Ammonia Stress Corrosion Cracking

What is Ammonia Stress Corrosion Cracking?

Ammonia Stress Corrosion Cracking (ASCC) is a phenomenon of material degradation that can occur in environments containing ammonia and subjected to mechanical stress. It is a specific type of stress corrosion cracking (SCC) where the presence of ammonia acts as a catalyst for the cracking process.

Ammonia Stress Corrosion Cracking (ASCC) can occur with:

  1. Copper alloys may experience stress corrosion cracking (SCC) when exposed to aqueous streams containing ammonia.
  2. Anhydrous ammonia can lead to SCC in carbon steel.

Materials prone to Ammonia Stress Corrosion Cracking (ASCC)

Materials prone to Ammonia Stress Corrosion Cracking (ASCC) include:

  1. Copper Alloys:
    • Copper-zinc alloys (brasses), especially those with a zinc content above 15%.
    • Admiralty brass.
    • Aluminum brasses.
  2. Carbon Steel:
    • High-strength carbon steel.

It is important to note that these materials are susceptible to ASCC when exposed to ammonia-containing environments, such as aqueous ammonia or anhydrous ammonia, depending on the specific material.

The susceptibility to ASCC can be influenced by factors such as alloy composition, residual stress, exposure conditions, and the presence of other contaminants. Proper material selection, design considerations, and control of environmental conditions are essential in mitigating the risk of ASCC in these materials.

Critical Factors for Ammonia Stress Corrosion Cracking (ASCC)

Critical factors associated with Ammonia Stress Corrosion Cracking (ASCC) include:

Copper Alloys:

  • Susceptible alloys can develop cracks when subjected to residual stress and exposed to chemically ammoniated compounds.
  • The susceptibility of brasses to ASCC is influenced by the zinc content, particularly when it exceeds 15%.
  • The presence of a water phase containing ammonia or ammoniacal compounds is necessary for ASCC to manifest.
  • Oxygen, even in small amounts, is essential for ASCC to occur.
  • An alkaline pH level above 8.5 is required for the initiation of ASCC.
  • ASCC can transpire across a wide range of temperatures.
  • Residual stresses resulting from fabrication or tube rolling procedures can facilitate cracking in alloys prone to ASCC.

Carbon Steel:

  • Carbon steel is prone to cracking when exposed to anhydrous ammonia with a water content below 0.2%.
  • Laboratory testing has demonstrated cracking at extremely low temperatures, reaching as low as -27 °F (-33 °C).
  • Crack growth rates and susceptibility to ASCC increase with rising temperatures, but cracking can still transpire under ambient or refrigerated conditions.
  • Stress relief measures following welding can eliminate susceptibility to ASCC in most common steels with a minimum specified tensile strength up to 70 ksi.
  • Even small amounts of air or oxygen contamination intensify the likelihood of cracking.
  • Elevated residual stresses arising from fabrication and welding processes heighten the vulnerability of carbon steel to ASCC.

These critical factors emphasize the conditions and variables that contribute to the occurrence and progression of Ammonia Stress Corrosion Cracking. Understanding and addressing these factors are essential for preventing and mitigating the risks associated with ASCC in materials susceptible to this phenomenon.

How to prevent Ammonia Stress Corrosion Cracking (ASCC)?

Prevention and mitigation strategies for Ammonia Stress Corrosion Cracking (ASCC) in copper alloys and carbon steel include:

a) Copper Alloys:

  • Choose copper-zinc alloys with a zinc content below 15% as they exhibit improved resistance to ASCC.
  • Utilize copper-nickel alloys such as 90-10 Cu-Ni and 70-30 Cu-Ni, which have very low susceptibility to ASCC. Below 120 °F (50 °C), these cupronickel alloys are practically immune to ASCC.
  • Control SCC in steam service by preventing the ingress of air.
  • Consider using 300 series stainless steel and nickel-based alloys, which are immune to ASCC.

b) Carbon Steel:

  • Prevent SCC in steel by adding small quantities of water to the ammonia stream (minimum 0.2% water content). It’s important to note that vapor spaces may have lower water content due to partitioning of ammonia in the water phase.
  • Employ effective stress relief techniques for welds to reduce residual stress, thereby preventing ammonia SCC.
  • Use low-strength steels with a minimum specified tensile strength below 70 ksi.
  • Prevent the ingress of oxygen into storage facilities as even low levels of oxygen (<5 ppm) can lead to cracking under certain conditions. Maintain oxygen levels below 1 ppm.
  • Prior to introducing ammonia into atmospheric and pressurized storage systems, purge oxygen using nitrogen.

Implementing these prevention and mitigation measures can significantly reduce the risk of Ammonia Stress Corrosion Cracking and help maintain the integrity and reliability of copper alloys and carbon steel in ammonia-containing environments.

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