Alclad

Alclad is a corrosion-resistant aluminium sheet formed from high-purity aluminium surface layers metallurgically bonded to high-strength aluminium alloy core material. These sheets are commonly used by the aircraft industry. The first aircraft to be constructed from Alclad was the all-metal US Navy airship ZMC-2, constructed in 1927 at Naval Air Station Grosse Ile. Alclad is a trademark of Alcoa but the term is also used generically.

Described in NACA-TN-259, of August 1927, as "a new corrosion resistant aluminum product which is markedly superior to the present strong alloys. Its use should result in greatly increased life of a structural part. Alclad is a heat-treated aluminum, copper, manganese, magnesium alloy that has the corrosion resistance of pure metal at the surface and the strength of the strong alloy underneath. Of particular importance is the thorough character of the union between the alloy and the pure aluminum. Preliminary results of salt spray tests (24 weeks of exposure) show changes in tensile strength and elongation of Alclad 17ST, when any occurred, to be so small as to be well within the limits of experimental error." In applications involving aircraft construction, Alclad has proven to have increased resistance to corrosion at the expense of increased weight when compared to sheet aluminum.

...
Wikipedia