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Fluid Journal : Fluid Journal 2008-2009
anything except nitrogen content and price, your supplier probably won't care about anything else either. On the other hand, fluid fertilizer dealers and UAN bulk tank owners may not want to depend solely upon their UAN supplier to manage the corrosion that could occur in their equipment. The potential for UAN corrosivity can vary greatly from producer to producer, plant-to-plant, and even day-to-day in the same plant or terminal. Before we discuss more about how to evaluate and minimize the corrosive quality of UAN you purchase, we must first understand more about why UAN is corrosive and the key corrosion mechanisms that occur when UAN is stored. Excess ammonia Of the components that make up UAN (urea, ammonium nitrate, water, and a small amount of ammonia), ammonium nitrate (AN) is the more corrosive component. AN's corrosive affect is buffered by the small amount of "excess" ammonia present; as little as 0.01 percent (100 ppm) to as much as 0.20 percent (2,000 ppm). Ammonia itself is not a corrosion inhibitor. But without this small amount of excess ammonia, UAN becomes extremely acidic and corrosion inhibitors cannot be applied cost effectively. One might wonder why extra ammonia is not always added to the UAN solution. It generally is not an issue of cost. Instead, excess ammonia content is usually limited by plant design and customer preference. Some UAN buyers prefer high ammonia levels in the UAN they purchase because their customers prefer that the UAN have a strong ammonia odor. Other buyers worry about odor complaints from neighbors if ammonia levels are too high during application. In addition, very high ammonia levels could be a safety hazard for personnel loading and unloading UAN. Furthermore, some herbicides and other tank mix products are sensitive to having too much ammonia in the UAN. With either preference, corrosion can be controlled as long as the ammonia stays in the UAN at adequate levels. The problem is there are many ways in which ammonia can be reduced to levels that no longer provide the buffering required. For example, consider a rail car full of UAN, as depicted in Figure 2. When full, the ammonia quickly saturates the small vapor space above the liquid level in the car. Even when warm, no significant amount of ammonia is lost, so the UAN remains fairly non-corrosive. However, upon arrival at a customer site, when the tanker is emptied, a thin film of UAN is left on the tank wall. At the same time, fresh air (that contains no ammonia at all) is sucked into the tank. When this happens, the ammonia levels in the vapor space must be reestablished. The only source of ammonia is the "thin film" left on the tank wall. If there is very little ammonia available, such as when excess ammonia levels in the bulk UAN are less than 100 ppm to begin with, the corrosivity of the small amount of UAN remaining on the tank walls can increase dramatically. Surface corrosion As UAN levels in storage tanks drop during the season, this same corrosive "thin film" is formed on the vertical wall as fresh air comes into the UAN storage tank. For this reason, fluid fertilizer dealers should ask about the typical excess ammonia provided by their UAN supplier. If there is a need to purchase a low ammonia product, the dealer should be aware that other steps may need to be taken to help minimize the impact of this low ammonia level. The type of corrosion produced by low ammonia levels is called "surface corrosion." The good news is that surface corrosion initially is not very damaging to the steel. However, it does contribute to other problems. In a simple demonstration, that can be done in one's own backyard, a steel corrosion coupon was put inside a well-sealed glass jar along with just enough UAN 32 to barely cover most of the coupon. In this example, an old coupon was used that happened to have a small corrosion spot on one side. The jar was sealed and left in the sun for 7 days (Figure 3) and during that time there was no sign of new corrosion. The jar was then opened to the air for about 3 hours and resealed and left overnight. The following morning, it was noted that severe corrosion had developed on the coupon (Figure 4). However, this generalized surface corrosion was not very damaging and was easily wiped off with a paper Figure 2. Example of how "thin UAN film corrosion" can occur.
Fluid Journal 2005-2007