The structure of electro-deposited chromium

Abstract

The visual appearance and crystal structure orientation of electro-deposited chromium have been studied for deposits prepared at current densities from 50 to 3000 amp./sq.ft., and temperatures from 12 to 85° C, using a standard chromic acid bath containing 250 g. CrO 3 per litre, and a ratio of CrO 3 to sulphate ion of 100:1. Some measurements were also made at 95° C. A diagram has been constructed showing the effect of temperature and current density on the appearance and crystal orientation of the deposit. The brightest deposits are characterized by a (111) preferred orientation, and no indication of any other fibre structure has been obtained. If the current densities are plotted against the logarithms of the temperature at which the brightest deposits are formed, a linear relation is obtained. With increasing variation of the conditions of deposition (temperature or current density) from those characteristic of the brightest deposits, two effects are produced: ( a ) an increasing number of particles of purely random orientation are present in the deposit, and ( b ) the perfection of alignment of the particles of preferred orientation becomes less. The residual stress present in electro-deposited chromium has been measured by the method of Stoney. If at a given current density the temperature of deposition is increased, the residual stress, which is contractile at first, rises sharply and may reach values as high as 110 tons/sq.in. This rise continues until the temperature is reached at which particles of preferred orientation first make their appearance. With further rise of temperature, the residual stress falls until it is practically zero at the temperature at which the brightest deposits are formed, and then rises again at higher temperatures. The hardness of electro-deposited chromium has been measured for deposits prepared over the range 25-90° C at current densities of 500, 1000, and 1750 amp./sq.ft. The deposits of completely preferred orientation have the greatest hardness, and this maximum hardness is the same for all three current densities, although the temperatures of deposition at which the maxima occur are of course different. The temperature/hardness curves at different current densities can be almost exactly superposed by a mere shifting o f the temperature scale, and it appears that the hardness is a property depending solely on the structure as revealed by X -rays, and is independent of the exact conditions under which the structure of a given type is produced.