INDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1933
23
IN CONTACT WITH VARIOUSMETALS TABLEV. PH OF WATERSOLUTIONS
-BOILEDDISTILLEDWATER-24 48 96 400 Hours Hours Hours Hoiirs 6.8 ... 6.9 6.7 9.9 9.9 9.9 9.7 8.5 8.3 7.8 7.4 8.4 8.4 8.6 7.4 6.8 7.1 7.5 6.7 6.7 6.6 6.9 6.6 6.9 6.7 6.9 6.8 6.9 7.1 7..3 7.1 6.7 6.7 6.7 6.7 6.8 6.8 6.8 7.2
I
METAL Blank Magnesium Cadmium Zinc Iron Aluminum Duralumi~i Nickel Copper Brass
0
Hour 6.7
... ... ... ... ... ... ... ... ...
0
Hour 6.7
.... .. ... ... ... ... ... ... ...
1 N (6%) NaCl 24 48 96 Hours Hours Hours 6.8 6.8 11.7 12.0 l2:4 9.6 9.4 9.6 9.2 9.5 9.8 6.9 6.4 6.4 6.9 7.0 7.0 7.1 7.1 7.0 8.9 8.8 8.8 7.2 7.1 7.1 8.1 8.1 7.8
change may also be quite rapid. A pursuit ship taking off with a ground temperature of 32” C. can climb to 20,000 feet in 15 minutes where the temperature will be about -18’ C. The machine used for the vibration test is shown in Figures 3 and 4. The frequency of vibration was 1750 and the amplitude of vibration 2.03 mm. (0.080 inch). It will be noted that this amplitude is eight times the maximum recommended by Zand. The panels were made of 0.040-inch (0.102cm.) heat-treated aluminum alloy (duralumin) riveted together with a 2.5-inch (6.4-cm.) lap. The finishes consisted of auto lacquers, airplane dopes, oil enamels, and glycerolphthalate enamels applied over two different oil-base primers. The coatings were applied by spray and then subjected to the following test procedure: 1. Baked for 24 hours at 100’ C. 2. Vibrated 25 million cycles at room temperature. 3. Vibrated 7.5 million c cles at -40’ C. 4. Exposed 60 days in $eatherometer. 5. Vibrated 20 million cycles at room temperature 6. Vibrated 11 million cycles at -40’ C. Although none of the panels failed from alternate bending until the final vibration a t low temperature (and only the most brittle finishes then), several of the finishcs showed a marked tendency to chip under slight impact. The test did not simulate sudden temperature changes, nor were any panels included of thicker films such as would tie used for
400 Hours 7.0 12.4 8.8 9.8 6.8 7.1 6.8 8.8 8.8 8.2
0 Hour 6.6
...
.. .. .. ... ... ... ...
... ...
3.5 N (20%) NaCl 24 48 96 Hours Hours Hours 6.6 6.7 11.6 11.6 12:o 9.7 10.0 9.4 9.2 9.2 9.1 6.4 6.4 6.6 6.6 6.5 6.7 6.5 6.7 6.7 8.9 8.8 8.8 8.6 8.6 8.8 8.8 8.7 8.8
400 Hours 7.0 12.2 9.4 8.4 6.7 7.1 6.9 8.8 8.7 8.7
highly polished finishes. Even though the results from the vibration test were rather negative, they did show that materials of satisfactory flexibility are not likely to fail from alternate bending due to vibration of the airplane, and that exposure to light and low temperature are conducive to failure. Paint coatings for aircraft, therefore, should possess, besides durability, the following characteristics: 1. High resistance to permeability by water. 2. ,Pigments which mill tend to passivate metals and prevent
corrosion. 3. Absence of pigments of alkaline reaction. The failure of primer 1 of Table I1 is believed due in no small part to the large percentage of calcium carbonate used as an extender. 4. Good adhesion to metal which may not be chemically clean. 5. High resistance to impact and bending even after exposure. LITERATURE CITED (1) Anonymous, “Anodic Oxidation of Aluminum and Its Alloys
as Protection against Corrosion,” Dept. Sci. and Ind. Research, His Majesty’s Stationery Office, London, 1926. (2) Edwards, J. D.,and Taylor, C. S., Trans. Am. Electrochem Soc., 56,27 (1929). (3) Evans, U.R., Ibid., 55,243 (1929). (4) Zand, S.J., S. A . E . Journal. 29,263 (1931). RECEIVDD August 29, 1933
Painting Aluminum JUNIUSD. EDWARDS AND ROBERTI. WRAY,Aluminum Research Laboratories, New Kensington, P a .
