Paint Technology and Tests

by Henry A. Gardner

When iron is brought into contact with moisture, currents of electricity flow over the surface of the iron between points that are relatively pure and points that contain impurities. These currents stimulate the natural tendency of the iron to go into solution, and the solution proceeds with vigor at the positive points. The air which the water contains oxidizes the iron which has gone into solution, and precipitates the familiar brown iron rust. Thus water, which acts as an acid, and air, which acts as an oxidizer, have combined together to accomplish the downfall of the metal.

Three Photomicrographs of Corroding Steel

Inhibition and Stimulation of Rust. It is obvious that if means could be devised to stop the solution pressure of iron and make it resistant to the flow of surface electric currents, rust could be prevented. Such methods have been devised, and to better illustrate how they operate, an analogy may be drawn between iron in water and shellac in alcohol.

It is common knowledge that when shellac is placed in alcohol, the shellac will force itself into solution in the alcohol, and form a clear, transparent lacquer. If, however, there should be mixed with the alcohol a quantity of water, it would be found that the shellac could no longer go into solution, and it would remain in its original condition. In the same way, if there be placed in water a small quantity of material, such as soluble chromates, or an alkaline substance like caustic soda or lime, it will be found that iron will no longer have a tendency to go into solution in this treated water, but will stay bright and clean. These materials which prevent the rusting of iron have been called by Cushman, who first advanced these explanations, “rust inhibitors,” or materials which inhibit rusting. The paint maker, realizing the importance of these rust inhibitors, is incorporating them into paints designed for the protection of iron and steel, and the success which paints of this type have met with from a practical standpoint is a justification of what was first called the “electrolytic theory,” which suggested their use.

By placing small, brightly polished steel plates into a mush of paint pigment and water, a determination may be made of the pigment’s effect upon the metal. Some pigments, under such conditions, cause rapid corrosion of the steel plates. Such pigments are stimulators of corrosion, on account of acid impurities which they contain, or because of their effect in stimulating galvanic currents. Many carbonaceous pigments are of this type. Other pigments have the effect of keeping bright the steel plates and preventing rust. Such pigments are of the inhibitive type, and their action is to check or retard the solution pressure of the iron.

The Effects of Moisture. It might occur to the reader that although paint pigments, when mixed up with water and brought into contact with the surface of steel, might show either an inhibitive or stimulative action, that it is by no means certain that the same tendency will be exhibited by pigments when they are properly mixed with linseed oil and laid out as a film upon the surface of steel. In answer to this, it may be well to state that almost no material used by mankind is absolutely dry. Linseed oil, as it is pressed from the seed, comes from the cells, carrying with it a certain small definite percentage of water, and it is quite certain that even the best linseed oil that goes into use is not theoretically dry. Everyone knows, of course, that oil and water do not readily mix and are, in fact, more or less repellent to each other. It is, however, true that, in spite of this, oils can carry quite a percentage of water, without the admixture being apparent to the eye. In addition to this, careful experiments have proved very conclusively that linseed oil films, even after they have oxidized and hardened, have the power to a certain extent of absorbing water from the atmosphere. It is, therefore, safe to say that no linseed oil film in a paint coating is dry all the time. As a matter of fact, there is abundant evidence to show that in rainy weather, and, in fact, when the humidity in the air is high, paint films have absorbed water. As the sun comes out and warms the paint coating, and the humidity content of the atmosphere falls, this water to a large extent evaporates out of the film, only to be taken up again when the weather conditions change. This action may be likened to a breathing of the paint film, that is to say, an indrawing of water under humid conditions, followed by an exhaling of water under dry conditions. With these facts in mind, it must be apparent that pigments laid out in intimate contact with the surface of steel are subjected at all times either more or less to the reactions produced by water contact. Furthermore, as it is a property of water to become saturated with the gases of the atmosphere, such as oxygen, carbonic and sulphurous acids, and other impurities, there is present in a protective paint film at all times the elements necessary to carry on the corrosive process and reactions.

An outline of Cushman’s original research work, upon which has been based the classification of pigments as inhibitors, stimulators, and inerts, is clearly presented in his report[35] as Chairman of Committee U of the American Society for Testing Materials, of which the following is an excerpt:

[35] Page 73, 1910 Proceedings of the American Society for Testing Materials.