an excess of ammonia, but after the stripper, it occurs until the temperature is reduced. Biuret is undesirable in fertilizer urea because it is toxic to crop plants, although to what extent depends on the nature of the crop and the method of application of the urea. (Biuret is actually welcome in urea when is used as a cattle feed supplement). Isocyanic acid HNCO and ammonia results from the thermal decomposition of ammonium cyanate , which is in chemical equilibrium with urea: This reaction is at its worst when the urea solution is heated at low pressure, which happens when the solution is concentrated for prilling or granulation (see below). The reaction products mostly volatilize into the overhead vapours, and recombine when these condense to form urea again, which contaminates the process condensate. Corrosion Ammonium carbamate solutions are notoriously corrosive to metallic construction materials, even more resistant forms of stainless steel – especially in the hottest parts of the plant such as the stripper. Historically corrosion has been minimized (although not eliminated) by continuous injection of a small amount of oxygen (as air) into the plant to establish and maintain a passive oxide layer on exposed stainless steel surfaces. Because the carbon dioxide feed is recovered from ammonia synthesis gas, it contains traces of hydrogen that can mingle with passivation air to form an explosive mixture if allowed to accumulate. In the mid 1990s two duplex (ferritic-austenitic) stainless steels were introduced (DP28W, jointly developed by Toyo Engineering and Sumitomo Metals Industries and Safurex, jointly developed by Stamicarbon and Sandvik Materials Technology (Sweden).) These let manufactures drastically reduce the amount of passivation oxygen. In theory, they could operate with no oxygen. Saipem now uses either zirconium stripper tubes, or bimetallic tubes with a titanium body (cheaper but less erosion-resistant) and a metallurgically bonded internal zirconium lining. These tubes are fabricated by ATI Wah Chang (USA) using its Omegabond technique. Finishing Urea can be produced as prills, granules, pellets, crystals, and solutions. Solid forms For its main use as a fertilizer urea is mostly marketed in solid form, either as prills or granules. The advantage of prills is that, in general, they can be produced more cheaply than granules and that the technique was firmly established in industrial practice long before a satisfactory urea granulation process was commercialized. However, on account of the limited size of particles that can be produced with