Why is oxide thickness on an Mg–Al alloy surface thicker than on a Mg-free alloy for the same thermal and mechanical history?

• A. The oxide is denser and grows more slowly.
• B. Mg reduces diffusion of oxygen.
• C. Aluminum content is higher.
• D. The oxidation rate of Mg-containing alloys is greater than for Mg-free alloys.

Answer: (D)