M24 Spring Washer Square Section DIN7980 CR3 BZP

Spring washers are used a a preventative measure to prevent a fixing nut from coming loose as it grips into the bottom of the nut and the washer when it is tightened.  So the spring washer must be used in-between the flat washer and the nut. These washers are DIN 7980 which are used to prevent the nut loosening wnen movement strain or vibration may occur.  It is always advisable to use a locking washer when a plain nut is used along with a flat washer.  Always replace the spring washer after undoing the nut to provide the best reduction in the nut disingaging. 

These are bright zinc plated fasteners which is a preventative measure from corrosion.  Avoiding breaking into the Bzp plating to avoid corrosion developing faster although locking washers tend to secure themselves by breaking into both the nut and washer, but the fit is tight so it reduces corrosion.  Also bear in mind using different materials can cause fusing through corrosion especially in damp conditions when mixed with aluminium or stainless steel materials.

More information is available from Central Fasteners regarding fastener details, but its always worth checking with a manufacturer's specifications or drawings if an alternative size, tensile strength or types of washers are used.