Internal Stress and Structure in Thin Chromium Electrodeposits

Summary

The earliest stages of stress development in chromium electrodeposits formed from two electrolytes (a standard ‘simple’ 250/2·5 chromic acid/sulphuric acid solution, and a more concentrated proprietary solution) appear to be insensitive to the plating conditions. The rapid development of a tensile stress on a number of substrates, including electropolished nickel and various electrodeposited fee metals, can be explained in terms of two competing mechanisms, the diffusion of hydrogen in the substrate, and the coalescence of chromium crystallites formed in the first layer of the deposit. Epitaxy between substrate and deposit is thought to affect the mode of stress development over the first 0·1 to 0·3 μm of the deposit, but it is proposed that its effect is rather to constrain the growing nuclei as they impinge laterally than to produce a direct misfit stress between substrate and deposit. Transmission electron microscopy has confirmed that the fine structure of the deposits is independent of the deposition conditions, and diffraction evidence is presented for the epitaxial effects postulated to explain the development of stress. No chromium hydrides were detected in deposits formed from Snavely-type solutions.