Technical possibilities at Metoba
Metoba offers the following techniques for processing Chrome plating :Rack-finishing
1. Characteristics and properties
Pure chrome is a bluish-white, tough, ductile and malleable metal that can be polished to a high gloss. With a normal potential of -0.71V, chromium in its active state is a relatively base element. However, since chromium surfaces become passive even at the slightest oxidative effects, the potential shifts up to a value of +1.3V, so that chromium assumes precious metal character. Chrome-plated workpieces are characterised by insensitivity to touch, high surface smoothness, low coefficient of friction, low tendency to stick and high tarnish resistance. Chromium coatings are very resistant to chemical attack by gases, acids, alkalis and salt solutions. Only hydrochloric acid, hydrofluoric acid, aqua regia and hot concentrated sulphuric acid strongly attack chromium. The wettability of chromium is low. This property additionally increases the corrosion resistance, as aqueous media are repelled by the surface.
The hardness of the chromium layers in our company is 800-1100 HV 0.1. The high hardness, the cracking and the good friction behaviour of the chromium coatings result in the above-average wear and scratch resistance of electrolytically deposited chromium. The chromium plating of metals in the layer combinations nickel-chromium and copper-nickel-chromium is standardised in DIN EN 1456.
2. Benefits and areas of application
For decorative chrome plating, a galvanic layer <1μm with a corrosion-protective intermediate layer of nickel or nickel-copper is used. An alternative surface to decorative chrome plating is the Achrolyte process.
3. Basic material
Thin chromium coatings used in bright chromium plating with thicknesses of <1µm, deposited directly on unalloyed and low-alloy steels, offer little corrosion protection for brass, aluminium, bronze, nickel silver, zinc and copper. For this reason, but also because it is difficult to achieve a high gloss on the base materials, decorative chrome plating is used almost exclusively in combination with nickel or copper-nickel coatings.
4. Required delivery condition
The goods provided must be largely metallically bright. Residual oils or residual impurities must be removable with conventional aqueous alkaline degreasers. Oils containing silicone, alkali/alkaline earth and high-pressure additives must be avoided at all costs.
Deposited chromium layers retain their shine, but do not form it.
In bright chrome plating, where layers of 0.1-1µm are applied, the texture of the intermediate layer is reproduced. Chrome layers require a layer of semi-gloss or bright nickel as a base, which is usually applied to a copper layer.
6. Alternative surface systems
The tin-cobalt alloy offered by Metoba is an excellent alternative to chrome plating and is suitable for for drum and rack plating.