The Theory of the Potential and the Technical Practice of Electrodeposition: III . Linear Polarization on Some Line‐Plane Systems

Mainly a review of that portion of the theory of the potential which relates to the flow of current within a system, the conducting boundaries of which are not at uniform potential. When electrolytic baths, the conducting boundaries of which are non‐uniform because of polarization, are considered in the light of this theory, a great deal of confusion is eliminated, and the parts played by various factors in determining the distribution of metal deposits over plated objects become apparent. One of the most difficult problems in electroplating is that of producing a deposit within the recesses of an object of complicated shape. It is easier to produce a deposit within recesses with some plating solutions than with others. Throwing power is usually treated in the literature of electroplating as a definite property of a solution without regard to the object to be plated. Throwing power involves several conditions, some of which are so related to the form and size of the object to be plated that they cannot be combined into a single property, the measurement of which will have any general significance. If of two plating solutions of the same general type the one which has the higher polarization and conductivity also has the higher current efficiency at the lowest practicable current density, it will generally produce the more uniform coatings on irregularly‐shaped objects, whatever their size or form. If polarization, conductivity, and current efficiency do not favor the same plating solution, it may be impossible to say which will give the more uniform deposit except by experiment with the objects to be plated. Polarization always tends to make the deposit on irregular objects more uniform, and it is always of more importance in its effect on the smoothness of deposits and on irregularities of small size than on large shapes. The theory helps to explain the filling of pits or engraved lines. The mathematical section contains or indicates the solving of the problems presented when polarization potentials are arbitrarily assigned to the simple boundaries considered in Paper II and also when polarization is assumed to be a linear function of current density for all but three of the same cases. These simplified cases are helpful in explaining the relations between the various factors which must be considered in practical electrodeposition.