API 571 Certification (Corrosion & Materials) Practice Test

Which materials are preferred when the O2 concentration is below 20ppb?

1. 400 series stainless and Alloy C-276

2. CS and LAS

3. 300 series stainless and Alloy 20

4. 316L SS and Alloy 825

The correct answer is: CS and LAS

In environments where the oxygen concentration is below 20 parts per billion (ppb), carbon steel (CS) and low-alloy steel (LAS) become preferred materials due to their specific corrosion resistance characteristics and cost-effectiveness. At such low oxygen levels, these materials can generally handle the corrosive environment effectively without significant risk of pitting or crevice corrosion, which can be issues in environments with higher levels of oxygen or chlorides. Low oxygen concentrations reduce the likelihood of oxidizing conditions that may cause corrosion in more reactive materials. Carbon steel is particularly useful in a variety of applications because it offers good strength and ductility. Low-alloy steel can also provide enhanced strength and toughness while maintaining good corrosion resistance, making these alloys suitable for environments where minimal corrodent activity is present. In contrast, higher-grade stainless steels and nickel-based alloys might be unnecessarily expensive and could introduce other issues, such as sensitization or intergranular corrosion in certain conditions. Therefore, the choice of carbon steel and low-alloy steel aligns best with the requirement for durability and practicality in low oxygen scenarios.