Sodium Tetrachromate

The essential constituent of the bath is sodium tetrachromate, Na2Cr40,3, which is, however, only stable at temperatures below about 25°C. This temperature should therefore not be exceeded in the operation of the bath. Current densities of 75-150 A/dm are used. The current efficiency of the bath is high (30-35%) so that the metal is deposited at the rate of about 1 tm/min. The deposits are normally matt in appearance, but are comparatively soft and readily polished. 

Sodium Tetrachromate, Na2Cr40i3.4H20, is similarly prepared. ... 

F. A. H. Scbreinemakers could not prepare lithium tetrachromate, Li2Cr40is but be showed that sodium tetrachromate, Na2Cr40i3.H20, can exist under the conditions indicated in Fig. 34. F. Mylius and R. Funk prepared this salt by evaporating a soln. of sodium chromate in an excess of chromic acid. Tbe garnet-red, deliquescent plates melt at 40°-50° with tbe separation of chromium trioxide. Tbe salt dissolves in water without decomposition. A sat. soln. at 18° has 74-60 per cent, of Na2Cr40i3, and has a sp. gr. of 1-926 and at 0°, 16°, and 22°, tbe respective percentage solubilities are 72-19, 74-19, and 76-01. 

Tetrachromate electrolytes The alkaline tetrachromate baths are used to a small extent chiefly for the direct chromium plating of zinc die-castings, brass or aluminium, since the solutions do not attack these metals . The original bath was developed by Bornhauser (German Pat. 608 757) and contained 300 g/1 of chromic acid, 60g/l of sodium hydroxide, 0-6-0-8g/l of sulphuric acid and 1 ml/l of alcohol. 

A proprietary tetrachromate bath has been used in Germany under the name of the D process. By the use of additions of magnesium oxide and sodium tungstate it is claimed that the current efficiency of the bath can be raised to as high as 35-40%. Other additives such as indium sulphate, sodium selenate or sodium hexavanadate enable bright deposits to be obtained. 

Specialty Chromium-Plating Baths. Chromic acid baths using sodium chromate and sodium hydroxide to form a tetrachromate (92) have had limited use. Porous chromium is used in lubricated wear applications, and is made by chemically etching regular chromium plate, sometimes with light grinding after the etch. Black chromium is used on solar collector surfaces (see PHOTOVOLTAIC CELLS Solarenergy). Baths are sulfate-free, and include fluosilicic acid or acetic acid (91). 

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