T
HE painting of aluminum for decoration or protection
presents much the same problems as the painting of other metals. However, because of the resistance of aluminum to atmospheric corrosion, painting has not generally been a necessity as in the cases of iron and steel. As a result little study had been devoted to the subject until the introduction of alloys, stronger but less corrosion resistant than commercially pure aluminum, made the study of protection imperative. This need arose particularly in the aircraft industry where lightweight sections heavily stressed must be assured against any deterioration. For most conditions of atmospheric exposure, no special technic is necessary in the painting of aluminum. For severely corrosive conditions, however, as against salt spray or intermittent immersion, investigation has developed special methods of surface preparation and types of paints which give greatly improved performance. Before taking up the details of painting practice, something should be said about the varieties of aluminum and its alloys which may be presented for painting. Commercially pure aluminum-for example, 99 to 99.5 per cent in purity, is available in all common wrought forms as plates, sheet, tubing, extruded shapes, and rod and wire, and is designated as
25. The wrought alloys are conveniently divided into two groups-the so-called common alloys and the strong alloys subject to heat treatment. The three best-known common alloys are 35 (an aluminum alloy with 1.25 per cent manganese), alloy 43s (containing 5 per cent silicon), and a more recent development, 4s (containing 1.25 per cent manganese and 1 per cent magnesium). The best known of the heattreated alloys is duralumin or 17S, which contains 4 per cent copper, 0.5 per cent magnesium, and 0.5 per cent manganese. d special product with exceptional resistance to corrosion, sold under the trade-mark “Alclad,” combines a central core of 17s with integral surface layers of high-purity aluminum. Another extensively ueed heat-treated alloy is 515, having as alloying ingredients 1.0 per cent silicon and 0.6 per cent magnesium. This brief list does not cover all the commercially used wrought alloys of aluminum, but, since their painting characteristics are not appreciably different, it is sufficient for present purposes. Aluminum alloy castings are sometimes painted, but mainly for purposes of decoration. Aluminum is commonly alloyed with copper, silicon, magnesium, manganese, nickel, and zinc. There are literally hundreds of aluminum casting alloys which are in use a t one time or another where their
24
INDUSTRIAL AND ENGIKEERING CHEMISTRY
special characteristics are demanded, either from the standpoint of foundry practice or service requirements. Here again there are no special differences noted with respect to their ability to take and hold paint, so that no detailed discussion of compositions is required.
Vol. 25, No. 1
SURFACEPREPARATION
Aluminum to be painted should be free from oil, grease, dirt, and other foreign material, just as in the case of other metals; where greasy substances are to be removed, solvent cleaning can be employed. It is difficult on a commercial scale, however, to secure an oil-free surface in this manner PAINTADHESION because grease removed by the solvent remains in diluted Generally speaking, aluminum and its alloys need paint form in the solvent used, and a thin film is left by evaporation. protection only against severely corrosive conditions. In It is a more common practice, therefore, to remove thin films these cases the determining factor in paint protection is the of oil by chemical treatment, such as with solutions of sodium adhesion between the metal and the paint coating and its phosphate and silicate. A rariety of cleaners of this chardegree of permanence. The most common cause for loss of acter have been developed which effectivelv clean aluminum paint adherence and blistering without attaiking it. In using is the penetration of moisture them, it is important that all through the paint coating and traces of the cleaner be reThe painting of aluminum and its alloys its r e a c t i o n with the metal moved by washing and that presents much the same problems as are inuolved surface. There are, therefore, the piece be a l l o w e d to dry thoroughly before painting. two factors i n v o l v e d : The in the painting of other metals. Surface preparam o r e impermeable the paint A still more effective surface tion is a n important factor, however, where coating, the less l i k e l i h o o d preparation can be obtained by protect ion against water and severely corrosive treatment with solutions conthere is of moisture penetrating conditions is demanded. This is not a problem taining phosphoric acid and the coating in a p p r e c i a b l e of roughening the surface, but qf rendering it a l c o h o l . One such proprieamounts; the second factor is tary compound has had wide the corrosion resist,ance of the passive. Anodic coatings of cerfain types are and successful use. It seems aluminum surface or its passiveffective in this respect, and good results are also ity. Aluminum owes its corroprobable from experiments here of chemical cleaners. secured with certain types that such a solution, in addition sion resistance to the almost inChromate-containing paints make effective primto cleaning the surface, leaves visible film of oxide which it iners, and aluminum paints are also good. The it in a passive condition as a stantly acquires when