API 571 Certification (Corrosion & Materials) Practice Test

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Which material is least resistant to naphthenic acid corrosion?

9Cr
2.25Cr
CS
6% Mo Alloys

The correct answer is: CS

In the context of naphthenic acid corrosion, carbon steel, commonly referred to as CS, is indeed the least resistant material among the options provided. Naphthenic acid, found in crude oil and associated with high-temperature services, can aggressively attack materials, particularly those with lower alloy content. Carbon steel lacks the alloying elements that enhance resistance to acidic environments. Alloys such as 9Cr and 2.25Cr incorporate chromium, which helps form a protective oxide layer on the surface, offering better resistance against corrosion. Similarly, 6% molybdenum alloys also contain molybdenum, which further improves corrosion resistance and strength at elevated temperatures. Overall, the base structure of CS offers minimal protection against the corrosive effects of naphthenic acid, rendering it vulnerable in environments where this acid is present.