API 571 Certification (Corrosion & Materials) Practice Test

The type of stainless steel that has high susceptibility to sigma phase embrittlement due to its ferrite content is primarily the 400 series stainless steel. This series, which includes ferritic and martensitic stainless steels, can form sigma phase, especially in the presence of high temperatures and prolonged exposure.

Ferrite is a body-centered cubic form of iron that is present in these alloys, and it can become unstable at certain temperatures, leading to the formation of sigma phase. This phase is brittle and can decrease the ductility and toughness of the material, making it particularly susceptible to embrittlement during service.

In contrast, the 300 series stainless steels, which are primarily austenitic, are characterized by having lower ferrite content and therefore have a lower risk of sigma phase formation. Duplex stainless steels do contain both ferrite and austenite phases, but they are designed to minimize phases that lead to embrittlement. Wrought nickel alloys are not typically associated with sigma phase embrittlement as they contain negligible ferrite content and have different phase stability characteristics.

Therefore, the 400 series stainless steel stands out for its susceptibility to sigma phase embrittlement, making it the correct answer.