Epitaxial Chromium Deposition

Abstract

On nickel, copper, iron, and steel substrates, chromium has been deposited epitaxially from a fluoride catalyzed chromic acid electrolyte at temperatures above 80~ Prominent planar facets on the deposit surfaces are parallel to chromium {110} crystallographic planes and have structural details indicative of growth by "bunching." Reflections from these facets differ in intensity, according to orientations of substrate grains, and suggest applications in metallography for study of preferred orientations and contrast improvement of "flat etching" metals.In the course of work reported previously (1), it was observed that appearance and porosity of 0.5 ~m (micrometer) thick chromium deposits depended on the crystallographic orientation of electropolished nickel substrates. It was inferred that the topography and deposit structure of the chromium were significantly affected by interfacial epitaxy. Epitaxy of chromium has previously been observed only in thin (<0.1 ~m) deposits on nickel (2-4) and on copper (5). Anticipating discernible effects on deposit structure, 15-30 ~m deposits from sulfate and sulfate plus silicofluoride catalyzed baths were examined microscopically and by x-ray diffraction. Only the structures and preferred orientations described by Kaiser and Wiegand (6) were found (7). Together with information previously available (8,9), these results indicate that the characteristic structural elements of chromium deposits are submicroscopic crystallites (10-7 to 10 -4 cm), and further investigation of the influence of epitaxy would require that electron microscopy and diffraction be used. No further work was done as it was obvious that epitaxial effects on deposit structure were limited to a very thin zone at the interface.The work was resumed when it was noted that surfaces of chromium deposited from a fluoride catalyzed bath at 85~ consisted of well-formed, apparently crystalline, facets. Strongly epitaxed deposits, it was found, can be plated on a number of different metals. This report describes the procedures employed in an exploration of the characteristics of such epitaxial chromium deposits, using metallographic and x-ray diffraction methods. The results obtained are discussed with reference to the deposition process. ExperimentalThe chromium plating bath, prepared from reagent grade materials and distilled water, contained 250 g/1 (2.5M) chromium trioxide and 5 g/1 (0.25M) fluoride ion (from hydrofluoric acid or sodium fluoride). The bath, in a Teflon beaker, was maintained at temperatures of 75~176 by immersion in a controlled temperature water bath. Anodes were cut from sheet lead and had areas of 120 cm 2. A polyethylene propellertype stirrer was used to provide mild agitation to minimize temperature and concentration gradients. Cathodic current density was varied from 300 to 1500 mA/cm~, but was usually 500-700 mA/cm 2. Power was obtained from a laboratory rectifier (filtered, singlephase full wave) or an electronically regulated constant voltage/current power supply.The cathodes used were nick...

Electrodeposition of Chromium

“…Hardesty [280] reviewed cases of epitaxial chromium deposition in crystalline form. By working at over 80 C in a solution containing 250 g L À1 CrO 3 and 5 g L À1 fluoride ions with 500-700 mA cm À2 , he demonstrated the continued growth of the crystals of an electropolished nickel basis metal in the cross section of a thick chromium deposit.…”