Surface Preparation and Painting of Magnesium Alloys A.
W.WINSTON, J. B. REID,
HE last five
@T
AND
F.H. GROSS, The Dow Chemical Company, )lidland, hlich.
Paint adhesion and protection against sea w-ater are the main problems in connection with the painting of magnesium allops. Paint does not adhere satisfactorily to the untreated metal and the remedies suggested to overcome this are shown. Pretreatment of the metal surface by means of a chemical treatment is regarded as most satisfactory. The preferred and most widely used treatment consists of a dip in a nitric acid-sodium dichromate solution. Various factors in the operation and control of this treatment and their effect on subsequent paint adhesion are described. The importance of obtaining matte iridescent coatings in preference to bright yellow coatings is stressed. The effect of the composition of the primer and finish coats on adhesion and protection is discussed. Adhesiveness of paint system is first consideration for inland service conditions ; corrosion resistance, obtained by using inhibitory pigments such as zinc chromate, is main requirement for marine conditions. Typical satisfactory paint schedules are listed.
years have seen a remarkable development in the utilization of the light alloys of magnesium, marketed as DOWmetal and the A. 31.series of alloys in America, a n d a s E l e k t r o n m e t a l i n Europe. These alloys generally contain from 88 to 98 per cent magne-ium and varying amounts of aluminum, zinc, and manganese. Lightness is their outstanding c h a r a c t e r i s t i c , t h e specific gravity being about 1.8 or, in other words, 60per cent of the weight of the aluminum-base a l l o y s . Their high strengthweight ratio and other desirable physical properties are respon4ble for the rapid increase in application of these alloys. The commercial forms of the metal of interest to the painter are -and castings, sheet, structural shapes, and die castings. Assemblies of these forms ranging in size froin small typewriter parts to fully enclosed automobile-carrvine trailers 30 feet in length may be presented for painting. In common with the development of other new engineering materials, some problems have arisen in connection with decorative and protective painting. The purpose of this paper is t o discuss these problems and offer toward their >ohtion a combination of surface treatment and painting procedure which is proving satisfactory in service. The first problem, and the one with which we are principally concerned i. that of securing permanent adhesion of the paint coating under normal conditions of outdoor exposure. Corrosion generally is not a factor here, the metal being visually unaffected by conditions under which paint embrittlement and peeling take place. The second problem deals with the paint protection of the metal against severe corrosive conditions such as exposure to sea water. “
Y
Paint Adhesion to Untreated Metal That paint does not adhere satisfactorily to untreated magnesium alloys when exposed outdoors has been definitely established for a number of years. Peeling occurs and is accompanied by embrittlement and sometimes blistering of the paint film. This phenomenon is not confined to these alloys, for i t occurs also on other nonferrous metals, particularly zinc and cadmium (6, 14). I n view of the similarity in behavior of paint over these three types of metals, it is
?#+
natural to look for a conimoii cauqe. The “alkaline peeling” theory (7) has been suggested by Whitmore (14) to explain paint failure on zinc., cadmium, and magnesium, and his data 4how the extent of the alkalinity developed on various metal surfaces immersed in water and salt qolutione. Magnesium metal rapidly acquires a thin coating of magneqium hydroxide on exposure to moist air (13). In water the formation of a thin film of magnesium hydroxide on the surface of the metal occurs almost immediately, as may be demonstrated by placing a piece of t h e m e t a l ( e v e n f r e s h l y machined) into water containing phenolphthalein indicator. Strips of paint peeled from an untreated magnesium alloy surface show a distinct a l k a l i n e reaction 15-ith phenolphthalein, and it is inferred that the alkalinity is due to magnesium hydroxide and is the cause of the paint peeling. The hydroxide, of course, may be present prior to painting or be formed later by penetration of moisture throigh t h e p a i n t film. The reaction between the paint constituents and the nietal is also a possible cause for the failure of adhesion, and will be discussed in some detail under “Primers.”
