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The excellent rust-inhibitive properties of chromates are generally recognized; zinc yellow, perhaps most important of the chromate pigments, is finding extensive use in metal primers for the protection of ferrous and nonferrous metals and alloys. Efforts to overcome the effect of a relatively high content of water-soluble sulfate as well as a tendency to blister under exposure to high humidity or water have resulted in the development of a new zinc chromate pigment, ZnCr04.4Zn(OH),.This zinc tetroxy chromate is much less soluble in water than is ordinary zinc yellow and hence yields more water-resistant H E outstanding corrosion-inhibitive properties of the chromate ion are well recognized by scientists and tech-T nologists charged with the production, design, and maintenance of metals and metal structures (1, 9, 10,11,16,16) although the exact nature of the protective film produced by chromates is still a point of controversy (3). The development of the stainless steels, with their relatively high content of chromium, and the increasing use of chromates in the pretreatment of metal articles (2, 8) to enhance their resistance to corrosion are but two of the many applications of the unique properties of this element and its compounds to the problem of conserving metals. Atmospheric Exposure Tests on Chromate PigmentsPaint technologists have not neglected the chromate pigments ( I ) , and their value in priming paints for metal protective service is more widely recognized today than ever before. Our own interest in the chromate pigments was stimulated by the excellent results frequently obtained during investigations of various metal-protective primers. Typical results for a representative series of such paints after 5.5-year exposure a t an angle of 45" facing south in a semiindustrial atmosphere are shown in Figures 1 and 2.The 5 X 10 inch (12.7 X 25.4 om.) panels shown in Figure 1 were cut from exposed, larger, copper-bearing black iron panels which had been finished with one coat (right half) and two coats (left half) of raw linseed oil primers containing the pigments indicated. To permit a better portrayal of the results of these tests, these sections were cut as shown in Figure 1. The small sections (1 X 2 inches, or 2.5 X 5.1 cm.) marked B were washed with soap and water to remove the dirt and grime which had accumulated during the exposure and show the condition of the paint films (Figure 2). I n order to determine to what extent the paints had protected the metal, paint films. Since it is essentially free from corrosion-inciting ions such as sulfate, none of its water-soluble chromate is needed to counteract the effect of eelfcontained harmful ions, and all is available for combating external corrosive agents. Laboratory fresh and salt water immersion tests, tide-water-level exposures, and electrode potential measurements have demonstrated the water resistance and corrosion-inhibitive properties of priming paints containing zinc tetroxy chromate. Exterior exposures indicate excellent durability and weath...

BOUT seven or eight years ago the attention of the paint industry was focused on certain types of unsatisfactory paint service due to early adherence failure. Since a large number of these failures had been traced to abnormal moisture conditions (S), improved priming practices as well as mill and back priming of house siding were suggested as a means of overcoming these troubles. The value of such schemes was assumed to lie largely in their ability to exclude moisture from the wood and to prevent swelling and shrinkage phenomena which were thought to be the source of the difficulties. To test these ideas, an investigation was instituted in which an attempt was made to correlate permeability of paint systems with their ability to prevent these early adherence failures (8). Some of the results obtained in this investigation are being reported because of the recent interest in permeability measurements.A large number of methods and instruments have been proposed for measuring the moisture permeability of paint films. Muckenfuss (6) placed a film spread on paper, wire cloth, or other porous support, over a pan of water and under a dish containing calcium chloride; all parts were sealed with mercury in such a manner that the water had to pass through the film in order to reach the desiccant. Both fresh and exposed coatings were measured in this manner (7). Several years earlier Gardner (2) had evaluated the water-excluding efficiencies of various paints applied to white pine boards by In an investigation of the possible relation between the permeability to moisture and the durability of various priming and three-coat painting systems on wood, it was found that the initial permeability to moisture alone cannot be used as a criterion of the protection rendered by such a system on prolonged exposure. The permeability to moisture of the ordinary multicoat paint system is low and, when weathered, was found not to increase appreciably until breaks in the film enabled moisture to enter the wood. > PERMEABILITY EXPRESSED A S A PERCENTAGE OF THE VALUE FOR UNPAINTED WOO0 0 10 2 0 30 40 50 --< I N 0 I 12.5 NO 3 7.5 NO 4 14.0 a NO 5 7.6 2 NO 6 9.5 n NONE 8.0 m NO I 5.5 w NO 2 6.0 J 2 NO 3 1 . 5 r W NO 4 8.0 K NO 5 6.0 2 NO 6 7.0 U w o. NONE 5.5The higher the number, the greater the failure. The.upper bar in eaeh group shows the initial permeability of the priming coat alone. The other three bars in each group show the permeabilities of three-coat systems consisting of primer and two coats of the same exterior house paint: bar 2, unweathered: bar 3 , weathered 1 year; bar 4, weathered 2 years.

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