COURTESY, NEWPORT ROLLlNa MILL COMPANY
Commercial Equipment for Bonderizing Aluminum Sheets
R. C. Gibson and W. S. Russell PARKER RUST-PROOF COMPANY, DETROIT, MICH.
T
HE increasing use of aluminum and its alloys, due t’o light weight, availability, and reduced cost, has emphasized the need for obtaining maximum protection against corrosion. I n this paper the term ‘Laluminum”will be taken to include sll alloys of the metal in wrought or cast form. Aluminum has a high affinity for oxygen; an oxide film is formed upon exposure to the atmosphere, n-hich prevents further sttack unless it is removed or penetrated by chemical or mechanical means. Most of the aluminum used in industry today is in the form of alloys which differ greatly in physical and chemical properties. The many and varied uses to which these alloys are put require, in most cases, some type of organic fi+sh for protection and decoration of the surface (6, 11). These finishes must be durable under the conditions t o which they are subjected, rctain a pleasing appearance, and p o t e c t the base metal from corrosion. Surface conversion processes are the only methods in use which prepare the surface of aluminum as a good base for an organic finish, Surface conversion coatings, formed by chemically converting the metallic surface, have as their main objective the formation of a stable and nonreactive coating, integral with the
base metal, which inhibits corrosion and increases the adherence of applied organic finishcs. The common methods of converting aluminum fiurfaces are acid etching, treatment with phosphoric acid-organic solvent mixture, production of chcmical oxide coating, production of electrochemical oxide coating, and formation of phosphate coating. -4cid etching t,rentments may coiiSist of immersion of the aluminum in phosphoric or chromic acid subsequent to cleaning in an inhibited alkali cleaner. The process is commonly referred to as chromodizing when chromic acid is used. Thin films of aluminum phosphate or a chromium containing compound are supposed t o be formed. The phosphoric acid-organic solvent mixture is a surface preparation made up of aqueous solution of phosphoric acid with organic grease solvents and emulsifiers. The mixture is usually spplied by brush-on or dip, and probably forms a thin film of nluminum phosphate on the surface of the metal (U. S. Army Corps of Engineers Specification T-1184-D). The chemical oxide method is illustrated by a two-step procesb in which the aluminum is immersed in a hot solution containing sodium carbonat,e and chromate followed by a water rinse and 1222
December, 1946
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
then a sealing treatment in hot potassium dichromate solution (4,10). The electrochemical oxide coating is formed by a n anodic treatment using as electrolytes aqueous solutions of acids such as sulfuric, chromic, boric, or oxalic ( 3 ) . Phosphate coating method '
A recently developed surface coating method consists in reacting the aluminum surface in a metal acid-phosphate solution containing oxidizing agents and a complex fluoride for accelerating the coating action (7, 8, 9). The metal surface is converted to a finely crystalline, nonmetallic phosphate coating of the proper texture, adapted to inhibit corrosion and increase the adherence of any applied paint film. Figure 1 presents photomicrographs of untreated and phosphate coated 24s-T aluminum sheet. Figure 2 shows the effect of phosphate coating on paint adhesion. An aluminum alloy panel was cleaned with an organic grease solvent, then the lower part of the panel was phosphate-coated (Bonderized). An alkyd baking enamel was applied and then ruptured lengthwise of the panel by the commonly used knife test. The poor paint adhesion on the untreated surface and the excellent performance over the phosphate coating are apparent. Operation of process
The phosphate coating process may be carried out by immeruon or spray application in the same type of equipment and with similar cleaning and rinsing cycles as are used for processing steel, zinc, and their alloys (1). A typical layout for the process consists of the following five stages: cleaner, water rinse, Bonderite, water rinse, and chromic arid or pho.sphoric-chromic acid rinse. All aluminum to be Bonderized should be free of grease, oil, or other foreign material. The cleaner stage may be solvent piash (naphtha, kerosene, mineral spirits, etc.), vapor degrease, mild alkali, inhibited alkali, or other methods used for cleaning metal. The choice of cleaner and of application by immersion or spray depends on the type of work being processed and the dirt likely to be encountered, In some cases hot water spray suffices for adequate cleaning. The water rinse following the cleaning operation removes the cleaning agent and prevents contamination of the phosphate processing solution. The aluminum surface is converted t o the phosphate coating by immersion in the processing solution for 30 seconds to 4 minutes a t 170-185" F. or by spraying the solution for 10 seconds to 2 minutes at 140-155" F. The processing time depends upon such factors as the type of aluminum alloy being treated and surface pretreatment required. The excess coating chemicals are removed by cold water rinse lasting 20-60 seconds. A final rinse of the coating in a 0.05% solution of chromic qcid or phosphoric-chromic acid mixture, followed by a dry-off stage, completes the cycle of preparing the metal surface for all types of paint.