Methods of Securing Adhesion The problem of securing permanent paint adhesion may be approached in two ways. The first is to develop special paints or paint schedules combining adhesiveness, resistance to alkalinity, great imperviousness to moisture, and strong inhibitory power. While this ideal paint has yet to be discovered, there is sufficient hope for success to warrant the considerable amount of work now being done along this line. Incidentally, if the cause of poor paint adhesion is essentially the same for all metals but varies in degree, it seems reasonable to assume that a paint which would adhere satisfactorily to untreat’edmagnesium alloys would adhere well to other metals. The second method of promoting lasting adhesion is through the surface treatment of the metal before painting. The possibilit’ies which suggest themselx-es are: 1. Mechanical roughening of surface 2. Natural weathering of surface 3. Anodic oxidation, or other electrolytic treatment 4. Application of dissimilar metal coating 5. Chemical treatment
1333
INDUSTRIAL AND ENGIKEERING CHEMISTRY
1334
TABLEI. EFFECTOF CHEMICAL TREATMEXT ON PAINT ADHESIOK (Alloy. Dowmetal F (Mg-4hl-0.3RIn); 3 , X 12 X 0.032 inch panels: atmospheric exposure a t Midland, Mich.)
Expt.
No. Chemical Treatment 1 Chrome-pickle; 1-min. dip in s o h . containing 1.5 Ib.
Paint Bchedulea
Paint Adhesion (Scratch) b 1 mo. 9 mo.
NazCr?O;.ZH:O 4- 1.5 Dints concd. HNOdaal.: room temp. 4 Chrome-phosphate: 20-min. dip in soln. containing 1 . :1 i\i'aHtPOa.HzO 1 Ib. NazCrzOr.2HzO/gal.: 9a-100° C. (203-21Zo,F.) ( 5 ) 3 Phosphate: 15-min. dip In s o h . containing 1 Ib. NaHzPOa (c. P.)/gal.: 95-100" C. ( 4 ) 3 Superheated steam: 15 min. a t 300° C. (572O F . ) ( I ) 1 Chrome-pickle. same as No. 1 4.5 Fluoride. 2 0 - d . dip in 48% H F (9) 3 Alum dighromate; 5-hr. dip in s o h containing 1.5% NazCrzOvZH20 1% potash alum 0.5y0 NaOH, 95-100" C. (12) 5 a 1 = 1 coat oil-base primer 2 coats oleoresinous synthetic resin enamel: 2 = alkyd-resin enamel. b Paint evaluations: 5 = good: 4 = fair+: 3 = fair: 2 = fair -: 1 = poor.
+
+
4
3 1 1 4.5 3
VOL. 27, NO. 11
factory. Bengough and Whitby ( 3 ) have shown a selenium treatment to be superior to some other8 with respect to corrosion resistance to sea water, rather than from the standpoint of paint adhesion qualities; but probably on account of the high cost of selenious acid the treatment is not in general use. The most successful chemical treatments developed to date are dips in solutions of nitric acid and dichromates, represented in the United States by the acid dichromate dip and by the chrome-pickle treatment.
Chrome-Pickle Treatment This treatment has been applied commercially on a large scale to magnesium alloys during the past few years. Some lack of adhesion was exDerienced with early paint jobs which was traced t o the nonuniform-nature bf the coatings. Extensive studies have pointed the way to correct solution control, enabling consistently satisfactory results to be obtained in the adhesion and life of subsequent paint systems. The process consists of a 15-second to 2-minute dip at room temperature in a water solution containing, when fresh, 1.5 pounds of sodium dichromate and 1.5 pints of concentrated nitric acid per gallon, followed by rinsing in cold, then warm water. This composition was adopted after studying the effects of variations in composition and temperature on the protective and paint base qualities of the coating, as shown in Table 11. As the bath is used the nitric acid and sodium dichromate decrease in strength (the former more rapidly than the latter) until the acid content reaches about 0.7 pint and the dichromate about 1.2 pounds per gallon. At this point, in order to continue to secure the proper type of coating, it is necessary to replenish the bath, bringing the sodium dichromate up to 1.5 pounds and the nitric acid to 1.3 to 1.4 pints per gallon. It was found advisable to maintain the relative acid concentration slightly lower in the revivified solutions to prevent the formation of bright coatings which afford inferior paint adhesion. In this way the bath may be rejuvenated five or six times before discarding, or until the by-products (magnesium, sodium, and chromic nitrates) become so concentrated in the solution as to render it impossible to secure satisfactory coatings. The time of dip must be increased gradually up to about 2 minutes as the solution ages. (Analytical control methods have been developed and are available to those interested.) Iridescent colored coatings with yellow and red shade
