The Pacific War Online Encyclopedia

Aluminum

Photograph of aluminum foil

Aluminum is a light, strong, somewhat ductile silvery metal, atomic number 13. It is the best conductor of electricity after copper and silver, and its alloys have a higher tensile strength than those of any metals except iron and copper. It is more resistant to corrosion than iron and has a better strength to weight ratio than steel, making it indispensable for construction of aircraft. However, aluminum does not wear as well as iron, being considerably softer. The Japanese used aluminum extensively for power transmission lines in order to conserve copper.

Aluminum is the most abundant metal in the earth’s crust, but it is so difficult to extract from its ores that the price was $374 per ton in 1941, much higher than the price of either steel ($43 a ton) or copper ($240 a ton). The principal ore is bauxite, a mixture of hydrous aluminum oxides, silica, and iron oxides formed in tropical climates when heavy rainfall leaches the more soluble minerals from aluminum-rich soil. Bauxite is purified by treating it with hot alkali, which dissolves the aluminum oxides while leaving behind the impurities. The solution is then treated with acid to precipitate nearly pure aluminum oxide. About a pound of aluminum oxide is extracted from each two pounds of bauxite. This is dissolved in molten cryolite (sodium aluminum fluoride) and reduced by electrolysis, a process requiring large amounts of electrical energy. Two pounds of aluminum oxide yield about a pound of aluminum metal.

Aluminum for air frames was generally hardened with about 4% copper, 1% magnesium and smaller quantities of manganese to make duralumin alloy. The Japanese further developed "Ultra Super Duralumin" with added zinc. These alloys required cold working followed by heat treatment and aging to bring out the full strength of the alloy, which was up to six times greater than that of pure unworked aluminum. Duralumin was susceptible to corrosion and was often protected with pure aluminum cladding.

The only significant natural source of cryolite in 1941 was the Ivigtût mine in Greenland, which was controlled by the Allies. However, the cryolite used in aluminum refining is recycled, so that relatively small quantities are required to maintain production, and Japan stockpiled sufficient cryolite before the war that it never became a limiting resource. Synthetic cryolite can also be produced from fluorite, a mineral that is widespread in nature. While this process was relatively new in 1941, the Germans, Americans, and Japanese all began developing synthetic cryolite production facilities as further insurance of their supply of this strategic material.

In 1941, bauxite was mined in Arkansas, the Caribbean, and South America. Smaller amounts were produced in India. The Arkansas production fell far short of American requirements, and bauxite was one of the seventeen strategic materials whose import was deemed critical. Production was monopolized by ALCOA in 1940, and the company was reluctant to expand production facilities for what might turn out to be a temporary surge in demand. It took the entry of Reynolds into the aluminum production market to force the expansion that would prove critical during the war.

The only source under Japanese control at the start of the Pacific War was in the Palau Islands, and Japan imported 212,587 tons of bauxite from the Netherlands East Indies in 1940. However, Japan seized the rich deposits in Malaya and the Netherlands East Indies early in the war. When these became inaccessible due to the submarine blockade later in the war, attempts were made to use aluminiferous shale from Manchuria, but this proved to be a poor source of aluminum, and production plummeted.

Japan had imported five to ten thousands tons of aluminum per year prior to 1934, but thereafter made strenuous efforts to increase domestic production. Abundant hydroelectric power aided the development of the aluminum smelting industry. Japanese production of aluminum in 1941 was 71,740 metric tons and peaked at 151,000 tons in 1944, while U.S. production was 309,100 tons in 1941 and peaked at 920,200 tons in 1943. At the time war broke out, Japan had stockpiled 254,740 tons of bauxite.

Thermite. Although by far the most important use of aluminum metal was in structural alloys, aluminum powder is a component of thermite, along with iron oxide and sometimes a binding agent. Once ignited using a high-temperature igniter, such as a burning magnesium ribbon, the aluminum rapidly reacts with the iron oxide to form aluminum oxide and molten iron at a very high temperature, just short of the vaporization temperature of aluminum, 4221 °F (2327°C). Thermite is used in civil engineering for welding where electric power for arc welding is not available, such as repair of railways in remote areas. Thermite found military uses as an incendiary (though it was eventually replaced by napalm) and for demolitions. A thermite grenade, such as the American M14, produced white-hot metal suitable for spiking guns or destroying engine blocks without explosives. In principle, thermite grenades could easily melt through the deck armor of tanks, though I have found no accounts of thermite grenades being used as antitank grenades in the Pacific.