what is the black material on the surface of wheel nut and bolt

In Chinese market, the most popular wheel nut and wheel bolt are black ones, especially the black chrome. You may ask, what is black chrome?
The black surface of black chrome is kind of oxidizing substances. Chemical liquid and iron ion liquid chemical reaction by electricity in the plating tank, then become black oxide adhere the surface of the nut, at this time the black was crude and matt dark.
In order to make the last layer of protection coating completely adhere together with black chrome coating, it must be baked through high temperature and electricity spray to form the final bright black.
Usually Black chrome products could pass more than 96-hour neutral spray test while maintains smooth and shinning black surface which means the black chrome product not rust in 5 years.
Also there are other ways to make a black surface, here is a table to compare different black surface: from www.wheelnutcn.com