+
+
3.5 3 coats
Experience with these shows that the various forms of mechanical roughening (sand- or shot-blasting, wire-brushing, or sanding) may offer some apparent increase in adhesion when evaluated by scratch tests. The adhesion, however, is apparent only because, when a bend test is applied, no appreciable increase in the adhesion is observed. Satural weathering, such as is recommended for zinc or galvanized metal, is of no value. Anodic oxidation on magnesium alloys has not yet been satisfactorily accomplished. Dissimilar metals have been applied by electroplating or metal spraying, but the method has been found impractical (IZ), Corrosion is accelerated on the base metal when a break in the coating of the dissimilar metal occurs, and the risk of the accelerated corrosion outweighs the advantages gained in paint adhesion. The chemical treatment method has been extensively studied in the United States and Europe with the result that several processes have been developed which are proving completely successful in commercial practice. The published literature (f?, 3, 10, 22) on various chemical treatments dwells principally on the increased corrosion resistance to salt solutions or sea water conferred on the alloy by the use of the chemical treatment, with or without paint coatings. Salt-solution or sea-water corrosion data obtained in the laboratory are valuable for predicting paint service performance on the coast or on ships. Such data should not be used to predict paint adhesion, as, for example, on the magnesiumalloy side panels of a truck used exclusively in the Middlewest. For this type of service the authors believe there is no reliable accelerated substitute for the standard outdoor exposure test. Prior to any chemical treatment, the part should be free from oil, grease, heavy oxide, or casting skin. Removal of oil or grease is accomAND OPERATIXG TEMPERATURE TABLE 11. EFFECTOF COMPOSITION plished by washing in organic solvents such as OF CHROME-PICKLE SOLUTIOK ON PROTECTIVE AND P A I S T BASE QUALITIESOF COATING carbon tetrachloride or naphtha. Oil or grease Coned. N a L h 0 - 2 H 2 0 , Pounds per Gallon may also be removed by means of strong alkaline " 0 8 , -Aa-Bba---, -Cccleaners such as are used for cleaning steel, since Pints/Gal. 0 . 5 1.0 1.5 2 . 0 0 5 1.0 1.5 2.0 0.5 1.0 1.5 2.0 the metal is not attacked by these materials. Bath Operated a t 25-30" C. (77-86' F.) 0.5 2d 2 2 2 ........................ Oxide films and casting skins are best removed by 1 . 0 3 2 2 2 3 . . . . . . . . . . . . . . . . . . . . . pickling in a 10 to 20 per cent sulfuric acid 1.5 3 . 5 2 2 2 3 4 3 ... 1 3 3 . 5 ... 2 . 0 3 . 5 4 2 . 5 2 1 1 3 1 1 2 2 3.5 solution, or mechanically by sand-blasting, wirebrushing, grinding, etc. 2 2 3 ...................... 0.5 2.5 Several of the more important chemical treat2 2.5 2 5 ......... 1 . . . . . . 1.0 3 1 . 5 3 5 3 2 . 5 3 4 3 3 2 3 3.5 4 ments recommended as paint bases have been 4 4 2 . 5 1 1 1 1 1 3 3 3 2 . 0 3 tested (Table I). The phosphate process proa Corrosion resistance, unpainted: duplicate specimens, 3 X 1 X 0.25 inch, extruded: vides s a t i s f a c t o r y a d h e s i o n where the part immersed 20 see. every 2 mip. i n 3 % ,NaCl, 24 hr., per day: values a t 9 days. b Corrosion resistance painted; triplicate specimens 3 X 1 ,X 0.25 inch, extruded. treated, such as a sand casting, possesses a Painted 1 coat black iron oxide oil-base primer + 2 coats alkyd-resin enamel. Tested as in naturally rough surface and is sufficiently rigid A . values a t 4 weeks. This rimer was later found inferior t o newer materials which, if uLed in above tests, undoubtedly yould have improved the ratings under B and C. to withstand flexing. On smooth metal surfaces, Paint adhesion after 1 year; slngle specimens 3 X 12 X 0.032 inch sheet. Painted 8 8 in B. Exposed in atmosphere a t Midland, Mlch. however, such as sheet material, which in addid 5 = good; 4 = fair+; 3 = fair; 2 = fair -; 1 = poor. All specimens, 1-min. dip. Dowtion to being smooth are subject to a flexing metal F (Mg-4.41-0.3Mn) used throughout. movement in service, the adhesion is less satis-
