Electroplating, Electrochemistry and Electronics - The 15th William Blum Lecture - Part 2
This article is the second of four parts of a re-publication of the 15th William Blum Lecture, presented at the 61st AES Annual Convention in Chicago, Illinois, on June 17, 1974. Dr. George Dubpernell reviews the history and extent of commercial plating, then delves into the electrochemical science, including potentials, overvoltage and connections to electronics.
Recipient of the 1973 William Blum
AES Scientific Achievement Award
Editor’s Note: Originally published as Plating & Surface Finishing, 62 (5), 436-442 (1975), this article is the second of four parts of a re-publication of the 15th William Blum Lecture, presented at the 61st AES Annual Convention in Chicago, Illinois, on June 17, 1974. A printable PDF version of Part 2 is available by clicking HERE. The printable PDF version of the complete 44-page paper is available HERE.
The writer first attempted61 to separate overvoltage into measurable component parts by the use of extremely thin platinum and palladium membranes, with hydrogen or oxygen being evolved on one side of the membrane only, and the potential of the other side checked for diffusion products of an electromotively active nature. No diffusion of electromotively active hydrogen or oxygen was detected. Instead, there appeared to be no diffusion whatsoever with the thicker membranes. When the membrane was made thin enough, some current penetrated solution in fine pores and charged the back side of the membrane.
In the discussion to Coehn's paper in 1899,57 Ostwald suggested that possibly overvoltage was due to the formation of atomic hydrogen at the cathode, and Bredig described his work with Haber on the alloying of sodium with lead, tin and mercury, as a possible clue to the nature of overvoltage phenomena.
Since no electromotively active hydrogen was detected in the first experiments,61 the writer became imbued with the idea that the high negative potentials of cathodic overvoltage were due to the electrodeposition of small traces of electronegative metals such as lithium, sodium, potassium, calcium, etc. This was not a new idea; the writer adopted it particularly from a paper by F. P. Bowden.62 It was also mentioned in another paper in 1927 by Sand, Grant and Lloyd.63
If this theory was correct, it appeared that a sufficiently pure solution of acid could be prepared containing no alkali or alkaline earth metals and that it would exhibit no overvoltage or a drastically reduced overvoltage, when the overvoltage was measured. An investigation was conducted to this end in 1950-1951 at the University of Connecticut using freshly distilled sulfuric acid in plastic containers.64 No clear cut results were obtained in measurements on platinum electrodes, and it appeared that sulfuric acid could not be purified to any great extent in this way,65 presumably because of the high boiling point of over 300°C.
The investigation was continued in 1967-1968 in the laboratory of Dr. D. J. Kenney at the University of Detroit. High purity sulfuric acid was synthesized by passing sulfur trioxide gas into doubly distilled water at temperatures around 40-50°C. A mercury cathode was used with a somewhat improved method of measuring the overvoltage, in that the solutions used were adjusted to the same conductivity, thus making the solution IR drop in comparative measurements at a given current density a constant factor, instead of trying to eliminate it.
It was found that when the pure sulfuric acid was used in the two-compartment measuring cell with the cathode compartment under a hydrogen atmosphere, the overvoltage hardly seemed to have been decreased at all. It was felt that some decrease in overvoltage must have been accomplished, so more careful measurements were made to determine the extent of the decrease, by comparison to the overvoltage in strong sodium sulfate solution. Neutral 1.5M Na2SO4 (pH 6.2) was used and compared to pure 0.2M H2SO4 having about the same conductivity (0.0966 mhos/cm vs. 0.0855 mhos/cm).
Surprisingly, the overvoltage plus IR drop was found to be identical in the two hydrogen-saturated solutions, within the limits of error of measurement, even though the cathode process was 100% hydrogen evolution in the sulfuric acid and 100% sodium deposition on the mercury cathode in the neutral sodium sulfate solution.
Overvoltage was found to increase almost linearly with current density at 25, 50, 100, 200 and 300 mA/cm2, but at each current density appeared identical in each solution, whether 0.2M H2SO4 or 1.5M Na2SO4. Typical results at 25 mA/cm2 (25 A/ft2) are given in Fig. 8. The investigation was reported in a paper at the 19th meeting of C.I.T.C.E. (Comite International de Thermodynamique et de Cinetique Electrochimiques) in September, 1968, but only the extended abstract has been published.66