API 571 Certification (Corrosion & Materials) Practice Test

Which materials are especially vulnerable to polythionic acid stress corrosion cracking (SCC)?

1. Alloy 600 and alloy 800

2. Stainless steels and titanium alloys

3. Copper alloys

4. High-strength nickel alloys

The correct answer is: Alloy 600 and alloy 800

Alloy 600 and alloy 800 are particularly vulnerable to polythionic acid stress corrosion cracking (SCC) due to their specific composition and microstructure. These alloys, which are primarily nickel-based, are known to be susceptible to SCC in environments where polythionic acids are present, such as those found in petrochemical processes or during the degradation of sulfur compounds. Polythionic acid SCC is driven by the presence of sulfide and stress, leading to the formation of cracks that can propagate through the material. In the case of alloy 600 and alloy 800, the nickel content makes them susceptible when exposed to acidic environments because the microstructural features can lead to the precipitation of phases that act as stress risers, exacerbating the cracking process. Another significant factor is that these alloys are often used in high-temperature applications where exposure to polythionic acids can occur, making their susceptibility even more relevant. This characteristic leads to a heightened concern for the integrity and safety of systems employing these materials in environments where polythionic acids might be generated or present. In contrast, other materials listed, such as stainless steels and titanium alloys, while they may have their own vulnerabilities, do not exhibit the same level of susceptibility to polythionic acid SCC