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Fluid Journal : Fluid Journal 2008-2009
towel. The amount of metal loss was small enough that even the original corrosion spot remained on the coupon (Figure 5). Sludge It has been determined that opening the jar resulted in the release of the ammonia from the UAN, which caused the corrosion that occurred on the surface of the coupon. The "Key Take Away" from this example is that although surface corrosion may not be very damaging, it does generate a large amount of corrosion sludge, and this sludge winds up on the tank floor! The reason this is important is that the generation of surface corrosion sludge leads to a major UAN corrosion mechanism called "under deposit corrosion." The chemistry that occurs with under deposit corrosion is complicated and a rigorous description is beyond the scope of this article. In brief, corrosion sludge prevents the normal flow of ions in the UAN solution and results in the formation of "pockets of corrosive ions" under the sludge on the tank floor. The buildup of corrosion sludge can also prevent corrosion inhibitors from accessing the steel under the sludge. The net result is the deep pitting that usually occurs on UAN tank bottoms. It is not unusual to see UAN pits as deep as a quarter inch develop in as little as 5 years when preventive measures are not taken. Examples of UAN sludge buildup and the resulting pitting corrosion are shown below in Figures 6, 7, 8 and 9. Managing UAN corrosion Fluid fertilizer dealers who handle UAN should assume a more active role managing UAN corrosion. There are mechanical, operational, and chemical means to do so, and all three areas should be considered at any UAN handling facility. Mechanical Drain and clean UAN storage tanks at least every two years. This will remove the surface corrosion sludge that leads to the more damaging "under deposit or pitting type corrosion." Never leave small puddles or "heels" of UAN in storage tanks for long periods of time. As in the rail car example cited above, this UAN can easily be stripped of the buffering ammonia and this results in acidity in the UAN and severe corrosion. Avoid leaving puddles of UAN Figure 4. Severe corrosion developed on coupon after loss of ammonia. Figure 5. Surface corrosion initially results in minimal damage. in steel piping. Blowing them clear with air just flashes off any remaining ammonia buffer, leaving thin films of acidic UAN on the pipe wall. Instead, consider a flush with aqua ammonia or, where practical, install corrosion- resistant pipe such as plastic or stainless steel. Secure the dome covers on all empty tanks or tankers that hold or transport UAN. This is particularly important for trucks and rail cars because it helps prevent the loss of excess ammonia. Dealers who carelessly leave this equipment open to the atmosphere increase the cost that all dealers pay for UAN transport and storage. Install a recirculation header in storage tanks. Corrosion sludge is bad, but sludge that does not move is even worse, as this Figure 3. Sealed jar left in sun for 7 days. Figure 6. UAN tank sludge.
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