The Electrodeposition of Cadmium from Cadmium Sulfate Solutions: Part I. The Effect of pH, Current Density, and Temperature on the Crystal Size of the Deposit, the Current Efficiency, and the Electrode Efficiency Ratio

Abstract

The electrodeposition of Cd from CdSO 4 solutions was studied, with the object of determining the effect of variation of pH of the solution, current density and temperature of the electrolyte on the crystal structure and grain size of the deposit; and on the anode and cathode efficiencies and the electrode efficiency ratio. The pH of a simple solution of CdSO 4 rises rapidly with progressive deposition, and requires to be buffered in order to maintain a uniform pH. At the same time, the crystal structure of the deposit becomes progressively less granular, but subsequently (at a pH above 6), more crystalline, dark and less adherent. An optimum range of pH is approximately 5 to 5.7. The effect of aluminum sulfate, sodium acetate, boric acid and sodium chloride as buffering agents was examined. The best buffer within the desired range of pH was a mixture of boric acid and sodium chloride. CdSO 4 solution with this buffer was adopted as a standard electrolyte. Increasing c.d. has only a small effect on anode and cathode efficiency, the variation being 1.6 per cent and 1.4 per cent respectively, and the electrical efficiency ratio is also little affected. There is however a marked effect on the grain size of the deposit, which consists of large crystal aggregates at low c.d. and is relatively fine grained and also more adherent at higher c.d. “Treeing” occurs when the c.d. exceeds 4.5 amp./s. dm. Increasing temperature has a relatively more marked effect on the electrode efficiencies than c.d. The anode efficiency rises notably at the higher temperatures examined, and the cathode efficiency decreases somewhat, resulting in a progressive increase in the electrical efficiency ratio. The effect on the deposit is at first in the direction of refinement of the grain size, but at higher temperatures, a coarser, crystalline structure develops. It is concluded that temperatures in excess of 40° to 50° C. are undesirable in depositing Cd from CdSO 4 solutions. The “structureless” type of deposit obtainable from cadmium cyanide solutions is unobtainable from CdSO 4 solutions in the absence of an “addition agent.”

Key findings

• ▸The pH of a simple solution of CdSO4 rises rapidly with progressive deposition, and requires to be buffered in order to maintain a uniform pH.
• ▸An optimum range of pH is approximately 5 to 5.7 for the electrodeposition of Cd.
• ▸Increasing current density has a small effect on anode and cathode efficiency, with variations of 1.6% and 1.4% respectively.

Keywords

Identifiers

DOI

10.1149/1.3493786

Journal

Transactions of The Electrochemical Society

Year

1931