-
0
d . .-
predoruinatiug a i d lusters from matte to briglit are obtainable. These features vary with the condition of the bath and the condition or corripositioii of the alloy, aud bear a definite relationship ta the paint adhesioii qualities of the cnating (Table ITS). The intensity of the color, as distinct from the actual color, of the coating is largely dependent on the time the article is exposed to the air in the interval between removal from the chrome-pickle solution and immersion in water. A t least a &second period of exposure to the air should he allowed to insure a full uniform shade and improved paint adhesion qualities. Addition of suiall quantities of sodium dichromate to the final wash water has been suggested to improve the color of the coating, but this practice liar been found detrimental to paint adhesion. The most iniportant siisgle feature in chrome-pickel coatings in relation to paiut adhesion is the type of etch obtained. A binocular microscope a t one hundred diameters has been found valuable in these exarninatioi The etch, as shown in Table 111,is a function of the compo ion and condition of the bath, the alloy composition, and also the time of dip. The type of etch is frequently indicated by the luster and color of the coating. Matte-gray to yellow-red iridescent coatings show under the microscope a pebbled surface or network etch of a depth and contour admirably suit.ed for good anchorage for a paint film (Figure 1). Bright yellow or bright iridescent. coatings show a relatively smootli surface with occasional round pits and little to no tooth for a paint film. The first type of etch invariably results in good paint adhesion and the second type in lair to poor paint adhesion. St is worthy of note that the bright yellow coatings provide better protection against sodium chloride solutions than the matte
l i t coatings. The rurttt.e coatings are generally produced i n fresh solntions, and t.l:e etch apparently is due to the selective dissolution of the aluminum-rich areas which, in the as-cast metal, occur adjacent to the grain houndaries. The bright smootli coatings are produced in used solutions, particularly if the nitric acid content (in pints per gallon) exceeds the sodium dichromate content (in pounds per gallon). h s shown in Table SII, solut.ion heat-treated alloys yield bright coatiuga rnuclr inore readily than the as-cast alloys. On chrome-pickling Dowmetal G , which contains about 10 per cent ahuninum and is in the sand-cast or heat-treated and aged condition, or which is die-cast, a gray to black powdery deposit of Mg.& compound may be observed on the surface of the metal. The occurrence of this deposit is believed to be due to the dissolution of the aluminum-rich areas in which the cornpound is embedded. Contrary t o expectation, the presence of small quantities of this compound on the surface is not detrimental to paint adhesion. I n the case of the aluminum-free alloys such as Uowmetal M, the chrome-pickle treatment produces a bright yellow or iridescent surface which shuws numerous small irregularshaped pits under the microscope. The continuous network found in the aluminum-containing alloy8 does not occur in the aluminum-free alloys and, incidentally, paint adhesion is better on the former than on the latter. As stated earlier, untreated magnesium alloys react alkaline to phenolphthalein almost immediately when immersed in water. A chrome-pickled surface, on the other hand, does not react alkaline for several days under the same conditions. This indicates that, while the nature of the etch is a major factor in paint adhesion, the presence of the passivating
INDUSTRIAL AND ENGISEERING CHEMISTRY
1336
TABLE111. RELATIOSSHIP BETWEEN ALLOY,COXDITIOX Dowmetal Alloy,--CompositionAlloy AI M n Zn Mg Condition
g;; A G H
10
' i ' ;:$] 0 . 2 . . . Rest 0 . 1 . . . Rest 0 . 2 3 Rest 0 . 2 . , , Rest 0.1 . . . Rest
..
.. .
8 10
Sheet; wrought
ChromePickleSoln.5 Newa Used c New
-
OF
VOL. 21,
CHROME-PICKLE SOLUTIOS, COATING, Coating Color
Luster
Matte M a t t e to semi-matte Jlatte
Gray t o yellow-red Yellow-red iridescent Gray t o yellow-red
G A G
6
8
.
