Structure of chromium deposits from plating solutions containing trivalent and hexavalent chromium

Abstract

The structures of chromium deposits on iron from plating solutions containing chromium in the hexavalent and trivalent forms have been examined and compared. The two deposits are referred to as hexavalent and trivalent deposits, respectlvely. The hexavalent deposits are microcrystalline and epitaxial, but the trivalent deposits are distinctly different. They are never epitaxial and only microcrystalline when deposited at high current densities (> 1000 A m⁻²). Evidence is presented from electron microscropy, X–ray diffraction, and differential scanning calorimetry that the trivalent deposits at low current densities are amorphous, with a crystallization temperature of approximately 350°C. The trivalent deposits also contain six times the amount of dissolved gases (oxygen, nitrogen, and hydrogen) as in the hexavalent deposits. There is some evidence that the trivalent plating solution consists of two plating systems: an ammonia–free system which deposits amorphous chromium and predominates at low current densities, and a system containing ammonia which deposits crystalline chromium and predominates at high current densities.

MST/374