Standard_Parts_Catalogue

Page 2170 | Surface Treatments Finishes in Alphabetical Order Anodizing Black Oxide Coating Chrome-Plating Anodizing is one of the most widely used methods for treating the surface of aluminum workpieces. In this sur- face treatment, an anodizing process is used in which the surface of the component is specifically oxidized electrolytically—the top layer con- verts into a stable oxide compound Al 2 O 3 . Changing the process param- eters allows the layer thickness to be varied between 5 and 25 μm and allows organic, inorganic, or electro- lytic coloration. The surface treatment takes place in an electrolytic tank, whereby the workpiece acts as the anode and the sulfur or oxalic acid filing is the cath- ode. Usually direct current is used, which creates a weak flow of current between the two electrodes. The hy- drogen ions created in this process stimulate electrochemical corrosion on the aluminum surface, during which released atomic oxygen reacts with the metallic aluminum to form a hard oxide layer. Anodizing is mainly used to give alu- minum workpieces better corrosion resistance. Introduction of dyes in the Al 2 O 3 layer also allows anodizing to permanently color code components or visually enhance them – for exam- ple, by means of a red color. Black-bronzed parts are only mini- mally protected against corrosion. The process is therefore, usually used to improve storage stability or for decorative reasons. When the workpieces are placed in the hot black oxide solution, a chem- ical reaction creates a mixed oxide layer consisting of FeO and Fe 2 O 3 with a maximum thickness of 1.5 μm. The dimensional accuracy is pre- served. The conversion layer is heat resistant up to about 300°C and is resistant to abrasion and bending, although it is too porous to provide adequate protection against corro- sion. This protection can be achieved through additional coatings for which the black-oxide layer acts as a primer. The process is standardized accord- ing to DIN 50938. Chromium layers with thicknesses between 8 and 10 μm are used for decorative purposes and are avail- able as glossy or matte chrome-plat- ing from Ganter. The process is galvanic process. Chromium ions are supplied from an aqueous solution with a chromic acid base. Usually a combination of layers are necessary, whereby which the chro- mium always forms the top layer. For example, Ganter uses two-layer chrome-plating with nickel as the first layer and chromium as the top layer. The three-layer process is also used. Here the first layer is copper, the sec- ond nickel, and the final layer is chro- mium. Chrome-plating is a comparably cost-intensive process that places high demands on occupational safe- ty and environmental protection due to the use of chromium(VI)-based electrolytes. Alternative electrolytes based on non-toxic chromium(III) are still in the testing phase.