Yoshida, H., Hirayama, H., Mochizuki, S., Inoue, M. et al., "Development of Black Surface Treatment Applicable to Bolts for Motorcycles," SAE Int. J. Engines 9(1):436-444, 2016.
A black surface treatment without using hexavalent chromium and applicable to bolts of motorcycles has been developed. The surface treatment also satisfies the requirements of bolts for motorcycles such as fastening performance, corrosion resistance, weather resistance and appearance of outer finish.There is a method of coating a black chemical conversion film using trivalent chromium on the zinc-plated surface as one of the surface treatments that do not use the hexavalent chromium. Such a black film, however, is thinner than the film using hexavalent chromium, and unable to ensure satisfactory black appearance. Meanwhile, although the dip-coating using a black paint can provide a sufficient black color, it is necessary to apply coating two times to eliminate an irregular color. As a result, thick paint films are formed on the roots of thread, making it difficult to fit into the internal thread. To simultaneously solve the above-mentioned two issues, two measures were executed. One was to apply the black film using trivalent chromium as an undercoat of dip-coating. The second was to develop paints that the amount of pigments in paints was adjusted on the assumption of the color of the undercoat. Thus a proper fit of thread was ensured when fastening in addition to a satisfactory black appearance even with the dip-coating only once. Also, the friction coefficient was controlled at an appropriate level by the selection of friction modifier. Moreover, the adhesion was improved by the enhancement of anchor effect of coating with the elimination of finishing process of the black chemical conversion film, and the requested corrosion resistance was achieved.