Accelerated Tests of Paint Finishes on Aluminum JUNIUS D. EDWARDS AND ROBERTI. WRAY,Aluminum Company of America, New Kensington, Pa. HE d i f f i c u l t i e s i n the
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way of developing satisfactory accelerated tests for paint finishes are generally recognized. Nevertheless, the need is so urgent t h a t t h e i r use is justified, providing the results bear some r e a s o n a b l e relation to service performance. In the use of thin aluminum alloy sheet on s e a p l a n e s , particularly on p on t o o n s , service conditions are met which may require paint p r o t e c t i o n . In and out of sea water, alternately wet and dried, paints on seaplanes are put to a searching trial. In examining paints for this service, a test has been developed which gives good results and has yielded valuable information. A description of this test, w h i c h h a s b e e n i n u s e since 1926, and some of the inf o r m a t i o n gained from it are p r e s e n t e d b e c a u s e it seems generally applicable to paints for metal which is subject to wetting and drying in service.
An accelerated test of paint ,finishes on alumin u m , which includes periodic immersion in sall water followed by atmospheric exposure in predetermined cycles, has been found useful in investigating painting practice on seaplanes. The apparatus consists qf large wooden tanks which are automatically filled with salt water at definite intervals. Boxlike structures of the aluminum alloy 178-T were used as test s&wecimens,exposed in the tank in such a manner as to be completely wet, inside and outside, when the tanks were filled. The results secured closely parallel the types of failure experienced in service. An investigation of various surface preparations showed the following order of excellence: anodic coatings, oxide coatings produced by means of chemical dips, treatment with phosphoric acid solutions, treatment with phosphate solutions, and cleaning with solvents. Investigations qf types of painting systems indicale that the best finishing system found to date involves the use of a n anodic coating and a phenolic resin-base zinc chromate primer applied and dried before assembly, followed by aluminum paint made with the same type of vehicle.
I)ESCRIFTION OF APPARATUS ’The test consists of immersion in-salt water for a set period, followed by exposure to the atmosphere and drying; the two conditions are repeated in cycles. The teat apparatus, shown in Figure 1, is located on the roof of a factory building a t Edgewater, N. J. Three large open wooden tanks approximately 12 feet long, 4.5 feet wide, and 18 inches deep contain the test specimens. A large water-supply tank located at the side and above the first t a n k f u r n i s h e s t h e water for filling the test tanks. Water from the Hudson River is pumped directly into the supply tank. A typical analysis of this water, made in August, 1932, w a s as follows: c1
SO4
cog PH
G./L 10.4 1.4 0.065 7.9
The water contains approximately 1.7 per cent of sodium chloride, about half the concentration found in sea water. When the water reaches a predetermined level in the supply tank, a large float trips a valve, permitting the water to flow into the uppermost test tank and fill it to a level about 3 inches above the specimens. As
FIGURE1. TESTAPPARATUS 5
t h e t a n k fills, a similar float rises until it trips a valve in this tank, the water flowing to the next tank and so on to the last tank, from which it is discharged into the river. It usually r e q u i r e s a b o u t 40 m i n u t e s t o fill t h e supply tank and about 10 minutes t o drain it. The specimens are t h u s immersed for a b o u t 10 minutes and are out of the water until s u b s t a n t i a l l y dry during each cycle. A new supply of water is used in each cycle, since the old water is returned t o the river.
