Much of the aluminum structure used on the Boeing Model 75 aircraft was protected from corrosion by an electroplating process called anodizing. This process produced a hard surface that withstood attacks of corrosion that would normally occur on bare aluminum. Surface corrosion can appear as oxidized film that will eventually progress into pitting corrosion and will destroy the part over time if left unchecked. To understand how aluminum corrodes we must delve into the chemistry of aluminum itself.
Pure aluminum is less corrosive than alloyed aluminum but alloyed aluminum is needed to improve its strength. To pure aluminum the element copper is added along with trace elements of manganese and magnesium. These so called alloying elements rarely exceed 6-7% of the total make-up of the aluminum, but increase its strength significantly. For instance, aluminum in its pure state has a tensile strength of 13,000 psi but if that pure aluminum is alloyed with copper and subjected to the heat treating process its tensile strength is increased to as high as 65,000 psi. Aluminum is divided into two basic general classes: 1 – The casting alloys (those suitable for casting in sand, permanent molds or die castings) and 2 – The wrought alloys (those that can be shaped by rolling, drawing or forging). Of these two the wrought alloys are the most widely used in aircraft construction, being used for stringers, bulkheads, skin, rivets and extruded sections. Figure 1 shows a typical cast aluminum engine component.
Right in Figure 1, a cast cylinder head made from aluminum alloy 142-T61. The ultimate tensile strength of this alloy is 40,000 psi. After machining this aluminum head is screwed on a highly machined steel barrel to produce a cylinder assembly. In Figure 2 is an example of die casting aluminum parts. The examples produce some recognizable parts, such as accessory drive pads for radial engines, navigation light bodies, pillow block mounts and even spark plug cooling cells for Wright Whirlwind radial engines.
The wrought aluminum alloys are identified by the numbers beginning with 1 through 7, the first digit identifies the predominant alloy type that produces the coding as: 1xxx (pure aluminum), 2xxx (copper alloy), 3xxx (manganese alloy), 4xxx (silicon), 5xxx (magnesium), 6xxx (magnesium and silicone) and 7xxx (zinc). Typical modern alloy numbers are: 1100 (pure aluminum-weldable), 2024 (copper alloyed-non weldable but heat-treatable), 3003 (alloyed with manganese-weldable), 5052 (alloyed with magnesium-weldable, non heat treatable), 6061 (alloyed with magnesium and silicone-weldable and heat-treatable) and 7075 (alloyed with zinc-non weldable but heat-treatable).
When Boeing was producing the Model 75, the aluminum designation was slightly different from today’s designation. The digit identifier was: 1 = pure aluminum, 24 = copper alloy, 3 = alloyed with manganese, 52 = alloyed with magnesium. The 6xxx and 7xxx alloys had not been formulated and tested during this time period.
Therefore, much of the aluminum alloy that Boeing used was 24S-T (copper alloy that has been heat-treated to increase strength, 3S-O (manganese alloy that was in its softest state and used for fuel and oil tanks that were welded), such as the fuel and oil tanks that were gas welded, as were many of the die cast aluminum fittings that were also welded. A Boeing maintenance manual states, “The fuel tanks are constructed of 3S aluminum alloy sheet, assembled by riveting and welding. All surfaces of the tank are beaded and the baffle rivets are sealed against leakage by welding. All seams are torch welded. Removable sump plates are provided at the rear of each side of the tank. To prevent uncovering the fuel tank outlets in a dive, auxiliary lines have been connected to fittings in the front end of the tank, ahead of the sump plates. Coarse mesh finger strainers are silver soldered to the outlet fittings at all four connections. The screens in the sump plates in the tank are separate from and may be removed for cleaning, etc., without disturbing the fuel lines. The filler neck is equipped with a cap and adapter assembly. The filler neck is so located that the expansion space may not be filled when the airplane is in the three-point position. The capacity of the tank is 46-gallons and is stenciled on the center section in view of each member of the crew.” The oil tank is described by the factory as, “The oil tank is of 3S aluminum alloy construction, with the total volume of 5.8 U.S gallons. However, only 4.4 U.S gallons is specified capacity required for the airplane. The remaining space cannot be filled due to the location of the filler neck. This tank incorporates the use of an Air Corps Type 39B4232 filler neck and cap assembly.”
