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Ammonium carbamate is a chemical salt resulting from the reaction of ammonia and carbon dioxide. The structure is: H2N-C(=O)-O(-)(+)NH4. Ammonium carbamate can be formed without any intermediates by passing ammonia gas over solid carbon dioxide (dry ice).

In the real world, there is often water present and this complicates matters. Carbon dioxide and water can react to form carbonic acid, H2CO3. Carbonic acid is a diprotic acid and can react with 2 equivalents of base, so, when carbonic acid reacts with ammonia, ammonium bicarbonate begins to form; HO-C(=O)-O(-)(+)NH4. As more ammonia is added, ammomium carbonate begins to form; H4N(+)(-)O-C(=O)-O(-)(+)NH4. These are equilibrium reactions.

As the concentration of ammonium carbonate increases, there is a conversion of some of the ammonium carbonate to ammonium carbamate. This is a dehydration reaction.

An industrially important chemical that is derived from ammonia and carbon dioxide is urea, which is used as fertilizer. Urea is ammonium carbonate that has given up two molecules of water; H2N-C(=O)-NH2. The fertilizer plants that manufacture urea have severe corrosion problems due to the ammonium carbamate intermediate. The corrosion is so severe that carbon steel cannot be used. Some parts of the process will corrode 316 stainless steel. Those heated sections are manufactured from special alloys.

Why does iron corrode in the presence of ammonium carbamate? Ammomium carbamate is hygroscopic and will attract moisture. Wetted ammonium carbamate will become an equilibrium mixture of ammonium carbamate and ammonium carbonate. This combination of salts and moisture attacks the protective oxide layer on the surface of steel with chelation-type mechanism. This exposes the underlying base metal for further attack.

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