Possibilities to enhance Technical Chromium Layers
15º Encontro e Exposição Brasileira de Tratamentos de Superfície 4º INTERFINISH Latino Americano Possibilities to enhance Technical Chromium Layers Klaus Leyendecker (Umicore Galvanotechnik GmbH, Germany) ¹ Abstract Chromium layers give surfaces brightness, high hardness and excellent abilities to withstand wear. But especially for industrial applications higher protection characteristics of coatings on top of steel against corrosion are requested, because conventional chromium layers tend to build micro-cracks. The problem increases, if additional lateral tensile stress is added. It often leads to macro-cracks. Umicore Electroplating explored several combinations of chrome with first layers to solve this pending task. The findings show surprising results: It is both possible to enhance corrosion resistance and reduce the thickness of the chromium layer. This leads to decreased costs and less environmental pollution. Introduction Chromium layers are commonly used as thin decorative coatings on sanitary items, furniture handles, interior and exterior of motor vehicles and many other items of daily needs. The layer thicknesses are typically between 0.1 and 0.5 μm. In the technical field chrome has become indispensable due to its high hardness and excellent resistance to wear. Applications can be found in the automotive field, hydraulic engineering, printing technology, mining and much more industrial sectors. Usually, for industrial applications these chromium coatings are plated up to several 100 micrometers without any intermediate layers on top of steel. Conventional chromium layers tend to the formation of microcracks due to internal stress. These cracks turn to macro-cracks, if additional lateral tensile stress is added e.g. by bending. If these ruptures reach the steel substrate, the chromium layer cannot give corrosion protection any more. The continuous growing demand for higher corrosion protection is Figure 1: Increasing the thickness, does not improve the usually met by raising the layer performance. Solution: Combine two layers to form a layer thickness. Under certain system. circumstances, however, this leads to an ________________________ ¹ Dr. Klaus Leyendecker, klaus.leyendecker@eu.umicore.com Umicore Galvanotechnik GmbH, Klarenbergstrasse 53-79, D-73525 Schwaebisch Gmuend, Germany 15º Encontro e Exposição Brasileira de Tratamentos de Superfície 4º INTERFINISH Latino Americano increase in the tendency to macro-cracking. Some other ways must be found to solve this problem. Technical approach One possible solution is to combine chrome with a suitable first layer. Here, a so-called splitting of the functions can be achieved: - The first layer takes care of the corrosion protection. - The second (chromium) coating takes over the wear protection. Suitable combination partners to chrome are: - Copper - Copper alloys (bronzes) - Nickel - Nickel alloys (nickel-phosphorus) Several combinations with a varied thickness were used and examined with commonly used and standardized methods of testing like Neutral Salt Spray Test (NSS), Copperaccelerated Acetic Acid Salt Spray Test (CASS), Corrodkote and bending. Results Both copper/copper alloys and nickel/nickel alloys showed good performance. Using copper/copper alloys as first layer the combination of yellow bronze (CuSn) achieved best results. The optimal layer thicknesses were 60 μm yellow bronze and 10 μm chrome. This layer combination leads to several further advantages: The thickness of chromium coatings and thus the costs can be reduced. This decreases the amount of toxic chromic acids in the company and waste water treatment effort and costs. Using nickel/nickel alloys as first layers best test results could be achieved with the combination of nickel-phosphorus and chrome. The nickel-phosphorus layer can be deposited with an electroless or an electrolytic process. Both processes achieved comparable results. It was possible to reduce the chromium layer from 50 μm to 10 μm. Benefits - It is both possible to enhance corrosion resistance and reduce costs with a suitable combination of a first layer and chrome as material for surface finishing coating. - Yellow bronze and nickel-phosphorus showed best results in comprehensive testing procedures. - The thickness of chromium layers can be reduced. Thus, not only production costs but also other environmental issues decrease.
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