exposed result of the formation of into moist air. The protective best top-coat protection is rendered by aluminum soluble aluminum phosphate. action of this film depends to paint. While such a surface treatsome extent upon its thickness, ment i s n o t a s e f f e c t i v e as but is also dependent on its consome of the oxide coating treattinuity, which in turn is affected by thk’ alloying constituents which may be present in the ments to be described, nevertheless it is quite satisfactory metal. When aluminum is oxidized by moist air, hydrogen and its lower cost dictates its use in many cases. By a substantial margin the best surface preparation is is liberated; if this gas liberation occurs beneath a paint coating and a t too rapid a rate to permit ready diffusion of obtained by oxide coating under conditions which give a hard, the liberated hydrogen, loss of adhesion and blistering may adherent, and impermeable oxide film on aluminum. The occur. The general conclusion drawn from extensive testing best films are obtained by anodic coating, and two general on all types of aluminum, under a wide variety of conditions, methods have been found satisfactory. The one method, is that paint adheres best to aluminum or aluminum alloy known as the Bengough process (2), involves anodic coating surfaces which are most nearly passive or corrosion resistant. in a 3 per cent solution of chromic acid under specified condiPaint, therefore, under certain conditions, such as intermittent tions of voltage and temperature. This method has found service in water, adheres better to pure aluminum than to extensive application in the aircraft industry, where thin some of the heat-treated alloys such as 175. This conclusion sections of duralumin have been given special protection would be of little practical use (since naturally the type of thereby, either when used bare or followed by painting. metal used is dictated mainly by the service conditions The other method, known as the Alumilite process (S),has rather than its ability to take paint) were it not for the fact reached the stage of development where its use for this purthat the corrosion resistance of the surface can be modified pose can be recommended. This method involves anodic and greatly improved by a variety of special surface treat- coating in electrolytes containing sulfuric acid, followed by special treatments m hich increase the protection rendered by ments before painting. The most effective of these treatments involves producing a the film. Oxide coatings, produced chemically without the thick and protective coating of oxide before painting. Sheet aid of electric current, in general are not as impermeable or as aluminum, as supplied to the trade, presents a smooth flat protective as the anodic oxide coatings. However, coatings surface. The product known as flat sheet has a bright mirror- of this character which give satisfactory results are nom like surface as a result of finish-rolling between burnished under development and test. The type of surface treatment which should be employed steel rolls. Another type of sheet, known as gray plate, which has the reputation of taking and holding paint some- in any case is therefore determined not only by the metal to what better than flat sheet, is finished by rolling against which it is to be applied and the character of the structure coated steel rolls. The steel rolls acquire a coating from which is to be painted, but by the service conditions to which contact with the aluminum itself; if this coating is allowed it is later to be exposed. I n the painting of metal surfaces to remain on the rolls instead of being continuously removed, it seems to be almost axiomatic that some surface roughening as with burnished rolls, the result is to give the gray-plate is necessary or desirable if adequate paint adherence is to be sheet a gray color and a microscopically rough surface. Be- secured. General experience here is that this factor has cause of the method of production, gray plate sells a t a been much overrated, as adequate paint adherence for many slightly lower price than flat sheet, and for purposes of paint- purposes can be secured on smooth, bright, flat sheet if the paint selected is of the proper type. The degree of roughing is just as good and perhaps slightly better.
uess of the surface is riot the only controlling factor in paint adhesion in cases where moisture penetrates the film, as has just been explained. However, roughening the surface, n.9 by sand-blasting, for example, undoubtedly provides better mechanical anchorage for a paint film, but once moisture has pe,netrated, corrosion may he more severe than if the original protective oxide film had not been removed by the sandblasting process. If roughening of the surface is found necessary, i t can advantageously be followed by anodic coating before painting. Sand-blasting is usually practical only for castings or very thick sections, since sheet may be warped in the process. ACCELEHATED TESTSOF I'mms
ON
coating, was obtained witli anodic coating. The other surface preparations were rated in the followiiig order: cleaning with phosphoric acid-alcohol solution, huffing followed by solvent, solvent-cleaning, caustic dip, scratch-brushing, In this particular t,est there did not appear to be any marked difference bebween the solvent-cleaned and tlie etched surface after exposure for 0 moiiths. Better adherence was :ilso securcd on the biiffed surfare than on the scratch-brushed surfiicr.
