Black Enamel

Insect pins come in two materials:  black enameled steel or stainless steel

Black enamel is the most commonly-used finish and provides good anti-corrosive properties under ordinary conditions.

When considering what is or isn't "ordinary" conditions, the most important consideration is humidity.  If you are concerned at all about the definition of "ordinary", you might want to consider using stainless steel pins. 

Also available in size 15 minutens from Austerlitz and Entochrysis.