Electroplating, Electrochemistry and Electronics - The 15th William Blum Lecture - Part 3
This article is the third 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 (6), 573-580 (1975), this article is the third 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 3 is available by clicking HERE. The printable PDF version of the complete 44-page paper is available HERE.
Hydrogen overvoltage in electroplating
The hydrogen overvoltage of the basis metal being plated is often a dominant factor in plating from both acid and alkaline solutions, and may even determine whether or not any plate at all is obtained. For example, cast iron, malleable iron, or high carbon steel surfaces may become difficult or impossible to plate after cleaning and pickling in preparation for plating, due to the low hydrogen overvoltage of the resultant surface. Such surfaces of pickled or acid dipped cast iron or high carbon steel rival the surface of platinized platinum used for the hydrogen electrode in having a low hydrogen overvoltage, making it so easy to evolve hydrogen at a low potential that it is difficult to deposit many metals.
Alkaline cyanide zinc solutions often could not be used for plating cast iron or malleable iron parts, as only hydrogen evolution was obtained, or at best the coverage was poor.104,105 One solution to the problem was to introduce a little mercury into the bath, or to dip the part in an acid mercury-containing dip prior to plating in order to increase the overvoltage of the surface, and this procedure was patented.106-108 It was also found that treating such surfaces in boiling hot, concentrated cyanide solution for 5 to 30 min would increase their overvoltage and permit them to be zinc plated.109 Sometimes the only remedy is to grind or polish off the low-overvoltage surface, or to plate over it with a heavy undercoating of another metal such as copper or cadmium. Two more recent references show that this type of behavior is still a problem.110,111
Chromium plating utilizes very strongly acid baths, and is only possible on bright, dense, highly-polished surfaces in general (which have a relatively high hydrogen overvoltage). This was clearly recognized by Haring and Barrows in 1927.112 If nickel surfaces are "passive," it may be impossible to chromium plate them. This can happen in a number of ways. Anodic alkaline cleaning tends to oxidize nickel and make it "passive," and should be avoided. Poorly-covered zinc die castings may have dull, streaky, dark nickel deposits in the recesses which are difficult to cover with chromium and even tend to prevent coverage on surrounding areas. The use of black nickel plate to produce low overvoltage electrodes has been patented.113
Bright nickel plate from solutions containing excessive amounts of brightener may be "passive" and impossible to chromium plate even though it appears satisfactory. Probably very small invisible amounts of nickel compounds on the surface are sufficient to give a low overvoltage condition, under the reducing conditions at the cathode. Similarly, Raney nickel powder is a catalyst used for hydrogenation, and very small quantities are effective.
The theory of the electrodeposition of chromium itself is probably very closely tied up with the theory of hydrogen overvoltage. A black, powdery coating of chromium probably has a low overvoltage and tends to inhibit further chromium deposition and promote hydrogen evolution. The success of the hexavalent chromium plating solution may be due to the ability of chromate to complex or otherwise render harmless such impurities as iron, copper, zinc, nickel, lead, etc.
This type of impurity is difficult to control in trivalent chromium baths. The removal of lead is a definite step in the operation of chromium ammonium sulfate baths used for the electrowinning of the metal,114,115 and, if such baths are shut down for more than a few minutes, a low overvoltage condition results on the cathode chromium surface which may take many hours of electrolysis to overcome. This period of electrolysis is probably in the nature of an electrolytic purification, and is also required in breaking in a new bath before the current efficiency can be brought up to the normal level of around 50% or more.
The current efficiency in chromium deposition is closely related to the hydrogen overvoltage of the surface and simultaneous hydrogen evolution, although 10 or 20% or more may also be lost in side reactions such as reduction of hexavalent to trivalent chromium, or of trivalent to bivalent. If the temperature of the solution is increased, the current efficiency is generally affected quite strongly.
Figure 23 from a report by N.E. Ryan116 shows typical behavior of 300 g/L chromic acid solutions catalyzed with sulfate and with fluoride additions, and electrolyzed at over 1000 A/ft2 for chromium production. It seems likely that the low current efficiency for heavy deposits in boiling hot solutions with sulfate is due to a low overvoltage condition, whereas in fluoride-catalyzed solutions a cleaner, more oxide-free surface is maintained which probably has a higher overvoltage and gives a higher current efficiency. Shluger and Mikhailova117 have shown that the oxide film on sulfate-catalyzed chromium deposits contains even more sulfate proportionately than is present in the solution, while Shluger and Osbenkova118 felt that there was no oxide film at all on fluoride-catalyzed chromium deposits.