Kinds of Corrosion
by: Bogart, L.G. Vande, 1939
Definition of Corrosion
Corrosion has been defined as "the action of eating or wearing away by slow degrees", a definition which is rather broad and which does not restrict the application of the word to the destruction of metallic materials. In this discussion the subject of special interest is the corrosion of metals, excellent definitions of which can be found in the publications of two of the outstanding authorities on the subject, Speller of the National Tube Co. and Evans of King's College, England. Speller defines corrosion as "the chemical action of certain external agencies on metals which cause their deterioration or destruction" followed by "reversion to more stable combinations, of which the metal ores as found in nature are familiar examples". Evans says that "the study of Metallurgy-the making of metals appeals directly to a comparatively limited number of persons, but the study of Corrosion-the unmaking of metals-has an interest for all who use metals, a far larger class". (reference)
The definition of corrosion as the "unmaking of metals" seems to be particularly apt. It is brief; it is descriptive; and it ties together the two subjects of corrosion and metallurgy, the studies of which have been mutually dependent upon one another. It does lack something of completeness, however; and there seems to be some justification for combining the two definitions and calling corrosion "the unmaking of metals by chemical action between the metal and the external agencies which constitute its environment".
A study of the literature pertaining to corrosion shows a tendency to divide the observed instances into a number of classes, with the implication that each is basically different from the others. For purposes of simplification it is going to be assumed here that there are, in general, two methods by which the unmaking of metals can come about. The first of these is typified by what happens when the metal iron is exposed to the atmosphere at elevated temperatures. Under these circumstances the iron reacts with and combines directly with the oxygen in the atmosphere to form the product which is commonly known as "mill scale" and which has essentially the same chemical composition as the naturally occurring iron ore called magnetite.
Other metals, when exposed to the same environment, behave in similar manner to form a whole series of oxides; and when exposed to certain other environments, may again react in a similar manner to form other compounds.
All of these cases are characterized by the fact that the corrosion products are the result of direct combination between the reacting elements, and are not the result of substitution of or displacement of one element for or by another. For want of a better name this type of corrosion may be called direct chemical corrosion; although the same name has been applied to cases which do not fall within this classification as here defined, but which fall instead within the second of the two classifications.
The second method by which corrosion proceeds differs from the first in that it is the result of displacement of one element in one phase (usually in an aqueous solution) by another element originally present in another phase (in the solid form as a metal or alloy) ; and, since this displacement of one element by another is invariably accompanied by a flow of electric current, corrosion of this type is called galvanic corrosion or electrochemical corrosion.
Of these two names the latter will be used and the former will be avoided in this discussion because of the tendency to apply the word galvanic to those special cases in which the corrosion of one metal is enhanced by contact with. a, second and widely dissimilar metal. In `this discussion the term "electrochemical corrosion" is not restricted to those cases in which the metal being corroded is simultaneously in contact with the corroding solution and another and dissimilar metal or some other solid and conducting material. It may be applied to almost all cases of corrosion which occur when metals-either pure, impure, or in the form of alloys-are exposed to pure water, aqueous solutions of water soluble materials, or mixtures of water with some other substance which is neither soluble in nor a solvent for water.
Because of the universality with which water is encountered it is obvious that this must include a large percentage of those instances which are of industrial importance. This does not mean that direct chemical corrosion is not important; for it is, both because of the losses which result from it and also because of the way in which it frequently affects the course of electrochemical corrosion. It proceeds in a rather obvious manner, however; and there seems to be little point in spending much time with it prior to the consideration of the individual metals and the consideration of actual cases of corrosion in specific environments.
Requirements of Corrosion Cells
According to the electrochemical theory of corrosion all cases which fall within this category take place through the instrumentality of corrosion cells, each of which is made up of a metal or alloy in contact with pure water or some water solution. Before such a cell can function there are three requirements which must be met. In the first place there must be, for each cell, two points on the metal surface which act as connecting doors between the metal and the solution in order to permit the free flow of electric current back and forth. Second, the components from which the cell is made must be capable of conducting electricity. Third, there must be present what Speller calls a "driving force" which is capable of initiating and maintaining a flow of current throughout the system.