API 571 Certification Practice Test 2025 – Comprehensive All-in-One Guide to Master Corrosion & Materials!

The reason all low alloy steels are not recommended for environments prone to wet hydrogen sulfide (H2S) cracking lies in their susceptibility to a specific form of cracking known as hydrogen-induced cracking (HIC) or sulfide stress cracking (SSC). In sour service applications, where hydrogen sulfide is present, low alloy steels can suffer significant degradation due to the corrosive effects of H2S, particularly when combined with stress conditions.

Low alloy steels typically do not possess sufficient resistance to withstand the combined effects of hydrogen exposure and corrosive environments. Their microstructure may allow for the diffusion of hydrogen, which can lead to internal embrittlement and loss of ductility. This makes them particularly vulnerable in sour service conditions where wet H2S is present.

In contrast, materials like high nickel cast iron, 316L stainless steel, and high silicon cast iron exhibit better resistance to hydrogen sulfide environments due to their composition and microstructural properties. For example, 316L stainless steel is known for its excellent corrosion resistance, particularly in sour environments, owing to its higher nickel content and low carbon levels which help minimize the risk of localized corrosion.