Anhydrous ammonia is a chemical compound composed of nitrogen and hydrogen, typically used as a liquid or gas. The term “anhydrous” indicates that it is without water. While pure anhydrous ammonia is not highly corrosive to many common metals, its corrosivity changes significantly and becomes a concern in the presence of moisture. This interaction can lead to material degradation, posing risks to equipment integrity and safety.
Understanding Anhydrous Ammonia’s Corrosivity
The corrosive action of anhydrous ammonia primarily stems from its reaction with water. When it contacts moisture, it readily dissolves and forms ammonium hydroxide (NH4OH), a strong alkaline solution. Ammonium hydroxide is highly corrosive to certain metals and biological tissues.
While pure anhydrous ammonia generally exhibits low corrosivity to mild steel and stainless steel, the presence of even small amounts of water or other contaminants can initiate significant degradation. A more serious concern is stress corrosion cracking (SCC), particularly in carbon steel equipment. SCC can occur when steel is exposed to anhydrous ammonia, especially in the presence of oxygen and varying water content.
The susceptibility of steel to SCC increases with oxygen levels, even at very low water concentrations. Air, which contains both oxygen and carbon dioxide, is a significant contaminant that can accelerate SCC in steel. This type of cracking is particularly problematic in pressure vessels and often appears near weld joints where residual stress is highest.
Impact on Materials
Anhydrous ammonia’s corrosivity varies significantly depending on the material it contacts, especially when moisture is present. Materials like copper, brass, bronze, zinc, and certain aluminum alloys are particularly vulnerable. The corrosive effect on these metals is due to the formation of soluble complexes, leading to material degradation.
Carbon steel and stainless steel are generally resistant to pure anhydrous ammonia. However, these materials can still be susceptible to SCC if moisture, oxygen, or carbon dioxide are present. Equipment made from carbon steel is widely used for storing and handling anhydrous ammonia, but it requires careful management of contaminants.
Austenitic stainless steels demonstrate good resistance across a range of conditions, including in the presence of some moisture or carbon dioxide. Plastics like polyethylene and Teflon also show good resistance to anhydrous ammonia. However, materials like rubber can become brittle and crack with exposure, especially at high ammonia concentrations.
Preventing Corrosion and Ensuring Safe Handling
Preventing corrosion in anhydrous ammonia systems centers on meticulous control of moisture and proper material selection. It is important to prevent water ingress into storage and transport systems, as even small amounts of water can lead to ammonium hydroxide formation and subsequent corrosion. For steel equipment, maintaining a water content of approximately 0.2% in the ammonia can actually inhibit SCC.
Selecting appropriate materials, primarily carbon steel or stainless steel, is fundamental for equipment like tanks and piping. Avoiding susceptible metals, such as copper and its alloys, is also crucial. For new vessels, stress-relieving welds through post-weld heat treatment can significantly reduce the risk of SCC by minimizing residual stresses.
Regular inspection and maintenance are necessary to identify and address any signs of corrosion or potential weaknesses. This includes routine pressure testing and visual checks. Ensuring equipment integrity directly contributes to safety by preventing leaks and structural failures. Proper ventilation and personal protective equipment are essential for personnel handling anhydrous ammonia.