A process is described which produces on aluminum and its alloys, either in sheets or castings, from a solution containing zinc phosphate, nitrate, and fluoborate, a paint-holding phosphate coating similar to that given iron, steel, zinc, and cadmium surfaces by Bonderite processes. In addition, it also coats zinc and steel and their alloys, an advantage to manufacturers who may have mixed production of these metals. This process requires no electricity, may be applied by spray or immersion in mild steel equip-
1223
I n some cases a pretreatment before Bonderizing is found beneficial to the formation of a fine-grained phosphate coating. The type of stock being processed, the cleaning used, and the type of coating desired determine whether a pretreatment is used. Pretreatment methods consist of alkali or acid pickling, application of a specially prepared disodium phosphate solution (5), or mechanical wiping action over the surface of the metal with a material such as cloth, rubber, or Tampico fiber or horsehair brushes. One type of equipment used for processing metal sheet or strip by immersion is shown on page 1222. The clean sheets of metal are fed into rubber-covered rolls which convey them onto a chain conveyer. The conveyer carries them through the phosphate solution and into another set of rubber-covered rolls. Spray water and spray chromic acid rinses follow the coating operation. At the exit end of each stage, excess solution is removed from the surface of the metal by a pair of rubber-covered rolls and thereby reduces drag-out losses. This type .of apparatus is ryidely used at present for Bonderizing zinc electroplate and galvanized iron ie the steel mills. Solution and coating
One type of solution in commercial operation consists of a dilute aqueous solution of zinc dihydrogen phosphate in balanccwith phosphoric acid and acid phosphates, an oxidizing agent such as zinc nitrate, and a complex fluoride such as zinc fluoborate. A typical processing solution has the following composition, in per cent by weight: Zn*+, 0.7; POr---, 1.0; NO*-, 2.0; BFa-, 1.0, All constituents of the coating solution may be widely varied without affecting the beneficial results obtained. The processing solution is acid to bromophenol blue and is controlled by maintaining a definite titration against standard alkali using phenolphthalein indicator. The bromophenol blue titration is known as free acid and the phenolphthalein titration as total acid. As the solution is worked, the zinc phosphate, oxidizing agent, and complex fluoride are consumed. Replenishing materials are added to maintain the processing solution a t the proper operating strength. These replenishing materials are compounded so that all constituents in the processing solution remain within a definite range when the total acid titration is controlled by the addition of replenishing material. The aluminum surface reacts with the phosphoric acid and acid phosphate, activated by nitrate and fluoborate; local supersaturation is produced at the metal-solution interface with deposition of a nonmetallic and finely crystalline coating, which consists essentially of acid phosphates more basic than the dihydrogen phosphate, with small but appreciable amounts of fluoride, aluminum, and various alloying elements. Analyses of typical phosphate coatings are given in Table I. The phosphate coating weights obtained vary from 100 mg. to 2 grams per square foot (929 sq. cm.) of surface area, depending on the method of application of the coating solution and the type of metal pretreatment used.
ment, and is readily adapted to conveyer production methods. Outdoor exposure tests, as well as accelerated salt spray, humidity, and soak tests, show the improved results obtained by converting the surface of aluminum alloys to a stable, nonmetallic, finely crystalline phosphate coating, prior to painting, to inhibit corrosion and increase the adhesion and resultant durability of applied organic finishes. This coating permits the use of a variety of paints which cannot be used effectively over untreated aluminum alloys.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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Vol. 38, No. 12
permits deeper draws, reduces product breakage, and lengthens die life (Figure 3). Paint t e s t s
The commonly used methods i ' ~ rcvaluatiiig durability of paint films over meta,l surfaces were employed in this investigation. Accelcrated laboratory tests consisted of salt spray, humidity, and soak tests. They were supplemented by outdoor exposure and paint adhesion tests. The salt spray tests show the effect of corrosion on the durability of paint films. Painted panels were scratched by a sharp instrument so as to expose a very thin section of the metal surface before they were placed in the salt spray room. The extent of paint film pcelFigure 1. Comparison of Untreated ( l e f t ) and Phosphate-Coated ing from this scratch mark due to corrosive ( r i g h t ) 24s-T iliiminum 411oy ( X 100) action or loss of paint adhesion is a relative measure of the degree of breakdoxrn. The salt spray room was operated with a standard Mixed m e t a l production 20% salt spray at 95' F. (A.S.T.M. Designation B-117-44T). The huinidity test, which shows the effect of moisture penetration The Bonderite solution, h i d e s coating aluminum, will also on the paint film, was conducted in a room a t 100" F. and 95lOO7, relative humidity. The soak test 'i~ as made by immersion coat zinc and steel and their alloys. Mixed-metal commercial a t 100" F. articles which are being successfully coated consist of aluminumThe outdoor exposure test, which approximates actual service base die castings, sand castings, permanent mold castings, zincconditions, was made by placing the scratched panels on insulated base die castings, wrought aluminum alloys, cast iron, and coldracks a t Morenci, Mich. (rural atmosphere), at a 45" angle facing south. The test panels used were cut 4 X 6 inches in size from rolled steel. Parts for typewriters, calculating machines, mixers, approximately 20-gage metal. A11 panels were solvent-cleaned to furniture, scales, motor housings, automotive sheet metal, reremove oil, grease, or foreign material which might be deleterious frigerator cabinets, metal wall tile, washers, etc., illustrate the to the paint film. All variations were tested in duplicate. type of production which is being treated by the phosphate r o a t ing process. l'he paint system> used on the test samples are described in
.
