Preferred orientations and microstructures of chromium electrodeposits

Abstract

Chromium was electrodeposited in the standard sargent bath under ranges of bath temperature from 35 to 75℃ and current density from 25 to 200 A/d㎡. All the chromium deposits had the bcc structure, and some of them had ＜111＞ and/or ＜211＞ preferred orientations (or textures), whose degree of development changed depending on electrolysis conditions. The degree of development of ＜111＞ and/or ＜211＞ preferred orientations increased with increasing bath temperature up to 65℃ and then decreased again above 65℃ at a given current density. It also increased with decreasing current density at a given bath temperature, and with increasing thickness of a deposit when electrolysis conditions were appropriate for development of the preferred orientations. Development of the ＜111＞ and/ or ＜211＞ preferred orientations was discussed based on various theories suggested so far. Microstructure of the cross section of chromium deposit was found to be closely correlated with its preferred orientation. Deposits with strong ＜111＞ -medium ＜211＞ preferred orientation had the field-orientated texture type structure. Those with the weak ＜211＞ (occasionally accompanied by weak ＜111＞) preferred orientation had the structure of striations (or laminations). Those with medium ＜111＞ - weak ＜211＞ preferred orientation had the structure mixed with the field-orientated texture type and striations. Those with random orientation had the unorientated-dispersion type structure. Microhardness of the chromium deposit had a relation with its structure, residual stress, and grain size rather than with its preferred orientation. At constant current density, deposits which were produced at the bath temperature to bring about structure of striations, high residual stress, or small grain size, had relatively high hardness.