This apparatus can, of course, be o p e r a t e d only during the months when t h e r e i s no danger of freezing, n o r m a l l y from about the middle of April until the l a s t of N o v e m b e r . For many coatings this period has been sufficient to produce substantial breakdown. Specimens which successfully withstand one season’s e x p o s u r e are returned to the tanks the following year and the test is repeated until failure occurs or the answer is obtained to the problem under investigation. The test specimens are g e n e r a l l y stored indoors between exposure periods. In a few instances speiimens have been continued in the test for 5 years. Various types of specimens can be exposed in these tanks, but experience has shown that the most comprehensive information can be secured by the use of a fabricated specimen containing riveted or welded joints and parts never exposed to the sun, simulating conditions inside a pontoon. In the earlier tests a fairly c om p 1i c a t e d specimen was employed (Figure 2) which embodied joints of dissimilar metals, metal-to-wood, etc., such as might be used in aircraft construction. Although much valuable information was secured through its use, the number of tests which could be made a t one time was limited because of its size and design. Consequently a much simpler specimen was adopted, consisting of a boxlike structure w i t h r i v e t e d s i d e s , open a t the bottom and vented a t the top. This box was 10 inches high, 10 inches long, and 7 inches wide, and was fabricated from Aluminum Company of America’s alloy 17s-T sheet; this is a heat-treated alloy similar to duralumin in composition and proper-
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ties, which is widely used in aircraft construction. About one hundred of these boxes can be exposed in the tanks during one season. By exposing them open end down, the inside fills with water as the tanks fill; the inside is thus always exposed to a very humid atmosphere a n d is never thoroughly dried, whereas the outside is alternately expo sed to sunlight and salt-water immersion. The boxes are supported in the tank so that the lower 2 or 3 inches of specimen are continuously immersed in the water which does not drain from the tank. This p. e r m. i t.s a .study FIGURE2. EARLYTYPE OF O f p a i n t b e h a v i o r at the water l i n e , which TESTBox is generally recognized to be a critical point. It has also often been found that the moisture conditions inside the box produce more rapid failure here than on the outside of the box. Where a large supply of sea water or brackish water is not available, as a t New-Kensington, another expedient is emdoved. T h e t e s t pieies are c a r r i e d on a frame which, b y mechanical m e a n s , is a l t e r Y?& nately raised from the tank and !p lowered in prede& t e r m i n e d cycles. B
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chemical action on the metal is a t least as important as any effect they have in removing dirt and grease. In one series of tests a number of boxes were cleaned with different chemical cleaners, thoroughly rinsed in running water, dried, and coated with two coats of aluminum paint. After one season’s exposure, the paint on all boxes had shown definite signs of failure, but the boxes cleaned with phosphoric acid cleaners were in noticeably better condition than those cleaned with phosphate. Three of these boxes are shown in Figure 3. The box on the left was cleaned with a phosphoric acid cleaner containing alcohol; the top face showed some paint peeling but the paint on the sides was almost intact. The middle box, cleaned with a moderately alkaline sodium phosphate cleaner, showed bad paint blistering and peeling from the top with many large blisters and peeled areas on the sides. The box on the right was cleaned with a sodium phosphate cleaner of less alkaline characteristics, the paint showing similar failure to that of the middle box. The best results in the way of surface preparation are secured by means of anodic oxidation. There are two methods in general use for applying such coatings: anodic coating in a chromic acid solution, and the Alumilite process of anodic coating in sulfuric acid solutions. Practically no difference has been found in the results secured with these two methods as surface preparation for painting. Both surfaces have proved to be excellent bases for paint for severe exposure c o n d i t i o n s . The bos in Figure 4 was anodically coated in ’ a chromic acid solution and given two coats of aluminum p a i n t . It was i n excellent condition even after 2 seasons’ exposure. A similar box a n o d i c a l l y coated i n s u l f u r i c 5URFACE PREPARATION
the tanks.
way was also in excelient c o n d i t i o n after a like neriod. A n o t h e r p a i r of boxes having the same treatment was in very good condition after 3 seasons’ exposure. Paints over anodic coatings have lasted as long as 5 seasons in this test. The box shown in Figure 5 has been in the test for 5 seasons. It was anodically coated in chromic acid solution and painted with a linseed oil-base zinc c h r o m a t e primer and one c o a t of aluminum paint.
Test boxes of 178-T alloy coated with two coats of aluminum paint after one 8eason’s exposure. Box on left t i h t e d with phos horic acid them with :odium phosphatetype cleaners &ore pain&
INVESTIQATION OF SURFACE PREPARATION As with other metals, the preparation of the surface of aluminum alloys is a very important step in securing satisfactory paint service ( I ) , particularly under the severe conditions of exposure to which seaplanes are subjected. The simplest type of surface preparation is that of solvent cleaning to remove oil and grease. Test and experience show that this treatment is quite insufficient for this type of service. Even with the most durable coatings, the paint blistered and peeled from the solvent-cleaned surface within one season’s exposure in the alternate immersion test. Less durable coatings showed failure within a month or two. Certain chemical treatments were found to give much better results than solvent cleaning. The most common materials used for this purpose contain either phosphates or phosphoric acid. The phosphate cleaners are normally used as aqueous solutions, while cleaners of the latter type usually contain a solvent such as alcohol to assist in the removal of grease. The phosphoric acid or phosphate apparently reacts with the aluminum surface, forming a very thin film of aluminum phosphate which renders the metal passiveless reactive to moisture. Of these two, the phosphoric acid type of cleaner has been proved to be the most effective. Although these materials are generally referred to as cleaners, their
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INVESTIGATION OF PAINTS ON ALUMINUM
FIGURE4. ANODICALLY COATED ~TS-T ALLOY TESTBOX AFTER 2 SEASONS’ EXPOSURE Two coats of aluminum paint.