All the aluminum structural attachplates for wings and center section plus many of the internal aluminum fittings were made from 24S-T (now 2024-T3) aluminum with a tensile strength of 62,000 psi.
When repairing or replacing components it is imperative that the mechanic determines exactly what type of material was used to make the part by the OEM. All aluminum structural parts will be made from heat-treated aluminum alloy, most likely 24S-T (2024-T3). All non-structural parts would be made from 52S (5052-H) aluminum alloy, however some nonstructural parts could be made of 2024-T-3.
To protect aluminum surfaces from corrosion the Boeing factory used three methods. They were by priming, by anodic treatment using sulfuric and chromic acid. Anodic treatment is an electroplating process that puts a hard oxidized coating on the surface of the aluminum, particularly those parts made from the copper alloyed 24S-T. To understand just how anodizing works it will be necessary to look at a schematic of the system. Figure 3 depicts a typical anodizing system.
Boeing used a chromic acid concentration of 9% by weight. The acid was mixed with pure water and a temperature in the tank was maintained at 95 degrees F. After the plating process was complete the parts were removed and placed in a rinse tank of water at a temperature of 180 degrees F. After the rinse was complete the parts were dried with an air hose and then thoroughly wiped with a dry cloth.
The sulfuric acid process was similar to the chromic process described above except sulfuric acid replaced the chromic acid. The sulfuric acid concentration was 10% by weight. This process was used to anodize fuel tanks and other components. Amperage in the anodizing system was not less than 1-1/2 amps and the amperage was controlled by a rheostat that adjusted amperage based on size and quantity of metal being anodized.
Sulfuric and phosphoric acid will turn the aluminum surface to a grey color and the surface will be very hard, thus able to resist corrosion attacks. Any aluminum surface that contacted fabric was not anodized or primed but rather left in its original surface condition. This included the fuselage stringer panels. Chromic acid anodizing will cause the surface of the part to be a gold color the longer the part is left in the tank. Today, phosphoric acid anodizing (PAA) and chromic acid anodizing (CAA) are still common procedures. Primer used by the factory was essentially zinc chromate tinted green – approximately 6-3/4 gallons was used on each airplane. The Boeing Company has developed a portable phosphoric acid anodizing process used to prepare a surface for bonding with adhesive film or low temperature curing adhesives. This process is called Phosphoric Acid Non Tank Anodizing (PANTA) and I have used it to prepare for bonding aluminum with epoxy adhesives. Below in figure 4 is the PANTA process as removed from the Boeing 767 Structural Repair Manual (SRM). The PANTA process described in figure 4 can be used to return an anodized surface to a small repair in an anodized part. The power supply used is a 6-volt lantern battery. The + side of the battery is connected directly to the part to be anodized and the – side of the battery connected to the cathode, which is the stainless steel screen. When the circuit is connected a series circuit is created through the gelled phosphoric acid coating that separated the anode from the cathode. Here, phosphoric acid solution is gelled using micro balloons or Cab-O-Sil that holds the acid in the place where you want it. This system really works as I have used it several times when bonding aluminum with both low and high temperature cures. I am assuming that this process could be modified to use a mild concentrate of sulfuric acid instead of phosphoric acid. And a mild solution of chromic acid could be used also. Incidentally, I have used the gelled phosphoric or chromic acid as a means to hold the acid in a specific location after a repair to an aluminum sheet. It even will work if the surface is nearly vertical, where liquid acid would run off. The phosphoric acid treatment is used to clean and brighten an aluminum surface prior to priming. After the phosphoric acid etch has been completed and the surface washed clean with water and dried, the surface then receives a chromic acid conversion coating, that microscopically etches the aluminum surface and provides a hard oxidized surface to resist filaform corrosion that can form under an epoxy painted surface.