A Theory for the Mechanism of Chromium Plating; A Theory for the Physical Characteristics of Chromium Plate

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Abstract
This paper describes a general study of the structural characteristics of chromium plate and of the mechanism of chromium plating. New fundamental data are presented on the crystal structure of chromium deposited from chromic acid solutions. In view of deposition conditions producing the various structural modifications, a hypothesis for the plating mechanism is presented. Solution composition and other plating factors can be adjusted so that chromium deposits either as metal in the body‐centered cubic (B.C.C.) crystal arrangement, as a hydride in the hexagonal arrangement, or as a hydride in the face‐centered cubic arrangement. The hydrogen atoms are interstitially placed in the hydrides. The body‐centered cubic plates are stable, but the hydrides are structurally unstable and decompose with shrinkage in volume to form B.C.C. metal and hydrogen. The hydrogen may escape or may be occluded in the plate. It is postulated that the hard chromium plate of commerce is “cracked” because of successive deposition of hexagonal hydride and its decomposition during plating. As this cracking occurs, “cathode film” is drawn into the shrinkage cracks to fill them with a compound of partially reduced chromium. These physical effects, their magnitude, and the speed of their occurrence are governed by the cathode film. The cathode film is postulated as consisting principally of a “colloidal membrane” of such composition and disposition that its isoelectric point is within the pH range operative at the cathode surface. At the low end of this range, body‐centered cubic chromium is deposited; in the intermediate pH levels, hexagonal hydride is deposited; and at the high end, face‐centered cubic hydride is deposited. Between these limits, mixtures of the various structures occur. X‐ray data are presented to support the concept that the hydrides are not merely metallic chromium containing “dissolved” hydrogen.

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