ALUMINUM
An accelerated test has been developed for evaluating the various kinds of surface preparations, as well as the various types of paint which niay be used, especially on the metal parts of seaplanes. Test specimens are built in the form oF small boxes (IO X 10 X 7 inches), open at the bottom and vented a t the top. These boxes are assembled by riveting and in a mariner closely simulating actual aircraft practice. The surfaces of these boxes are prepared in various ways and paint coatings applied. In studying various methods of surface preparation, the same paint coating is used on all boxes. After the coating is thoroughly dry, the specimeup are placed in large wooden tanks located on the roof of R plant a t Edgewater, N.J., opposite h'ew York, N. Y. I3rackish water from the Hudson River is pumped into a supply timk urhich automatically drains, when full, to the first test tank, thence to the second and third tanks, and is finally discharged. The water in each tank rises to a l e d about 3 inches above the specimens before it passes to the next tank. The specimens are thus alternately met and dried a t about 45-minute intervals. The extrrnie lower portions of the specimens are contiriuously immersed. A s the apparatus is located out of doors, dlie specimens are also coiitinuously exposed to the weather in addition to alternate immersion in the salt water. This test, of course, cannot be made during freezing weather. In many instances failure of the poorer coatings occurs in 2 months, but many of the better coatings have Ritlistood 2 or 3 years of such test. In Table I are summarized the results obt,ained with .iarious types of surface preparation.
1.
ANUIXULLYCOATF:~ 17ST Box WITW Two COATS P A I N T AETER Two SEasONS (12 MowsmS) IN EDGEWATER Tesm
IlNliM
(There
WAS
little ehiingr during the ttrird aeaeon.)
I ' l t l ~ r ~PAINW u PUR A ~ ~ u h i i s t i ~ As previously indicated, it has riot been found tliat special painting practice is required for painting nluiuinum under ordinary conditions of service. Ifowever, as in the case of steel or any other metal, the value of the painting system is dependent upon the primer employed. This is particularly true where severe exposure conditions iiro encountered. A good priming coat for aluniinum, whether it be the c o ~ m e r cially pure metal or one of the heat-treated alloys, should meet the follo%kg requirements: Tlle primer should have good resistaim to moisture penetration and lrence prevent surface react,ioii. I n some cases the primer should crmtain a pigment possessing corrosion-inhibitive qualities. It should also sliow satisfactory adhesion to the metal suriace arid a t the same time present a surface permitting satisfactory adhesion of the succeeding coats of paint. Meeting tlie requirement of a moisturi:-reaistant film, nliimimtm paint made with a suitable vehicle stands high oii the list of primers for aluminum. Aluminum paint also shoms good adhesion to the metal and presents sufficient "tooth" for succeeding coats. For general a,pplication, aluminum paint has been found to give satisfactory service In auotlier series of testa of clear eoatiugs 011 17HT pa~lnls both as primer and top coat 011 aluminum. having various surface prepamtiom, soincwliat similar reFrom the standpoint of corrosion-inhibitive properties, sults were secured. These panels were given a single coat of a the chromate pigments are, in general; most suitable ( 1 , 4 , 6). clear lacquer and exposed to the weather for 6 months, a t a Of these, zinc chromate is prohahly the best. Excellent re45" aiigle facing south. The best surface prepnratiorr, as sults have been obtained with zinc cliromate primers o~ determined by nietal protectioii and the appear;mce of the aluminiim under particularly severe conditions of exposure.
26
INDUSTRIAL AND ENGINEERING CHEMISTRY
The selection of the vehicle for a priming paint for aluminum is important. I n the case of aluminum paint it has been found that a very long-oil varnish vehicle is most sui& able. A more distensible paint film is desirable in painting aluminum than in the case of steel because of the slightly higher coefficient of expansion of aluminum. This high expansivity becomes a factor only after aging has markedly hardened and embrittled the paint film. The vehicle also is an important factor in determining the moisture-proofing properties of the paint and should be selected with this factor in mind. These considerations are equally true whether aluminum bronze powder or other pigments, such as zinc chromate, are employed. Some of the newer synthetic-resin vehicles have been found to be particularly well adapted as vehicles for priming paints for aluminum. Of these, varnishes made from Bakelite and glyptal resins appear to be the best so far tested. Zinc chromate primers made on a glyptal resin varnish base have been found to be among the most satisfactory primers for aluminum. Iron oxide primers containing certain amounts of zinc chromate also constitute an effective combination. A specification for a primer of this type has been adopted by the Bureau of Aeronautics of the Navy Department. It has given satisfactory performance both in exposure tests and in actual service. I n the box tests conducted a t Edgewater (Table I), a number of different priming paints were compared, with interesting results. I n these tests the zinc chromate primer made with a glyptal vehicle gave the best results of any primer tried. Iron oxide-zinc chromate primers were next in effectiveness. Aluminum paint made with long-oil varnish vehicles ranked third. I n a series of weather-exposure tests conducted a t New Kensington, it was found that two coats of aluminum paint gave satisfactory protection to aluminum alloys for a period of 5.5 years. I n this test, blue lead also ranked well as a priming paint. Red lead, however, was somewhat inferior to either of the other two paints mentioned. These tests were not extensive enough, however, to warrant rating the lead pigments for priming aluminum.
