THE SWITHFIELD STREET BRIDGE, PITTSBURGH. Pa. The aluminum floor structure as uell a8 the steel
18
finished u l t h a n aluminum paint top coat
Painting Aluminum and Its Alloys JCNIUS D. EDWARDS AND ROBERT I. F R A I Aluminum Research Laboratories,
New Kensington, Pa.
I
S PREVIOUS communications on the
hesion and qerviceability on the different alloy.. Generally speaking, paint performance ib about the same on the subject of the painting of aluminum and its alloyq, several points have been emdifferent alloys of aluminum; there are, however, niinor differences to be observed in paint adhesion on the different phasized (8,3 ) . Preparation of the surface is important alloys, which may be important for specific types of service. &ere service conditions are severe, particularly where salt These differences, moreover, are of considerable interest water immersion may be experienced. The production of a from the theoretical side, since they may throw wme light passive surface before painting is desirable in these cases. upon the question of what determines paint adhesion to The use of a primer pigmented with an inhibitive material, metals and how it is lost. Their practical importance is such as zinc chromate, is effective in promoting adhesion diminished by the fact that the alloy is usually selected beunder severe service conditions. Top coats of high moisture impedance and durability, such as aluminum paint, are essential t o long-con~ tinued protection. may be JIinor differences in paint adhesion have been noted on different The p a i n t i n g of wrought alloys of aluminum, and some observations on the correfor decoration or protection; in either case satisfactory adhesion and life of lation of paint adhesion and alloy composition are presented. the paint coating is 'lost The production of passive surfaces, as best exemplified by certain tests on the p a i n t i n g of a l u m i n u m anodic coatings, minimizes any differences between alloys and alloys have been made with alloy 17s or duralumin, as it is frequently termed. gives an excellent base for holding paint, particularly where subl l l o y 17s has been e x t e n s i v e l y emjected to immersion in water or very humid conditions. Even p b e d in s t r u c t u r a l work, and one without special surface preparation, aluminum alloys hold paint such use is in very thin "Ieet sections very satisfactorily in comparison with other structural metals. for p o n t o o n s a n d w i n g surfaces on In the painting or repainting of surfaces which have been corroded, s e a p l a n e s . Painting tests have also preliminary treatment with chromate solution has been found been carried out with other alloys of aluminum, s o t h a t a n i n t e r e s t i n g effective. comparison can be made of paint, ad1143
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cause its physical prc,pertie> tit tlie job and not liecaose of
iriinor differences iir paint-holding cliaracteristics. \Vit,ii d l t h i s ilircussirm of paint adhesion and painting techiiic, it u m ~ l dbe inerirrect t r i give the impression that it is difficiilt bo paint aluininuiri wtisfactorily. As will be shown in a later section, paints generally adliere to and protect aluminuin and ibs alloys with greater ease than do the same paints on steel or some &or metals. It is not a difficnlt metal to pnint; b u t as in the ease of otlrer metals the search for the best practice is oE Imctiml interest.
Paint Adhesion
Alurriiriiirn .2lloys
011
Siorrie of the conirnercially wed wrought alloys of aliiniinuirr are listed in Table I; alloy 25, cominercial alurniriuin with a purity of about 99.2 per cent, is not gtmerally considered ari alloy, altliougli included ill t.he table. .. TA1lI.K
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I. NoMrs*r, cour~asri.is OF Ar.1.oua
ihy"
Copper
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WllooGrlr AlrT?\