In selecting a satisfactory primer for aluminum seaplanes, both the pigment and the vehicle have been found to play an important part. Of the pigments tested as primers, iron Oxide by itself proved rather ineffective,redlead was not found satisfactory under
January 15. 1935
ANALYTICAL EDITION
the test conditions, while aluminum bronze powder proved fairly effective. The chromate pigments were definitely better than any of the other pigments tested. Of the chromate pigments, zinc chromate gave decidedly the best results, regardless of the type of vehicle employed,. Zinc chromate shows decided inhibitive properties in priming coats on aluminum; this may be ascribed to its soluble dichromate content. Even s m a l l a m o u n t s of dissolved potassium dichromate in salt water have been found to stop corrosion on aluminum surfaces. Combinations of zinc chromate a n d r e d oxide of iron, such as that described in U. S. Navy P r o p o s e d Specification P23b, while not generally as good as zinc chromate alone, FIGURE5 . ASODICALLYhave given very good results, COATED 17s-T ALLOY better than those o b t a i n e d TESTBox AFTER 5 SEA- with b a s i c l e a d c h r o m a t e SONS' EXPOSURE alone or in combination with Oil-base zinc chromate primer other p i g m e n t s . Zinc chroand one coat of aluminum paint. mate in c o m b i n a t i o n w i t h o t h e r p i g m e n t s h a s some advantages over zinc chromate alone,- such as better hiding and lower cost, and under some conditions can be used to good advantage. For water immersion, particularly salt-water immersion, the vehicle of the primer must possess a high degree of moisture-impermeability. Tests have shown that some of the synthetic resin varnishes, particularly some of those containing phenolic resin, possess very high impermeability to moisture ( 3 ) . Tl-e vehicle must also show good adhesion t o $he surface. The best painting procedure is logically to put
FIGURE6. INTERIOR OF Two ALUJIINUMPAIKTED 17s-T ALLOYTESTBOXESAFTER ONE SEASON'S EXPOSURE Bpar varnish vehicle used on box on left and phenolic resin varnish on box on right.
t h e metal surface in as nonreactive condition as possible and then apply a durable series of paint coatings of minimum permeability to moisture. In a series of tests conducted in 1932, aluminum paint primers made with several different types of varnish were compared. Paint made with an ordinary SO-gallon ester gum spar varnish showed rather bad failure in one season's .exposure, whereas aluminum paint made with a phenolic resin varnish gave very good results, with the exception of some blistering near joints. The interior of these boxes is shown in Figure 6 and illustrates the type of failure observed.
n
These tests were partially repeated the following season, using primers made with zinc chromate. Although much less failure was observed in all cases, the phenolic resin-base primer again gave the best results, a glycerol phthalate-base zinc chromate primer being a close second. The former showed no failure after one season's immersion. Tests made to determine the effect of applying the primer before and after assembly invariably showed that less failure occurred in joints which were protected by the priming coat applied and allowed to dry before assembly. Riveted joints cannot be made absolutely water-tight mechanically, and therefore should be protected with paint if possible.
FIGURE7. COMPARISONOF LACQUER AND SYNTHETIC RESIN-BASEENAMEL 17s-T alloy teat boxes treated with phosphoric acid-type cleaner, after 2 seasons' exposure.
The selection of finishing coats of paint for aircraft work is also governed by their moisture-impermeability and adherence and, in addition, their resistance to sunlight. Here again the proper selection of both pigment and vehicle is highly important. Of the various pigments tested, aluininum bronze powder has been found to give the most satisfactory results. This might be expected, since aluminum paint shows excellent moisture-impermeability and is but little affected by sunlight. Its adherence to the priming coat is generally very satisfactory. The vehicle used, however, is quite important. Synthetic resin-base varnishes gave the best results in aluminum paint when used as primers; this also applies to the finishing coats. Some pyroxylin lacquers pigmented with aluminum bronze powder have also given very good results when applied over a suitable primer but pyroxylin lacquers as a group were found to be inferior to synthetic resin enamels. This was confirmed by Whitmore (8). In Figure 7 two boxes are shown, illustrating the greater durability of the synthetic resin enamel after 2 seasons' exposure. The box on the left was finished with a green pyroxylin enamel, one of the best tested, while the one on the right was finished with a green synthetic resin enamel employing the same type of pigment. Very little failure occurred on the box coated with synthetic resin enamel, whereas the lacquer-coated box failed rather badly a t all joints and edges. The box shown in Figure 8, which was in practically perfect condition after 2 seasons' exposure, was finished with aluminum paint made with a synthetic resin varnish applied over a synthetic resin-base zinc chromate primer. The effect of drying time between coats of paint or before immersion after applying the final coat is a very important consideration. This factor was not thoroughly investigated in these tests, as it was believed that allowing a sufficient drying period would eliminate the variable of drying time. It has been shown in other tests, however, that paint tends to become more impermeable as the film hardens. In many instances about one month may elapse after the finishing of
