Some Experimental Observations on the Effect of Addition Agents on Stress and Cracking in Rhodium Deposits

Abstract

The origins of internal stress in electrodeposits are briefly reviewed. The effects of aluminium and magnesium sulphate and of selenic acid on the stress and cracking of rhodium deposits from sulphate electrolytes are described. Aluminium and magnesium additions eliminated cracks in deposits up to at least 0·0005 in. thick, but no corresponding reduction in tensile stress (40–50 ton /in²) was observed in measurements on flexible strip cathodes. With the addition of 1 g/1. of selenic acid to electrolytes containing 50 ml/1. of sulphuric acid thin deposits (<0·0001 in.) showed many fine cracks penetrating to the basis metal, but these tended to close up with increasing thickness, and were not detected electrographically at thicknesses of 0·0002-0·0005 in. These results are correlated with observations of the deflection of flexible strip cathodes, which indicate that the stress in deposits of this type is initially tensile, but becomes compressive and finally approaches zero after deposition of approximately 0·0002 in. of rhodium. Addition of copper (1 g/1.) enhanced the stress in deposits from aluminium and magnesium containing electrolytes, but had no marked effect on the stress variation in deposits from electrolytes containing selenic acid.