FIKISHING COATSFOR ALUMINUM Once the metal is properly primed, any durable exterior paint or enamel may be employed for finishing coats. Here again, material long in oil is especially recommended; any desired color can be applied. Where the color of natural aluminum is satisfactory, aluminum paint will be found to give about the most satisfactory life of any coating. Whether used as a primer or top coat, aluminum paint will show its high moisture-proofing efficiency and opacity to Iight because of the specific nature of this film. The opaque metallic flakes of aluminum effectively retard the passage of light and moisture. The synthetic-resin enamels in various colors also form effective top coats. Lacquer enamels may also be used for this purpose, especially lacquer pigmented with aluminum powder. If lacquers are to be used, however, they should be of the most elastic type if satisfactory results are to be secured, The bituminous-base paints are frequently used on aluminum. Some of these paints with sufficient distensibility show satisfactory adherence as primers as well as top coats. They are high in moisture-proofing efficiency and are frequently used on the interior of duralumin seaplane floats and for waterproofing joints. They may be used alone where the black color is not objectionable, or may be pigmented with aluminum powder. Their tend-
Vol. 25, No. 1
ency to alligator or check is greatly minimized when they are so treated. Because of their resistance to alkaline attack, they may be used in back-painting aluminum used in construction where it is in contact with mortar or other alkaline materials. Generally speaking, a paint coating will show greater durability on aluminum than a similar paint coating applied to steel. A paint coating which may allow rust to become apparent on steel within 2 years may show good results on aluminum for twice that period before failure is noted. I n some of the tests conducted a t Edgewater, the built-up boxlike test structures contained both aluminum alloy and steel, and the same paint was applied to all surfaces. The durability of the paint coating was appreciably greater on the aluminum alloy than on the steel surface. I n the case of one box which had been given a chemically applied oxide coating prior to painting and then two coats of aluminum paint, the aluminum surface showed only a few slight blisters with tiny corrosion pits, whereas the steel was badly rusted. Even in the case of a box which had been merely cleaned with solvent, the aluminum was showing only small corrosion pits with some peeling of the paint film, whereas the steel again was badly rusted and the paint was peeling off in most places. After three seasons of 6 months each of accelerated testing a t Edgewater, two coats of aluminum paint over anodically coated 17ST were in excellent condition, showing but slight blistering at one end of the box with very slight pin-point corrosion pits on the top (Figure 1). Another anodically coated 17ST box, primed with zinc chromate followed by one coat of aluminum paint, was in even better condition, as it showed only slight corrosion in the joints of the box. These same coatings were applied to Alclad 17ST anodically treated and even better results were secured during the same length of exposure. CLEARFINISHES ON ALUMINUM When it is necessary to maintain a polished finish on aluminum sheet or castings, transparent coatings may be employed. These coatings may be of either the lacquer or varnish type, but must be distensible and resistant to sunlight. Pyroxylin lacquers ordinarily lack in durability unless they contain appreciable quantities of lighbresisting resins. Lacquers of this type are now available, however, and excellent results have been obtained in two-coat work for periods up to one year on panels exposed at 45' facing south. The tendency of the ordinary type of varnish to turn yellow makes its use questionable for this purpose. However, some of the newer synthetic-resin varnishes have been found to resist yellowing satisfactorily for substantial periods. These varnishes are usually of the modified glycerol-phthalate type; they show even greater durability than the lacquers, and their adhesion to the aluminum surface is excellent. Certain of the vinyl resins have also given satisfactory performance. LITERATURE CITED (1) Anonymous, h o c . Am. SOC.Testing Materials, 10, 73 (1910). (2) Bengough, G. D., and Stuart, J. M., U. S. Patent 1,771,910 (July 29, 1930). (3) Bengston, H., U. S. Patents 1,869,041-2; Gower, C. H. R., U. S. Patent 1,869,058 (July 26, 1932), and pending patents,
all assigned to Aluminum Colors, Inc., Indianapolis, Ind. (4) . , Gardner. H. A,, Am. Paint Varnish Mfrs.' Assoc., Sci. Sect. Circ. 296 (1927). ( 5 ) McCloud, J. L., IND.ENG.CHEM., 23, 1334 (1931). RRCEIVED August 29, 1932.
