Metal Pests

Originally limited to only tin, this condition turns the host metal into a brittle and dull dust. Metal pests work very slowly and through different mechanisms. Tin pest, the original pest,  occurred only in very pure tin which was exposed to freezing temperatures for a long duration of time. Gold pest works when gold and aluminum are in direct contact with each other, resulting in a purple dust. 
Many of the newer pests are kept a secret leaving only speculation regarding their inner workings. It is unknown how they are manufactured as this is kept as a very coveted secret by the governments which have the ability to create them. 
Often times, in the delivery of the pest to the host metal, the oxide layer above the metal must be removed. This leads to pests often being accompanied by an acid, base or reactive compound. Alternatively, bullets may be used as a means to burrow into the host metal.

Transmission & Vectors

What is known is that the pests are only transferred between specific metals via direct contact.

Causes

Many of their causes are unknown.

Symptoms

Metal will lose their shine and crumble.

Type

Supernatural