Clad sheet in which thin layers of pure aluminum or another more active alloy are roll-bonded to a high-strength aluminum alloy. The layers act as sacrificial anodes and protect the core from corrosion.
Japanese name: アルクラッド
Alclad appears in 1 practice question across 1 topic, including Nonferrous metals and alloys.
Field: Nonferrous metals and alloys
Answer: B. The surface layer is more active (less noble) than the core, so where the core is exposed at scratches or cut edges, the layer dissolves first and protects the core electrochemically
Cu-containing 2000-series alloys are strong but prone to localized and intergranular corrosion around Cu-bearing precipitates, so their corrosion resistance is poor. Layers of pure aluminum or Al-1%Zn alloy a few percent of the sheet thickness are therefore roll-bonded to both faces. This cladding is more active than the core, so where the core is exposed at scratches or cut edges, the cladding dissolves first like a sacrificial anode and protects it. If the cladding were nobler, the exposed core would instead corrode faster through galvanic corrosion, so the option of a noble layer is wrong. Melting on heating to fill gaps is the role of the cladding on brazing sheet. Because the cladding is weak, the overall strength and fatigue strength of the sheet are slightly lower, and the sheet thickness is chosen with this in mind.
Sacrificial anode Galvanic corrosion Al-Li alloy Beryllium copper GP zone Temper designation Tough-pitch copper γ'' phase