US Patent 7976258 Screw and screw fastening structure

There is provided a screw treated by a chemical conversion coating of trivalent chromium having a torque characteristic approximately equivalent to a screw of a hexavalent chromate treatment. For the purpose, on a surface of a bolt, a zinc plating layer is formed by a zinc plating treatment. On the layer, a chemical conversion coating of trivalent chromium is also formed by a treatment of chemical conversion coatings of trivalent chromium. The coating is formed to be thin and soft so that the zinc-based plating layer can be exposed on the surface due to rubbing off of the chemical conversion coating of trivalent chromium at a contact portion to a fastened member of the bolt by tightening the bolt.