Aluminum, Earth’s most abundant metal, is rarely found in a pure state.  Highly reactive, with a voracious appetite for oxygen, the element appears in nature as alumina, natural aluminum oxide.  By the early nineteenth century, British scientist Sir Humphry Davy had joined the growing list of chemists trying to isolate the metal.  He failed, but he left the metal with a name.  Well, three names, actually.  Davy’s first choice was alumium, a name he soon switched to aluminum, the name now commonly accepted in the US.  This he subsequently changed to aluminium, as the metal is known in Europe and the British Commonwealth. 

Although aluminum was successfully isolated in 1825, by 1886 the world’s total production was still less than 100 pounds (45 kg). That year, experiments in the US and France independently developed a new process that could produce nearly pure aluminum at commercially acceptable prices.  Aluminum was soon being cast into engine blocks which weighed less than a third as much as similar steel units.  Although the surface of each aluminum block rapidly formed a thin coating of aluminum oxide, the coating remained on the surface.  (Similar oxidation on steel would eventually rust through the entire block.) 

Pure aluminum was not strong enough to form aluminum sheet; aluminum alloys could be successfully produced, but proved susceptible to intergranular corrosion.  The alloys required priming, usually with the same oil-based primers used for steel and iron.  (The Air Corps’ 1938 list of approved primers showed the use of commercial products named red oxide, plum, light grey, and pink.) 

The Aluminum Company of America (Alcoa) developed a new type of  aluminum alloy sheet in the late 1920s.  Named Alclad, the sheet was rolled with a thin layer of pure aluminum, protecting the internal alloy from corrosion. 

By the early 1930s, a zinc chromate primer had been developed, apparently by the Ford Motor Company.  The primer proved popular in commercial aviation, but was foolishly rejected by the military for nearly four years.

 

 

Alclad, Aluminum Primers and Paints

 

Chronology

The following notes follow the early development of the Army’s (and, to a lesser degree, the Navy’s) interest in powdered aluminum, Alclad, and various primers and paints.   

20 Jun 1924   Army spec 3-44A for Bronze Powder, Aluminum allows maximum of  .15% copper and .05% lead. 

20 Jul 1926    Army spec 3-100 lists protective coatings for steel as zinc plating, cadmium plating, “Parkerizing, [phosphate treatment]” Indian Red (Fe₂O₃) oil-based primer, or 1 lb polished aluminum powder in 1 gal spar varnish followed by 2 coats air drying enamel.  Protective coatings for aluminum to be Indian Red primer followed by 2 final coats of aluminized spar varnish.  Exterior of cowling can be not less than 2 coats spar varnish or colored enamel. 

06 Oct 1930    Ford memo notes that the Air Corps rejected zinc chromate primer. 

15 Dec 1930  Army spec 3-100B makes first mention of Alclad 

21 Apr 1931    Report lists finishes for aluminum & alloys:  1 coat oil base metal primer & 2 coats of pigmented nitrate dope or 3 coats of semi-pigmented dope or 1 coat of iron oxide chromate primer with two coats of oil enamel. 

17 Nov 1931   Spec 3-100C notes that aluminum cowling interiors need not be painted.

13 Oct 1934    Panagra memo notes use of Ford zinc chromate for about 3 years and DuPont zinc chromate for about 1 year. 

03 Dec 1934  Panama Air Depot memo requests zinc chromate primer; Air Corps has used it for at least 1 year with satisfactory results. 

28 Mar 1935   Berry Brothers zinc chromate primer is approved; no specification is yet prepared, since material settles out in the can. 

25 Jul 1935    Memo lists 3 approved zinc chromate primers; Ford primer is now made by Sherwin-Williams. 

09 Oct 1935    Spec 3-100F notes no additional paint coating required on exterior surfaces of Alclad skins. 

01 Sep 1936 Materiel Division memo desires standardization of cockpit colors.  A-17 has light green, C-33 has a dark shade of green. 

26 Sep 1936  Spec 3-100G makes first mention of zinc chromate primer in specs!  Oil base metal primer still approved 

14 Nov 1936   Instructions for use of zinc chromate primer notes adding 4-oz of aluminum powder to 1-gallon of primer for second coat. 

29 Mar 1937   Wright Field memo recommends 1 brush coat of bronze green lacquer or enamel to eliminate glare on noses of C-32, C-33, and C-34. 

11 Aug 1938   Spec 3-100H notes interior finish to be 1 coat of iron oxide primer or zinc chromate primer, followed by one or more coats of the same primer containing aluminum powder, OR 1 coat of either primer followed by one or more coats of aluminum enamel or lacquer, OR one or more coats of aluminum bituminous paint.  Cockpit colors not mentioned. 

16 Nov 1938   Materiel Division report to Navy mentions that the Army has used zinc chromate primers exclusively for the past several years. 

