API 571 Certification (Corrosion & Materials) Practice Test

Which materials are vulnerable to corrosion under insulation (CUI)?

1. 400 series Stainless Steel

2. All grades of Duplex Stainless Steel

3. Carbon Steel and 300 series Stainless Steel

4. Nickel Based Alloys

The correct answer is: Carbon Steel and 300 series Stainless Steel

Corrosion under insulation (CUI) is a significant concern in industries where materials are exposed to moisture and thermal insulation, leading to hidden corrosion that can progress without being detected until significant damage occurs. Carbon steel is especially susceptible to CUI because it can readily corrode when moisture accumulates beneath insulation, especially if protective coatings break down or if there are water leaks. The corrosion products can create conditions that favor further corrosion, such as localized pitting or even general rusting. The 300 series stainless steels, while more resistant to corrosion than carbon steel, can still experience CUI, especially if the protective passive layer is compromised. For instance, if there is a chloride presence—common in many environments—this can lead to pitting or crevice corrosion, making these materials particularly vulnerable. In contrast, other materials listed, such as 400 series stainless steel, duplex stainless steel, and nickel-based alloys, while they have some corrosion resistance, do not typically face the same levels of vulnerability to CUI under typical circumstances. These materials can have better performance in many environments, including when exposed to moisture, making them less likely candidates for CUI-related issues compared to carbon steel and 300 series stainless steel.