API 571 Certification (Corrosion & Materials) Practice Test

What type of corrosion occurs beneath the metal surface of nickel alloys at high temperatures?

1. Galvanic corrosion

2. Sulfidation

3. Pitting corrosion

4. Intergranular corrosion

The correct answer is: Sulfidation

Sulfidation is a specific type of corrosion that occurs when metals, particularly nickel alloys, are exposed to sulfur-containing environments at elevated temperatures. This process typically results in the formation of metal sulfides, which can lead to a significant degradation of the material properties. At high temperatures, nickel alloys can become susceptible to sulfidation because of their interactions with sulfur compounds in the environment. The reaction between nickel and sulfur forms nickel sulfide, which can penetrate beneath the surface, thus impacting the integrity of the alloy over time. Understanding this phenomenon is critical in fields dealing with high-temperature applications such as petrochemical processing, where such corrosion can lead to catastrophic failures if not managed properly. In contrast, the other types of corrosion mentioned do not specifically pertain to the sub-surface degradation characteristic of sulfidation in nickel alloys under high-temperature conditions.