Ammonia (\(\text{NH}_3\)) is a compound of nitrogen and hydrogen. Whether it is corrosive depends on the specific metal, the form of the ammonia, and environmental factors. It is important to distinguish between anhydrous (water-free) ammonia, stored as a liquid under pressure, and aqueous ammonia (ammonium hydroxide). Aqueous ammonia, which contains water and dissolved oxygen, is the primary source of common ammonia-related corrosion issues.
The Chemical Mechanism of Ammonia Corrosion
The corrosion caused by aqueous ammonia involves a process known as complex ion formation, where the ammonia molecule acts as a ligand. A ligand is a molecule or ion that donates electrons to a central metal ion, forming a coordinate bond. This mechanism is responsible for the rapid degradation of specific metals.
The metal’s protective oxide layer is first penetrated, often assisted by an oxidizer like dissolved oxygen. The exposed metal ions, such as copper ions, then react with ammonia molecules to form highly stable and soluble metal-ammonia complexes, like the deep blue tetraamminecopper(II) ion. Since these complexes are soluble, they are washed away from the metal surface, continuously exposing the underlying metal and leading to accelerated material loss.
The presence of both water and oxygen is necessary to drive this destructive cycle efficiently. Water creates the aqueous solution where the reaction occurs, and oxygen acts as a depolarizer, continuously oxidizing the metal. Anhydrous liquid ammonia is far less corrosive because it lacks the necessary water and oxygen to facilitate the complexing reaction.
Highly Susceptible Metals and Alloys
The most significant corrosion threat from ammonia is directed at copper and its alloys, including brass and bronze. These materials are highly susceptible because the copper ion readily forms the stable ammonia complex, leading to rapid material degradation. Alloys like brass, a copper-zinc alloy, are particularly vulnerable to Stress Corrosion Cracking (SCC).
Ammonia SCC occurs when a susceptible metal is simultaneously exposed to a corrosive environment and a tensile stress, either residual or applied. For copper alloys, this failure typically manifests as brittle cracking, often progressing along the grain boundaries. The cracking is most severe in a moist, ammoniacal vapor environment. Here, the concentration of ammonia, moisture, and oxygen promotes the formation and dissolution of the copper-ammonia complex at the crack tip.
The vulnerability of brass is directly related to its zinc content; higher zinc percentages generally increase the alloy’s susceptibility to SCC. Another related failure mode in brass is dezincification, where the zinc component is selectively leached out, leaving behind a weak, porous copper structure. Zinc itself can also corrode through a similar complex ion mechanism when exposed to aqueous ammonia.
Aluminum exhibits a more complex reaction profile with ammonia. In very dilute aqueous ammonia solutions, aluminum often shows a reduced corrosion rate due to the formation of a stable, protective aluminum hydroxide or oxide film. However, in concentrated or high-purity anhydrous ammonia, aluminum can still be active. This is especially true when galvanically coupled with other metals like steel.
Resistant Materials and Mitigating Factors
Materials that do not readily form soluble ammonia complexes are considered resistant to this type of corrosion. Carbon steel and austenitic stainless steel, such as the 300 series alloys, are the primary materials of choice for most ammonia service applications. These metals are resistant because their native oxide layers, primarily iron oxide or chromium oxide, are not easily dissolved by the ammonia ligand.
Stainless steels are notably resistant to ammonia SCC because their chromium content forms a tenacious, self-healing oxide layer (\(\text{Cr}_2\text{O}_3\)). Carbon steel is inert to aqueous ammonia, but it is susceptible to SCC in anhydrous liquid ammonia if contaminated with small amounts of oxygen. This particular form of SCC is mitigated by the deliberate addition of a small percentage of water, typically around 0.2%, which acts as a protective inhibitor.
Controlling the environment is a primary strategy for mitigating ammonia corrosion in susceptible materials. Since oxygen is a necessary reactant, eliminating air or dissolved oxygen from the system can significantly slow or stop the corrosion process. The concentration of ammonia and the system temperature also affect the corrosion rate, with higher values generally increasing the corrosive potential.
For structural components that cannot be made from resistant alloys, specialized protective coatings can be applied. These include various types of epoxy, vinyl ester, or zinc-epoxy coatings that create a physical barrier between the metal surface and the corrosive medium. Process controls like post-weld heat treatment (PWHT) are also used for carbon steel to reduce the residual tensile stresses necessary for SCC to occur.