Anhydrous ammonia (AA), composed of nitrogen and hydrogen (\(\text{NH}_3\)), serves as a high-concentration nitrogen fertilizer and an industrial refrigerant. The term “anhydrous” means “without water,” which defines its corrosive nature. When kept in a completely dry state, AA is generally non-corrosive to most common metals used in storage and transport equipment. However, the presence of even trace amounts of moisture fundamentally changes its chemical behavior, turning it into a highly reactive substance. Understanding this transformation is paramount for selecting appropriate equipment and ensuring system safety.
The Chemical Mechanism of Corrosivity
The corrosivity of anhydrous ammonia is not a property of the pure compound itself but depends entirely on its strong affinity for water. When moisture is introduced into a containment system, the ammonia gas readily dissolves and reacts with water molecules. This reaction results in the formation of ammonium hydroxide (\(\text{NH}_4\text{OH}\)), a powerful alkaline solution. This newly formed solution is the true corrosive agent, aggressively reacting with metal surfaces and leading to chemical degradation. Impurities, such as oxygen or carbon dioxide, can further accelerate this corrosive process. Consequently, equipment must rigorously exclude moisture, as even trace amounts initiate the production of this strong base, compromising system integrity.
Materials Most Vulnerable to Damage
A select group of materials exhibits a severe and rapid reaction when exposed to moist anhydrous ammonia and must be avoided in handling applications.
Copper and Alloys
Copper and its alloys, including brass and bronze, are exceptionally susceptible to chemical attack from ammonium hydroxide. The corrosive reaction with these metals can lead to stress corrosion cracking (SCC). Ammonia-induced SCC causes normally ductile copper-based materials to fracture unexpectedly under mechanical stress. This failure mode involves the formation of fine, intergranular cracks that penetrate the material without significant visible surface loss.
Galvanized Steel
Materials coated with zinc, such as galvanized steel, are also vulnerable to alkaline attack. The zinc coating is chemically incompatible with ammonium hydroxide and corrodes rapidly, removing the protective layer and exposing the underlying steel. Only non-galvanized components should be used in systems containing anhydrous ammonia.
Cast Iron
Cast iron is generally avoided due to its poor resistance to the corrosive effects of the ammonia solution. The destructive reactions with these vulnerable materials necessitate their exclusion from all components, including valves, gauges, fittings, and pipes, that contact the chemical.
Safe Material Selection for Storage and Handling
Safe storage and handling rely on the exclusive use of materials proven to resist the corrosive effects of ammonium hydroxide.
Metallic Components
Carbon steel is the material most commonly used for storage tanks, piping, and nurse tanks in agricultural and industrial settings. This metal does not readily react with the alkaline solution formed by wet ammonia, providing a robust containment option. Stainless steel, particularly grades 304 and 316, is often employed when greater resistance to environmental factors or impurities is required. Low-alloy steels are also used for high-pressure pipelines, offering a balance of corrosion resistance and mechanical strength. Ductile iron is chemically stable and used for certain fittings.
Non-Metallic Components
Certain non-metallic materials are employed for seals, gaskets, and hoses. Components must be made from specific ammonia-rated materials for chemical stability and flexibility, such as:
- Synthetic rubber
- High-density plastics
- Polytetrafluoroethylene (PTFE)
Regular inspection and timely replacement of these non-metallic parts are necessary to maintain system integrity against leaks.