API 571 Certification (Corrosion & Materials) Practice Test

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Which material is generally the least resistant to Ammonium Bisulfide corrosion?

Alloy 800
CS
Alloy 625
410 Stainless Steel

The correct answer is: CS

Carbon Steel (CS) is generally considered the least resistant material to Ammonium Bisulfide corrosion. This is due to the inherent properties of carbon steel, which lacks the alloying elements present in more resistant materials like Alloy 800, Alloy 625, and 410 Stainless Steel. When exposed to corrosive environments, carbon steel is prone to oxidation and pitting, especially in the presence of acidic compounds such as Ammonium Bisulfide. The corrosion mechanisms can lead to significant material degradation, which is not experienced to the same extent with the other alloys mentioned. Alloy 800, for example, is formulated with nickel and chromium, which enhance its resistance to sulfur compounds and can withstand higher temperatures and pressures in corrosive environments. Similarly, Alloy 625 contains a high amount of nickel and molybdenum, offering excellent protection against a variety of corrosive media, including Ammonium Bisulfide. 410 Stainless Steel, while less resistant than the nickel-based alloys, still provides a better performance against corrosion than carbon steel due to the presence of chromium which forms a protective oxide layer. As a result, carbon steel stands out as the least resistant material when it comes to Ammonium Bisulfide corrosion.