09 Sep 1938  Spec 98-24113A for the first time lists Yellow Green for cockpits which are open or under sliding enclosures or Flat Bronze Green for closed cockpits, “the top and sides of which form part of the fuselage structure” (sides above windows and ceilings to be aluminum.) 

15 Aug 1939   Overhaul instructions for B-18: aluminized spar varnish over 1 coat zinc chromate primer; Pilots’ and Radio Operator’s compartments finished with Pine Green Duco 246-30966; exterior anti glare panels (starting with aircraft 36-343) Bronze Green; interior details and fittings Light Gray Duco 258-38141 and Dark Gray Duco 258-38142. 

19 Jan 1940   Navy Spec SR-15C identifies “cockpit green” color as “dull bronze green lacquer, Army color #9.” 

28 Nov 1940   Navy memo notes that Spec L-12a “...has been revised to replace the current Dull Bronze Green...with [Dark Green #30] lacquer...” 

17 Dec 1940 Army memo accepts Dull Dark Green for cockpits and anti-glare. 

01 Feb 1941   Army memo cancels list of approved oil base metal primers; they are replaced by zinc chromate primer. 

03 Feb 1942   To conserve aluminum pigment, Navy memo recommends tinting second coat of zinc chromate primer with Indian Red, lampblack, or other suitable indicator.  Light gray can be used on fabric surfaces where aluminum dope has been used. 

31 Aug 1942   Curtiss-Wright memo reports Berry Brother 174-G-13 to be used as “Curtiss Cockpit Green.” 

10 Sep 1942  Navy memo proposes to replace dull dark green with new green standard for cockpits.  Proposed color can be procured as lacquer, or made from zinc chromate primer, tinted with black pigment.

11 Sep 1942 Army Spec 3-100H drops aluminum from formula for yellow green finish. 

21 May 1943   Wright Field memo notes: “The color for Interior Green 611 is considered more yellow than the pigmented zinc chromate currently used in Army aircraft...” 

17 Nov 1943   Army Spec 98-24113B calls for flat black on interior parts which reflect in canopies, turrets, etc and medium green for interior portions of cockpits which are visible to crew members in flight, and are subject to direct rays of the sun.  Seats and drapes are still Dull Dark Green. 

19 Feb 1944   Wright Field notes that B-17, B-25, P-51, and AT-6 have been manufactured without protective coatings on internal surfaces for approximately 2 years.  Deviations granted to increase production. 

29 Jul 1944    Wright Field notes that B-29 interspar upper wing panels are not built with Alclad; panels are anodized with 1 coat of zinc chromate primer, then sprayed with two coats of aluminized lacquer

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Formulas for Aluminizing Coatings

The Army used a number of formulas for “aluminizing” various coatings.  What follows is a partial list of those recipes: 

1 gallon spar varnish                                -  1 pound polished aluminum powder
1 gallon spar varnish                                -  1.5 pounds aluminum powder
1 gallon spar varnish                                -  1.5 pounds aluminum paste
1 gallon clear varnish                               -  12 oz aluminum paste 

1 gallon of clear dope                               -  4 to 6 ounces aluminum powder
1 gallon of clear dope                               -  8 ounces aluminum powder
1 gallon of clear dope                               -  1 pound aluminum paste 

1 gallon oil base primer                             -  1 pound aluminum powder
1 gallon oil base primer                             -  1.5 pounds aluminum paste 

1 gallon bitumostic paint                            -  1 pound aluminum powder
1 gallon bituminous paint                           -  1.5 pounds aluminum powder
1 gallon bituminous paint                           -  2 pounds aluminum paste 

1 gallon clear lacquer                               -  8 to 12 oz extra-fine aluminum powder
1 gallon clear lacquer                               -  10 oz aluminum powder
1 gallon clear lacquer                               -  1 lb aluminum powder
1 gallon clear lacquer                               -  12 oz aluminum paste
1 gallon clear lacquer                               -  1 pound aluminum paste

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Formulas for Tinting Zic Chromate

A similar variety existed in formulas for tinting Zinc Chromate.  Navy, Air Corps and AAF records show the following additions to 1 gallon of zinc chromate primer: 

aluminum powder: 4 oz
aluminum powder: 6 oz (or aluminum paste: 8 oz) 

aluminum powder: 4 oz; black enamel: 1 pint (1/8th gallon); toluene: 1 gallon
aluminum powder: 4 oz; black enamel: 1/10th gallon
aluminum powder: 4 oz; black enamel: 1/10th gallon; toluene: 1 gallon
aluminum powder: 6 oz (or aluminum paste 8 oz); black enamel: 1/10 gallon; toluene: 1 gallon
aluminum paste: 4 oz; black enamel: 1/10 gallon; toluene: 2 gallons
aluminum paste: 2 oz; thinner: 1½ gallons 

Indian Red paste: 2 oz; thinner: 2 gallons 

carbon black tinting paste: 2 oz
carbon black tinting paste: 8 oz
black tinting paste or black enamel: 1/10th gallon; toluene substitute: 1 gallon