These Forgefix Flat Penny washers, have a 25mm outside diameter.
The washers are bright zinc plated for increased corrosion resistance, and have been given the name 'Penny Washer' due to their similarity in size to an old penny.
Washers provide a smooth surface for the nut or bolt to bear on, making it less likely to loosen as a result of an uneven fastening surface.
The main purpose of a washer is to distribute the load under a bolt head or the nut face.
Zinc coatings prevent oxidation of the protected metal to create a longer lasting finish.
It does this by forming a barrier and by acting as a sacrificial anode if this barrier is damaged.
Zinc oxide is a fine white dust that (in contrast to iron oxide) does not trigger a breakdown of the substrates surface integrity as it is formed.
The zinc oxide, if left intact, can act as a barrier to further oxidation, in a way similar to the protection afforded to aluminum and stainless steels by their oxide layers.