion resistance. Square washers have a square shape with four equal sides and right angles. These are typically used together with screws or bolts to distribute the load evenly over a larger surface and prevent the screw or bolt from sinking into the material.