In the process of nitriding, which compound forms on the surface when aluminum is present as an alloying element?

• A. Aluminum nitride.
• B. Iron nitride.
• C. Aluminum oxide.
• D. Aluminum carbide.

Answer: (A)

In nitriding, nitrogen reacts with alloying elements that readily form stable nitrides on the surface. Aluminum has a strong affinity for nitrogen, so when aluminum is present, the surface tends to form aluminum nitride. This AlN layer is hard and thermally stable, providing an protective, adherent coating that enhances surface hardness and guards against further diffusion. Aluminum oxide would form mainly from oxidation, not the nitriding environment, and aluminum carbide would require carbon exposure as in carburizing—neither is the primary outcome of nitriding with aluminum present. Iron nitride can form on plain iron or iron-rich alloys, but the presence of aluminum steers the surface chemistry toward aluminum nitride.