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Corrosion Inhibitive Pigments

or Relation of Pigments to the Corrosion of Iron by Cushman & Gardner, 1910

The many theories which have attempted to explain the rusting of iron, during the last century, have stimulated a large amount of original research on the relation of various pigments to the corrosion problem0. In the course of the investigations undertaken, the subject of protective coatings for iron and steel was naturally brought into prominence and received a considerable amount of attention. (reference)

The study of protective coatings for iron has led many paint manufacturers, as well as scientific investigators, to make a closer study of the causes of corrosion. It is evident that the electro-chemical explanation of corrosion must have a direct bearing on paint problems.

In the course of researches carried on by one of the authors upon the corrosion of iron, it was found that certain substances in contact with iron possessed the property of exciting electrical action and stimulating corrosion, while still other substances exhibited a tendency to inhibit or prevent corrosion. Bichromates of soda and potash were found to be the most eminent examples of the so-called "inhibitives," and it was found that as long as steel or iron remained in contact with these salts, even in fairly dilute solution, rusting could not take place. This naturally suggested the preparation and use of slightly soluble chromates as pigments. It was found that the chromic acid salts could be precipitated with certain other compounds to produce chromates applicable as pigments. A series of these compounds were prepared and tested; but it soon became apparent that while some afforded protection to steel, others did not.

The raw materials used, the method of preparation, the amount and character of the contained impurities, together with other factors, had a marked influence upon the efficiency of these compounds. It was even found that a series of chrome salts, all of which theoretically should have possessed inhibitive value, were in many cases actually stimulative.

Testing Pigments in Water Suspension

The results obtained from these investigations led to a series of experiments, first suggested by Thompson, in which the effort was made to determine the relative inhibitive value of a number of typical pigments for application to iron. A series of fifty of the most important pigments used in the manufacture of paints was therefore procured and submitted to test. The apparatus in which these pigments were tested consisted of a series of eight-ounce bottles, into each of which was placed a measured amount of pigment, afterward adding the same amount of distilled water to each bottle. Into each bottle was then placed a strip of steel, numbered and carefully weighed. The bottles were then connected up in series by the use of rubber stoppers and bent glass tubing, through which a constant current of washed air was passed for a period of three weeks.

At the end of this time the train of bottles was disconnected, the steel plates removed, washed off, carefully brushed with a tooth brush, to remove any extraneous or adherent pigment particles, and afterward dried and carefully reweighed. A loss in weight represented the amount of iron which was removed by corrosion. From the results obtained it was possible to tentatively divine the pigments into three groups.' The pigments which caused the most active corrosion were termed "rust stimulators," those which showed the least action were termed "rust inhibitors," while those which were found to be intermediate in the group were called "inerts."

The results indicated the position of each pigment tested, and the results proved so interesting that it was decided by Committees "E" and "U" of the American Society for Testing Materials to carry on a supplementary series of experiments. A considerable quantity of pigments similar to those used in the first test were collected in the open market, and samples of each, from the original package, were put in cans, carefully marked, stamped, and forwarded to members of the Committee, who had volunteered to repeat the work. A series of small plates of steel of the same size, cut from the same sheet of metal, and all stamped with numbers running from 1 to 50, were also supplied to each investigator. A description of the apparatus used for the test was also sent to the members of the Committee.

The results obtained by the various investigators, working independently and in different laboratories, were afterwards compared at a special sub-committee meeting, called for the purpose. The agreement between the results obtained by the several investigators corroborated the results of the original work, and appeared to indicate that the theory of inhibitives should have some practical application.

Loss of steel in grams in tests carried on pigments to ascertain their value as rust inhibitors.