Nonferrous metals and alloys
Sheet of alloys such as 2024 used for aircraft skins is often supplied as clad sheet (Alclad), with thin layers of pure aluminum or an Al-Zn alloy roll-bonded to both faces. Which is the main role of these layers?
- AThe surface layer is nobler and harder than the core, so even when scratched it acts as a protective film that stops the core from dissolving
- BThe 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
- CThe surface layer melts on heating and fills gaps at joints, preventing fatigue cracks from starting around rivet holes
Show answer and explanation
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.