ADHESIOS
AND PAINT
-
Etch ( a t 100 diam.) Deep network Network Deep network
Net work Used Matte to semi-matte Yellow-red iridescent Deep, irregular New Matte Pale iridescent Cast; s o h . heatSmooth, shallow Used Bright Yellow or pale iridescent treated Deep, irregular 8 0.2 , , Reet Caat: soln. heat-treated New Matte Gray t o iridescent Deep, irregular 10 0.1 , , , Reat and aged; die-cast Used Matte Gray t o iridescent Isolated round pits M 1.5 Rest As cast; sheet; New Bright Yellow or iridescent Isolated round pits Used Bright semi-matte Yellow t o pale brown wrought a Containing 1.5 Ib. NazCrzOr.ZH~0and 1.5 pints HNOa per gal.; used solutions about 1.3 to 1.4 pints "03 per gal. b ~~w an active solution which has not been revivified by additions of HNOi and dichromate. e Used a n active solution which may have been revivified from one t o several times.
A
1
so. 11
Paint Adhesion Good Good t o fair Good Good to fair Good Fair Good Good Fair Fair
I
chromate ion on the surface of the metal is also of considerable iniportance, particularly in the light of the "alkaline peeling" theory. While no data are shown here, it has been found that the chrome-pickle treatment properly applied is at least equal to the best of other recommended surface treatments and superior to most as a paint base for corrosive conditions wch a9 sea water.
tion, since the metal does not require much in the way of protection; in the latter the main requirement of the paint system is protection. Numerous tests have been run using proprietary paints to determine the best paint schedules. The results of one such test are given in Table IV which shows, in a general way only, some of the variables in formulation which affect adhesion and Drotection. This test, however. demonstrates that several paint systems possess good adhesion in atmospheric TABLEIV. RELATIONSHIP BETWEEN PAIXTSYSTEM ASD ADHESIOXASD tests, and several possess good protecPROTECTIOX OF DOWMETAL F (Mg-4814.3Mn) tion in 3 per cent sodium chloride s o h tion, but only some of these schedule. (All panels chrome-pickled prior t o painting) are good for both purposes. Primers Adhesion Ratings Per Cent (Scratch)" Failureb in Table IV which have the same pigPrimer -Finish Finish Finish ment and vehicle are different from each No. 1 Vehicle No. 1 No. 2 Pigment NO other in method of manufacture and Tung oil-phenolic 2 2 1.5 1 Zinc chromate Tung oil-phenolic 2.5 Q 10 2 Zinc chromate ratio of ingredients present, which es3 20 Tung oil-phenolic 3 Zinc chromate 25 Oil-base natural resin 4.5 4 Zinc chromate plains the differences in protective value Oil-base ester-gum phenolic 5 4.5 5 Red iron oxide of apparently identical systems. Tahle Oil-base ester-gum phenolic 45 3 4 6 Red iron oxide Linseed oil 3 4 100 (1 n m j 7 Red iron oxide IV also shows that one finish may be Oil-base synthetic 5 5 65 8 Red iron oxide inore compatible with the primer than Tung oil-phenolic 3 3 . 5 50 ( 6 weeke, Black iron oxide 9 3 4 Oil-base alkyd 40 10 Black iron oxide another.
:
3
2 2 10 Red iron oxide-zinc chromate Tung oil-phenolic 3 3.5 100 Red iron oxide-zinc chromate Oil-base natural resin 1 3.5 85 Red iron oxide-zinc chromate Oil-base natural resin 4 4 85 Red iron oxidelead chromate Linseed oil 4 4 Linseed oil 100 15 Iron hydrate Alkyd 5 4 65 16 Lead chromate-red lead 4 4.5 Tung oil-phenolic a5 17 Lithopone Oil-base alkyd 3 5 80 18 Titanox B Linseed oil 4 2 100 19 Red lead Linseed oil 5 4.5 90 20 Zinc dust Linseed oil 4.5 5 5 21 Portland cement illkyd 3 0 25 22 Pyroxylin 2 2 30 23 Pyroxylin 4 3 ? 24 6: Coal tar 1 1 3 25 a After I3 months of atmospheric exposure. Finish No. 1 = 2 coats gray oil-base alkyd-resinenamel; finish No. 2 = 2 coats oil-base ester-gum varnish 2 pounds of aluminum per gallon. b After 2 months in 3% NaC1; alternate immersion as described in Table 11.
11 12 13 14
I
+
Primers inay be applied over freshly chrome-pickled surfaces without any further treatment; but, if dust or oil has collected on the surface, it should be removed with naphtha or solvent. Proprietary metal cleaners which contain phosphoric acid, organic acids, or soaps generally impair or destroy the value of the chrome-pickle coating as a paint base.
