Zinc-nickel coatings can provide corrosion protection on par with hexavalent chromium coatings, but without the environmental disadvantages.
The European Union’s End-of-Life Vehicles Directive No. 2000/53/EG prohibits the use of hexavalent chromium in materials and components used in vehicle manufacture for the last three years. this spelled the end to the use of hexavalent chromium (cr-vi)-containing coatings previously used for corrosion resistance in passenger cars and vehicles weighing up to 3.5 tons. and many experts feel it’s only a matter of time before the directive encompasses off-highway equipment as well. so considering the extensive use of hydraulic fittings used in most mobile equipment, the exit of cr-vi will affect suppliers, vehicle manufacturers, and recycling.
After years of electroplating Cr-VI onto components for corrosion resistance, industry is demanding replacement materials in recognition of the need to reduce, and eventually avoid, releasing quantities of hazardous materials into the environment.
Since early 2007, Voss Fluid GmbH, Wipperfürth, Germany, has been replacing its previously used yellow chromated surface finish with a proprietary Cr-VI-free zinc-nickel material that now encompasses its entire tube fittings line, flange connections, and hydraulic valves and accessories. The new finish exceeds market requirements and offers high environmental compatibility and a clean appearance.
Long-term reliability
Other companies introduced a zinc thick-film passivation containing nano particles, but Voss focused research and development using a yellow chromated (A3C) surface. In salt spray tests, the surface coated with A3C is exposed to a salt mist that makes its corrosion resistance effectiveness visible. Even when suffering surface damage from handling and installation, the A3C coating exhibits high corrosion resistance against the formation of white rust and subsequent red rust.
In Voss Fluid’s R&D department’s opinion, surfaces of zinc with passivation and sealing layers offered inadequate protection and did not offer an equivalent self-healing effect. In spite of having the same zinc base coating as used for the yellow chromation and additional passivation and sealing layers, the degree of corrosion resistance drops rapidly after the sealing layer has been damaged. The R&D team sought greater corrosion resistance than that offered by a zinc base coating only — which is essential for tube joints.