Drawing aid
Production experience has shuwii that the Bonderite coating is very effective as an aid to the deep drawing of aluminum. TI
'I'able 11. Photographs of painted paneb after salt spray and outiloor exposure tests are shown in Figures 4 to 8. Figure 4 compare' the corrosion resistance and paint adherence irt toui different paint iystrm\ on Bonderized 2S-l/.JX, 5 2 8
Figure 2 ( L e f t ) . Effect of Phosphate Coating on Paint Adhesion
Formed blank, Bonderized
First draw, Bonderired
Figure 3 .
Second draw, Bonderired
Third draw, Bonderized
Fourth draw, headed, trimmed Bonderized
Fifth draw
.Iluminuni Cylinder Drawn with Aid of Phosphate Coating
Sixth draw
December, 1946
INDUSTRIAL AND ENGINEERING CHEMISTRY
1092 Hours in Salt Spray
A
A
1225
9 Months Out of Doors
B
C
D A Panels of Aluminum Alloy 2S-'/sH
B
B
C D A Panelu of Aluminum Alloy 52S-'/rH
B
B
C
D A Panelm of Aluminum Alloy 24s-T
B
c
I)
Phoaphat'eCoated
Untreated
A
C
D
Figure 4. 'Condition of Treated and Untreated Aluminum Alloy Panels, Painted with Systems A , B , C, and D , after Salt Spray and Outdoor Exposure
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INDUSTRIAL AND ENGINEERING CHEMISTRY
1226 17s-T
2S-'/,H
24s-T
61s-T
52S-l/*M
7.5s-T
R-301-0
Plioaphatr-
Coated
Paint System E (Zinc Chromate Primer)
l'lioaphatr-
Coatrd
L'ntrratrd
Paint System F (Alkyd Type Primer)
Figure 5 .
Condition of Painted Alurninuni i l l o y Panels after 502-Hour Exposure to Salt Spra?
PhosphateCoated
Untreated
52S-'/sH -__--
---
R-301-0
Paint B 1015 Hours i n S i l t Spray
Figure 6.
52S-Iid3
R-301-0
Paint D , 1015 Hours i n Salt Spray
52 S-'/2H
R-301-0
Paint G, 1015 Hours in Salt Spray
52S-'/aH
R-301-0
Paint D , 9 Months of Outdoor Exposure
Condition of ilurniuum Alloy Panels Painted with Three Types of Orjianic Finishes
INDUSTRIAL AND ENGINEERING CHEMISTRY
December, 1946
1227
PhosphateC0.d
Untreated
Steel
2S-'/tH
24s-T
R-301-0
Figure 7. Condition of Panels Painted with System A , after 744-Hour Exposure to Salt Spray
PHOSPHATE COATING ANALYSES, IN PER CENT Immersion-Coated Spray-Coated A1 alloy A1 alloy 2s 24s-T Steel 2s 248-T Steel 39, fin 37.0 32.6 31.4 38.1 32.9 PO4 38.2 38.8 42.1 41.8 36.7 41.0 F 2.7 4.1 1.4 1.9 3.3 AI 1.2 3.2 ... 1.0 3.0 Fe G5 .. 75 8.2 1.8 !.! 412 LU .0.. .4 ... ... 4.0 Mlg 0.1 0.4 ... 0.3 1.0 Cr" 0.04 0.03 0.1 0.4 0.8 0.7 0 Chromium ie present because the coating has received a chromic acid rinse. The remainder of the coating contains an undetermined amount of water, part of which is present as water of crystalliastion of zinc phosphate.
