API 571 Certification (Corrosion & Materials) Practice Test

Ammonia Stress Corrosion Cracking is most severe in which type of alloys?

1. Copper-Zinc alloys

2. Aluminum alloys

3. Stainless steels

4. Carbon steels

The correct answer is: Copper-Zinc alloys

Ammonia Stress Corrosion Cracking (ASCC) is particularly severe in Copper-Zinc alloys, often referred to as brass. This susceptibility is primarily due to the presence of zinc in the alloy, which can lead to significant stress corrosion cracking when exposed to ammonia. The mechanism involves the interaction of the ammonia with the copper matrix and the zinc, resulting in the formation of microstructural features that make these alloys particularly vulnerable to cracking under stress. Copper-Zinc alloys exhibit a combination of high strength and ductility, which can be compromised in the presence of ammonia, especially in environments containing moisture. The presence of ammonia disrupts the protective oxide layers and aggravates localized corrosion processes. This phenomenon poses a notable risk in applications where these alloys are exposed to ammonia, leading to cracking and potential failure. Other materials such as stainless steels, carbon steels, and aluminum alloys have varying degrees of resistance to stress corrosion cracking in ammonia environments, often due to their different chemical compositions and microstructures. Therefore, it is critical for engineers and material selection experts to recognize the high susceptibility of Copper-Zinc alloys to ASCC to prevent potential failures in relevant applications.