Black chromate coatings

#There are commercially available single-dip immersion processes for producing chromate conversion coatings including jet black chromate conversion coatings, on metal substrates such as zinc or cadmium, zinc or cadmium die-castings, zinc or cadmium plate, aluminium or copper. Such coatings can afford high corrosion and abrasion resistance, resist stains and fingerprints, and serve as an excellent base for paint and other organic finishes. Black chromate coatings are expected to have utility on a large scale as radiation adsorbers in solar energy devices. Zinc plate properly coated with a black chromate coating will with stand in excess of 100 hours of standard salt spray solution without white corrosion. In addition, chromate coatings can provide considerable lubricity where sliding action is required. We currently offer a black chromate coating process under the Trade Mark Enthox ZB.992. Our fluid, like other fluids commercially available for the same purpose, consists of an acid solution of hexavalent chromium containing a small proportion of dissolved silver.

#Other chromate coating solutions and processes are available for a variety of purposes. Blue passivate coating solutions provide a relatively thin film with moderate resistance to corrosion, e.g. less than 24 hours in the British Standard 1224 Salt Spray test before the appearance of white corrosion products of zinc. Iridescent passivate coating solutions provide thicker film with up to 96 hours Salt Spray resistance. Olive drab passivate coating solutions provide thick chromate films with a salt spray resistance of the order of 300 to 400 hours. Each different coating solution has an optimum immersion time, and the immersion of objects in the solution for longer or shorter times gives rise to inferior results. This invention is applicable to all these chromate coatings, but is of particular advantage in relation to coatings having a salt spray resistance of greater than 24 hours.

#All these processes and solutions have been in commercial use for many years.

#The following Examples illustrate the invention.

#A good quality black chromate finish will withstand 100 hours salt spray testing (B.S.S. 1224/1970, ASTM-B 36868). However, to illustrate the effectiveness of the post-blackening dip process, articles were processed such that without the post-blackening treatment the zinc coating would only withstand 48 hours salt spray.

#Similar samples were processed in the same black chromating solution for 2 minutes and washed thoroughly. The control samples were then dried at room temperature.