API 571 Certification (Corrosion & Materials) Practice Test

Which material is least resistant to wet H2S cracking?

1. Martensitic SS

2. CS

3. Alloy C-276

4. High Silicon Cast Iron

The correct answer is: CS

Carbon Steel (CS) is least resistant to wet H2S cracking due to its inherent properties and microstructure. Wet H2S environments can lead to environmental cracking, particularly in materials that lack specific alloying elements that enhance resistance to such conditions. Carbon steel is primarily composed of iron and a certain percentage of carbon, which makes it susceptible to sulfide stress cracking (SSC) and hydrogen-induced cracking (HIC) when exposed to wet H2S atmospheres. Unlike more resistant materials, such as alloys that contain nickel or molybdenum, carbon steel does not offer enhanced resistance to these types of corrosive cracking mechanisms. The other materials listed have various properties that improve their performance in corrosive environments. For instance, martensitic stainless steels may have improved strength and some level of resistance due to alloying elements like chromium. Alloy C-276 is specifically designed to withstand harsh environments, including H2S stress cracking. High Silicon Cast Iron, while not commonly used, has properties that can give it a degree of resistance against specific corrosion challenges, particularly localized corrosion. Overall, when considering the exposure to wet H2S, carbon steel stands out as the material most vulnerable to cracking and degradation due to its lack of protective alloying