TABLE I.
... ...
Zinc plate, 1 K 10-6 inch
Zinc plate, 5 X 10-6 inch
Hot-dip galvaoized iron
Figure 8. Condition of Panels Painted with System B , after 200-Hour Exposure to Salt Spray
well as over aluminum alloys was greatly enhanced by phosphate coating. Humidity test of 504 hours using a refrigerator paini system was satisfactory on Bonderized steel, 2S'/$H, 24S-T. and R-301-0. Untreated 24S-T and R-301-0 failed; steel and 2S-'/ZH showed blistering. Salt spray tests of panels shown in Figure 8 demonstrate the effectiveness of the Bonderite treatment on zinc electroplate and galvanized iron. I n the soak test of 339 hours the interior cabinei paint system was satisfacto;y on the coated panels but failed nD the untreated stock.
TABLE 11. DESCRIPTION OF PAINT SYSTEMS t/zH, and 24s-T aluminum alloys with the untreated alloys as tested by salt spray and outdoor exposure. All of the phosphate coated aluminum alloys were satisfactory with all paint systems in a 741-hour soak test; the untreated 24S-T and 52S1/2H showed early failure and 2S-l/2H gave considerable blistering. Darsey (2) reported that the durability of paint films applied on 2S-'/*H aluminum alloy, as shown by an 18-month Florida exposure test, is not influenced by the degree of blistering that the Blms exhibit in the humidity test. He found that the degree of blistering does not materially affect the durability of paint films applied over almost noncorrodible metals. However, it should be noted that the paint adhesion on uncoated, heat-treatable alloys such as 245 is not so satisfactory as that obtained on 2 s (commercially pure aluminum). Figure 5 shows salt spray tests which compare a zinc chromate primer with a modified-alkyd primer-surface applied over phosphate-coated and untreated aluminum alloys; both primers had a finish of yellow alkyd exterior enamel. The zinc chromate primer tended to prevent formation of visible salt spray corrosion products on the untreated alloys but did not promote good paint adhesion. Paint durability on all alloys except 2S-1/2H was poor in this test. Both paint systems were satisfactory over all of the Bonderized aluminum alloys and the untreated 2S-'/zH alloy. Figure 6 demonstrates the good paint-holding properties of phosphate-coated 52S1/zH and R-301-0 panels compared to the untreated alloys with three types of organic finishes. The phosphate-coated panels in Figure 7 were processed on a commercial Spra-Bonderite line in a large automotive plant and, after being painted, were submitted t o a salt spray test along with untreated metal panels. Durability of the paint film over steel as
Paint System A . Refrigerator Prime coat Finish coat B . Interior cabinet C. Sign Prime coat Finish coat D . Sheet metal E.
Prime coat Finish coat F. Prime coat Finish coat G. Automotive body Prime coat Finish coat
Baking Schedule Min. O F
Composition
30
60 I5
350 260 325
Oil-modified alkyd Yellow alkyd exterior 1 coat urea-formaldehyde modified alkyd Zinc chromate primer Yellow alk d exterior enamel Modified-% d primer-surfacer Yellow alkyiexterior enamel
30 30
350 260
1 coat oil-type dipping primer 2 coats nitrocellulose black lacquer
25 30
Oil-modified alkyd Alkyd urea-formaldehyde 1 coat urea-formaldehyde
+
+
60 260 Air-dry 30 250 30 228 30 250 400 180
LITERATURE CITED
(1) Darsey, V. M., IND.ENG.CHEM.,27, 1142-4 (1935). (2) Ibid., 30,1147-52 (1938). (3) Edwards, J. D., Monthlv Rem. Am. Electroplaters' SOC.,26, 518 (1939). (4) Edwards, J. D. (to Aluminum Co. of America), U. S. Paten1 1,946,152(Feb. 6,1934). (5) Jernstedt, G.W.,Trans. Electrocha. SOC.,83, preprint (1943). (6) Merkle, C.R. E., Steel, 116, No.19,109 (1945). (7) Tanner, R. R., and Lodeesqn, H. J.,(to Metal Finishing Research Corp.), U. s. Patent 1,911,726(May 30,1933). (8) Thompson, J. 5. (to Metal Finishing Research Corp.), Ibid. 2,234,206(March 11, 1941). (9) Thompson, J. S. (to Parker Rust-Proof Go.), Ibid., 2,312.85h (March 2,1943). (10) Wray, R. I., Aviation, 40,83 (1941). (11) WraJr, R. I., Ind. Finishing, 21, No.2, 25 (1944).
