API 571 Certification (Corrosion & Materials) Practice Test

Which alloy is particularly susceptible to hydrogen stress cracking in HF environments?

1. Stainless Steel

2. Low Alloy Steel

3. Aluminum

4. Carbon Steel

The correct answer is: Carbon Steel

Hydrogen stress cracking, also referred to as hydrogen embrittlement, is particularly relevant for carbon steels in hydrofluoric acid (HF) environments. This susceptibility arises due to the combination of hydrogen absorption in the carbon steel and the stress present in the material. In HF environments, the corrosive effects can generate hydrogen atoms, which then diffuse into the steel, leading to weakening of the metallic bonds and facilitating cracking under tensile stress. Carbon steels typically have a higher ferrous content, which enhances their capacity to absorb hydrogen compared to other alloys. Their microstructure can exacerbate this problem, as the presence of microstructural flaws or hard phases in the steel can act as sites for cracking under stress, particularly in the presence of hydrogen. While other materials, like stainless steel and low alloy steel, can also experience hydrogen-related issues, they generally exhibit improved resistance due to their alloying elements which can passivate the surface and reduce hydrogen absorption. Aluminum is not typically exposed to HF environments in the same way and does not suffer from hydrogen stress cracking in the same contexts as carbon steels do. Understanding these material behaviors is crucial for ensuring the integrity and safety of equipment operating in challenging environments like those involving HF.