Painting As stated earlier, the main problems in connection with the painting of magnesium alloys are adhesion and protection. The chrome-pickle treatment overcomes these problems to a very large extent, but further improvement is obtainable by selection of the proper paint schedule for the service conditions to be encountered. Two broad classes of service conditions are recognized: (1) inland atmospheres and (2) sea water or chloride-containing atmospheres. In the former the adhesion of the paint system is the principal considera-
Primers
The primer is the most important consideration in the paint system for either adhesion or protection. This is indicated in Table IV and is substantiated by other tests. Relatively fen of t h e h u n d r e d o r so p r o p r i e t a r y primers which have been tested have been found satisfactory, particularly from the standpoint of adhesion. With the cooperation of various paint manufacturers an effort was made to determine what factor in the composition of these primers was common to all and accountable for their superior adhesive qualities. The presence of too many variables made it iinpossible to obtain a true comparison, but tests are now in progress in which all phases of formulation are kept constant except the one being studied. It was suggested that reaction between the paint and the metal was responsible for poor adhesion. Tests on numerous paint constituents, as well as on a number of proprietary primers and paints, disclosed that the untreated metal reacted in various degrees, forming a dark adherent deposit of a greasy layer of magnesium compound with all materials tested, exclusive of some thinners. No consistent relationship, however, was observed between the degree of attack and adhesive quality of the paint. PRIMER PIGMENT. The type of pigment in the primer appears to have little effect upon adhesion, although much work
SOYEIIBER, 193.5
ISDUSTRIrlL A\ D ENGINEERISG CHETIISTRT
Service Interior (normal)
Primer 1 c o a t air-dried
Surfacer 1 coat air-dried (optional)
Exterior-normal weathering (Truck a n d trailer bodies, certain aircraft parts, etc.) Marine," continuous moist air, or where corrosion protection is desired. (Seaplane, ships, etc.) Baking schedules (tools, typewriter parts, etc.)
1 coat air-drled
1 coat air-dried (additional
1 coat air-dried zinc c h r o m a t e phenolichase primer
primer or finishing coat optional) ioptional 1 c o a t air-dried for smoothness'i
1 or 2 coats, depending snioothness desired
1 coat baking primer
'1 For maximum adhesion and salt water resistance. bake each coat of primer, surfacer, and finish.
reiriaiiis to be done on this subject. If loss of adhesion is due t'o reactions resulting from the penetration of moisture through t,he paint film,. it would appear logical to use pigments in the primer which inhibit such reactions or n-hich a t least are inert'. As a check upon the possible influence of the primer pigment, a large number of tests were run where strips of hare untreated metal were immersed in pastes of various piginents in distilled water. A11 of the leaded pigments (with one exception), black iron oxide, antimony oxide, and some organic toners appeared to promote corrosion seriously; lithopone, barytes, barium- and calcium-base titanium pigiiients caused slight corrosion; titanium dioxide, zinc oxide, lanipblack, aluminum powder, basic lead chromate, red iron oxide, asbestine, and most other extenders were inert. Zinc chromate alone was definitely inhibitive in its effect. Mixtures of some of the corrosion-accelerating pigments with the inert inaterials and with zinc chromate confirmed the supeT.IBLEVI.
Oil type
ISFLUENCE O F VEHICLES . 4 S D RESIXS O S PROPERlIEs OF PRIMERS A P P L I E D TO DOWMET 4L Drying Hard- Adhe- Water Primer Time ness m i n Resistance" Hours 24-72 12-24 1-6 2-8 2-6
Oleoreainous l l k y d resin Modified alkyd resin Modified phenol-formaldehyde resin Vinyl resin Baked only Chlorinated rubber hydrocarbon resin 0,25-2 0 A s determined by alternate immersion i n 3%
Poor Fair Good Good Good Good
Good Good Good Good Fair Good
Fair Fair Fair Fair Good Good
Good PcNor Tery good NaCl solution.
