Polarization - The 13th William Blum Lecture
This paper is a re-publication of the 13th William Blum Lecture, presented at the 59th AES Annual Convention in Cleveland, Ohio, on June 19, 1972. The theory of electrochemical polarization and its relation to what is seen in the plating tank were discussed.
Thomas Percy Hoar
Department of Metallurgy and Materials Science
University of Cambridge, England
Recipient of the 1971 William Blum
AES Scientific Achievement Award
Originally published as Plating, 61 (2), 35-42 (1974)
Editor’s Note: This paper is a re-publication of the 13th William Blum Lecture, presented at the 59th AES Annual Convention in Cleveland, on June 19, 1972. A printable PDF version is available by clicking HERE.
When I was invited to give this lecture, I accepted with the greatest pleasure not only because of the honor done to me by your Society in making me the recipient of your most generous Award, but also because of the honor we can jointly do today to a man who, more than anyone else, put electroplating on the scientific map. William Blum had many predecessors and has had many more successors in the art and science of electrodeposition: important as their achievements have been, from Faraday and Elkington to Edwards, Brenner and a host of others, many of them here today, Blum's achievement was in one respect the most important of all. He turned what had been an art, a mystery, into a science worthy to be studied at the National Bureau of Standards. The past 100 years has seen the chemico-metallurgical arts turned one by one into sciences: that the change in the status of electroplating came a little late in the day is due in large part to the fact that Blum was born a little later than Bessemer, Thomas, Le Chatelier, Siemens, Osmond and Heycock.
In seeking a subject for this discourse I was mindful that it might be appropriate to bring before you some of the general ideas that the recrudescence of fundamental electrochemistry has thrown up and to relate them where possible to a few of the cathodic and anodic processes used in practical electrodeposition. "Polarization" suggested itself as an obvious broad title for a review of a field in which Blum himself did some of the pioneer cultivation from the electrodepositors' point of view.
"Polarization," in its simplest electrostatic sense, means "the separation of electric charge into + and - poles." We use the adjective "polarizable" in this sense in electrochemistry when we speak of an "easily polarizable ion," meaning an ion that under the influence of a strong electric field becomes dipolar by displacement of its electrons relative to its positive nucleus, and so readily adsorbable in the strong field at a metal/solution interface, or tightly held as a ligand by a charged cation; and also when we refer to a "perfectly polarizable electrode," meaning one that can sustain a considerable and varying degree of dipolar charge across the interface without passing any electric current. Generally, however, we use the term "polarization" in a looser but more specialized way to refer to the change of electric potential difference across an electrode interface (or across a thin film, or along a length of conductor) brought about by supplying (or subtracting) electric charge at one side (end) and not the other, so that the equilibrium initially existing across the interface (film, conductor) is disturbed and electric charge begins to pass through it.
That formal definition, of alarming length, includes all kinds of overpotential - "activation," "concentration" and the two sorts of "resistance" overpotential, as we 'Department of Metallurgy and Materials Science, University of Cambridge, England now know them conventionally. We shall fortunately not need to discuss semantics any further, and we shall proceed to look at our ideas of polarization or overpotential as they have developed historically through experiment.