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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to show some peeling with corrosion of a plane before it is placed in actual serthe metal after one season in the accelervice. An average of a month's drying ated test; green synthetic resin enamel time was also allowed after applying the showed similar results. In a general way, final coat of paint to the test specimens it can be said that exposure for one season before s t a r t i n g t h e test; from 1 to 2 in the accelerated test is equivalent to 3 weeks' drying time was allowed between years or more of exposure under the atcoats, In tests of this kind it would be mospheric conditions prevailing a t Kern desirable to investigate varying drying Kensington. Coatings mliich last longer periods, in order to establish the mini$ than one season in this test would he mum drying time allowable. expected to possess very long life under It is difficult to state what one season land service conditions. in the accelerated test means in years of service, because service conditions vary !I.CKSOWLEDG~IEST so widely. However, cornparatiye tests have been made with many of the coatThe original design and as5embly of the test a p p a r a t u s d e s c r i b e d in this ings, both in air exposure and in the acFIGURE8. TEST Box OF 17s-T celerated test described. For example, AFTER sEl\soss, Expaper were w o r k e d out in 1923 by POSCRE members of t h e T e c h n i c a l Direction two coats of a l u m i n u m p a i n t m a d e Treated ,,ith phosphoric acid Bureau of A l u m i n u m C o m p a n y of with an 80-gallon ester gum spar varnish, which showed failure on 1%-T in one followed by synthetic reein-base zinc' America, especially E. Blough, C. F. (5 montlls) in the accelerated test, chromate resin-base primer,aluminum iollolved by paint. synthetic Nagel, Jr., and F. V. Hartman, with the assistance of the engineering staff of the have shown o n l y b e g i n n i n g f a i l u r e after 3 years of normal atmospheric exposure on panels ex- Glenn L. Martin Company. posed at an angle of 4.5" facing Eouth. In another case, an LITERATURE CITED oil-base iron oxide primer followed by one coat of aluminum paint had begun t o show failure in 2 season5 in the ac- ( 1 ) Edwards, J. D., and n'ray, R. I., ISD. ESG. CHEM.,25, 23 (1933). (2) Whitmore \I R. fbid. 23 19 (1933). celerated test, whereas a similar coating exposed to normal (3) R. i, kan,;\ R.,Ibid., 25, 842 (1933), air conditions was still intact after 3 years' exposure. The September 19, 1831. Presented before the Division Paint green pyroxylin lacquer preyiously nlentionedi which 1 ~ sand Varnish hell lie try a t the 88th >feetine; of the American Chemical still intact after 3 years' atmospheric exposure, had begun Society. Cleveland, Ohio, September i o t o 14. 1934.
and
A Rapid Method for Dialyzing Large Quantities of Protein Solution GILBERT C. H. STONE, College of the City of New York, New York, N. Y.
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URING the course of preparing large quantities of pure proteins, it was found necessary to dialyze batches of 5 to 10 liters of the colloidal ampholytic solutions. The simple
a good-sized paddle. The solution may be preserved with toluene, chloroform, or thymol, the rapid evaporation of which is prevented by covering the crock with a large piece of cardboard perforated by the necessary holes. Because of the continual motion of the solution and t h e c o n s t a n t flow a n d r e m o v a l of t h e dialysate, the motor Srlrver process is rapid. Tubular Cellophane is easily obtained, and when wet is very strong. Breakage of the sacs, due to internal pressure such as may occur in the usual methods of dialysis where the medium to be dialyzed is placed within the sac, is eliminated, because in this case the mater passes out of the bags into the solution. It was found that 10 liters of a solution consisting of a protein dissolved in 10 per cent ammonium sulfate could be f r e e d f r o m t h e s a l t , as far as possible by ordinary dialysis, in the a b o v e a p p a r a t u s in 24 hours-18 with tap water and G with distilled water. The author is indebted t o C. A. Marlies for helpful suggestions.
apparatus herein described was designed to carry out the dittlyzing process efficiently. Figure 1 shows a c o n t a i n e r of capacity sufficiently g r e a t t o hold t h e p r o t e i n s o l u t i o n . P i e c e s of 5-cm. (2-inch) tubular Cellophane, of a b o u t 25-cm. (10-inch) length, are folded a t one end and clamped closed by means of screw c l a m p s , fitted with two-hole r u b b e r stoppers and Water bent glass tubes, and connected in series. They are placed in the protein solution in the crock, and cold tap water is run through them. The weight of the screw clamp is sufficient to k e e p t h e water-filled bags subCrock merged in the solution. After most of the electrolytes have been removed from the p r o t e i n s o l u t i o n , a slow s t r e a m of d i s t i l l e d w a t e r is run through the bags instead of the tap water. The solution in the crock is kept in motion by means of a slow-moving stirrer, motor-operated, and fitted with FIGURE1
RECRIIVDOctober 4, 1934.
