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

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 bodycentered 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.

Phase Diagram and Superabundant Vacancy Formation in Cr-H Alloys

“…1-3).) Both these hydrides were originally formed by cathodic electrodeposition of Cr, 4) but one of them (hcp) was later synthesized by direct reaction with high pressure H 2 gas as well, 5) and the bcc-hcp boundary was determined in the range of H 2 pressure up to p(H 2 ) = 1.6 GPa and temperature up to T 700 K. 6,7) The structure of the hcp hydride was determined by neutron diffraction to be of NiAs type with Cr atoms occupying the As sites, 8) and its composition was close to x = [H]/[Cr] = 1.0. The hydrogen concentration of the fcc hydride appeared to be higher (x ≥ 1.0), and was initially identified with dihydride of CaF 2 type by analogy with other metal-hydrogen systems, 9) but in fact rather variable depending on the condition of sample preparation.…”

Effect of chromium plating on the endurance limit of steels used in aircraft

“…In the present investigation, the densities of tubes of chromium were determined for each of the following conditions: "as deposited, " after heating at 193 0 0 (in boiling ethylene glycol) and after a second heating in h elium at 440 0 O. The densities wer e 6.95 gjcm 3 as plated, 7.01 gjcm 3 after h eating at 193 0 0 , and 7.09 gjcm 3 after heating at 440 0 O. Brenner [9] indicated that changes in the volume of chromium on heating were isotropic. H ence, heating at 200 0 0 would tend to reduce the volume of the plated chromium.…”

“…Large quantiti es of hydrogen are produced at the cathode, and the chromium plate has b een shown to contain as much as 128 volumes of hydrogen per volume of chromium [5,6]. Theori es explaining the manner in which the cathodic hydrogen is h eld in the chromium [7] have been proposed but are not pertinent to the discussion at this point. Zapffe [8] has explained brittlen ess observed in bend tests on wire plated with chromium as due to hydrogen.…”

Synthesis of nanoporous chromium film by DC electrodeposition on aluminum substrate

“…The nano/micro Cr particles are randomly distributed on the Al substrate with different sizes because the barrier layer is totally dissolved at some positions and partially dissolved at others. Concurrently with Cr deposition, hydrogen bubbles are arising from H + reduction at totally dissolved barrier layer regions of the (PAA/Al) cathode and escape from the surface as H 2 gas [37,38]. At high current density, large number of hydrogen bubbles is created on the PAA/Al substrate during the Cr deposition (Fig.…”