riority of this latter material as an inhibiting agent. Xhile the test conditions are not, comparable to those in a paint coating undergoing exposure to weather, i t is believed that primer pigments for this service should be chosen from the inert group and that corrosion-accelerating materials should be abbent. When exposure conditions are severe and where protection is of first importance, t'he primer should contain zinc chromate. This conclusion is in h i e with tbe recent'ly deyeloped corrosion inhibiting primers, such as the Xavy P-27, where zinc chromate constitutes the entire pigment,. The inhibiting action of zinc chromate is probably due to a slight solubility in water. This solubility may be the cause of a reduced adhesion on Ilowmetal, noted by Fuller (8) in teste in which the pigment was the only variable. PRIMER VEHICLE. From the information on composition available, it was observed that the vehicle used in various primers had considerable influence on the adhesion to the metal. It was not possible, however, to point out any one type of vehicle as being the best since several types exhibited satisfactory adhesive qualities. The type of resin used in a
011
1337
Finish 1 or 2 c o a t s good-(luality oil p a i n t or e n a m e l ; la?q u e r finishes, if used over a compatible surfacer 2 coats air-dried enamel, semi- or full s y n t h e t i c
T o t a l Minimum Coats
2 or 3 coats air-dried phe-
4
nolic-base varnish cont a i n i n g 1 . 5 2 Ih. aluminun1 powder per gal. 2 or 3 coats baking enamel (1 coat crystal finish IS proving satisfactory for light service)
-1
4
2 or 3, depending on finish desired a n d service conditions
vehicle appear> to exert appreciable mfluence; Table VI repre-ent- the coiiclusions on those materials tested. The properties shown are generally true of these materials, rvhether they are applied to magnesium alloys or to other surface\ (11). The ratio of pigment to vehicle has an influence on adhesion. Indication. are that a 1 to 1 or 2 to 1 ratio gn'ei niavimum adhe\ioii in a formulation.
Finishing Coats The use of a surfacer intermediate between the primer and the finish coats makes a noticeable improvenient in the length of time that adhesion is retained, probably because of the increase in impermeability. Similar improvement' is observed when, instead of a .surfacer, an additional coat of finish is used; but an advantage in sinoothneas of the completed job is a n incentive to use a surfacer rather than a n additional finish coat. The choice of finish coats is largely dependent, upon the conditions to be encountered. Maximum durahility and weather resistance are desirable in combination with impermeability. It has been the writers' experience that semi- and full synthet'icmaterials satisfy these conditions. The recominendat'ions of most reputable paint manufacturers ivith regard to their enamels and surfacers can be followed. I t is advisable, in order to insure compatibility, that the products of one manufacturer be used throughout the schedule. Finishes baeed on phenolic-type resins have been found to possess good water resistance and are best for exposure in\ d y i n g occasional contact with sea water. Table T' lists 5ervice conditions and shows the type of paint system which in practice has been found most, suited for each. In all cases the metal v-ap properly chrome-pickled prior to painting. Literature Cited (1) Backer, ('. B., E. S . P a t e n t 1,481,788 (.ipril 17, 1923). ( 2 ) Bengough, G. D., a n d W h i t b y , L., J . Inst. .Ilt!taZs, 48, 147 (1932). ( 3 1 Bengough, G. D., a n d W h i t b y , L., Trans. Insi. C'hsin. Eng. (London), 11, 176 (1933). 14) B u r d i c k , E. C. ( t o T h e DOK ('heniical ( 2 0 . ) . Lr. S. P a t e n t 1,709,894 (.ipril 23, 1929). ij) Burdick, E. C , , a n d Gross, W. H. (to T h e Dow Chemical Co.), U. S. P a t e n t 1,947,122 (Feh. 13, 1934). (6) C'orbin, M.H., ISD.ENQ.CHEM.,25, 32 (1933). (7) ET-ans, U. R., Trans. Am. Electrochein. SOC.,55, 243 (1929). i8) Fuller, JV. R . , p r i v a t e communication. (9) Keller, L. J., U. S. P a t e n t 1,574,289 (Feb. 2 3 , 1926). (10) McCloud, J. L., ISD.EKG.CHEM.,23, 1334 (1931). (11) P l a m b e c k , A . O., Can. Chem. &. . V e t . , 18 (3), 50 (1934). (12) S u t t o n , H., a n d LeBrocq, L. F., J . Inst. .IIetaZs, 46, 54 (1931). (13) W h i t b y , L . , Trans. Faradaa S O C . , 29, 844 (1933). (14) W h i t m o r e , M. R., ISD. ENG.C H E X . .25, 19 (193:3). RECEIYED July 18, 1933. Presented before the Division of Paint and Varnish Chemistry a t the 89th Meeting of the American Chemical Society, New York, N.T., -4pril 22 to 26, 1935.
