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UAN Corrosion Management: …

UAN Corrosion Management:

When there is a lack of attention and little effort on maintenance, corrosion caused by fertilizers (including UAN) can result in financial losses, environmental damage, and even loss of life. However, these potential risks can be greatly reduced with only a small amount of effort and minimal expense on the part of the dealer or grower.

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  • 1. Fluid JOURNAL Official Journal of the Fluid Fertilizer Foundation Winter 2009 Vol. 17 No. 1, Issue #63 Phil Bureman and Dr. Craig Myers Take UAN Corrosion Seriously If a tank leaks, you may be liable for water contamination in ground and surface water. Summary: When there is a lack of attention and little effort on maintenance, corrosion caused by fertilizers (including UAN) can result in financial losses, environmental damage, and even loss of life. However, these potential risks can be greatly reduced with only a small amount of effort and minimal expense on the part of the dealer or grower. Figure 1. Collapsed storage tank. in UAN are usually detected Managing corrosion be liable for environmental and through sampling and testing and workplace safety violations, and are corrected prior to shipment to UAN owes much of its popularity to the entire business and/or business any customer. However, in some its ability to mix easily with herbicides reputation could be put at risk. facilities and terminals, no systems and other fertilizers, thus enabling or few systems are in place to detect a one-pass application on the farm. Why care? these corrosive properties. Since It also allows for improved safety The majority of UAN suppliers strive the results of UAN corrosion events versus anhydrous ammonia or to produce high quality UAN that may take some time to appear, the dry ammonium nitrate. It is easy is only minimally corrosive. When supplier may not always be aware to handle and apply accurately, manufactured and inhibited properly, that there is a problem. If corrosion plus it is not so likely to be stolen UAN corrosivity can be minimized. in their facility can be minimized--- for illegal use. Nevertheless, it is However, when made improperly, for example by having tank liners, important to remember UAN can UAN can corrode carbon steel very epoxy coatings, and stainless steel be corrosive to most metal surfaces quickly. Fluid fertilizer dealers should or plastic pipe in place---there may when stored. When not properly be aware that many UAN producers be less incentive for them to monitor managed, UAN corrosion can cause and brokers have taken steps to corrosion quality serious damage to pumps, pipelines, reduce their exposure to the risk storage tanks, barges, rail cars, tank Fluid fertilizer dealers should seek of UAN corrosion by installing tank trucks, and agricultural equipment. out quality UAN producers and liners, applying epoxy coatings, and UAN corrosion can cost UAN brokers and ask what systems they replacing carbon steel piping with manufacturers, sellers and users in have in place to ensure UAN quality. stainless steel. many ways, including damage to Dealers should ask producers In some cases, tank liners are and brokers about their sampling equipment and the cost of cleaning, applied simply to provide secondary procedures and ability to promptly removing, and disposing of UAN containment as required by some analyze those samples. Also, ask corrosion sludge. state regulations. In other plants, how these additional quality checks For fluid fertilizer dealers, UAN UAN producers and brokers have can help minimize risk and make corrosion can result in even more learned through experience that them more money in the long term. severe problems. For example, if upsets can result in significant UAN from better quality suppliers a UAN tank leaks, or worse yet if damage to storage systems in a very may cost a little more, but it is it collapses (Figure 1), significant short time. Such upsets are rare but probably worth the higher price. amounts of product and property they do happen. Corrosive properties Remember: If you never ask about could be lost; the dealer might
  • 2. 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. Figure 2. Example of how “thin UAN film corrosion” can occur. Excess ammonia Of the components that make up stays in the UAN at adequate levels. into the UAN storage tank. For UAN (urea, ammonium nitrate, water, The problem is there are many ways this reason, fluid fertilizer dealers and a small amount of ammonia), in which ammonia can be reduced should ask about the typical ammonium nitrate (AN) is the to levels that no longer provide the excess ammonia provided by their more corrosive component. AN’s buffering required. UAN supplier. If there is a need to corrosive affect is buffered by the For example, consider a rail car purchase a low ammonia product, small amount of “excess” ammonia full of UAN, as depicted in Figure the dealer should be aware that present; as little as 0.01 percent (100 2. When full, the ammonia quickly other steps may need to be taken to ppm) to as much as 0.20 percent saturates the small vapor space help minimize the impact of this low (2,000 ppm). Ammonia itself is not a above the liquid level in the car. Even ammonia level. corrosion inhibitor. But without this when warm, no significant amount of The type of corrosion produced by small amount of excess ammonia, ammonia is lost, so the UAN remains low ammonia levels is called “surface UAN becomes extremely acidic fairly non-corrosive. However, upon corrosion.” The good news is that and corrosion inhibitors cannot be arrival at a customer site, when surface corrosion initially is not very applied cost effectively. One might the tanker is emptied, a thin film of damaging to the steel. However, it wonder why extra ammonia is not UAN is left on the tank wall. At the does contribute to other problems. always added to the UAN solution. same time, fresh air (that contains In a simple demonstration, that can It generally is not an issue of cost. no ammonia at all) is sucked into be done in one’s own backyard, Instead, excess ammonia content is the tank. When this happens, the a steel corrosion coupon was put usually limited by plant design and ammonia levels in the vapor space inside a well-sealed glass jar along customer preference. Some UAN must be reestablished. The only with just enough UAN 32 to barely buyers prefer high ammonia levels in source of ammonia is the “thin film” cover most of the coupon. In this the UAN they purchase because their left on the tank wall. If there is very example, an old coupon was used customers prefer that the UAN have a little ammonia available, such as that happened to have a small strong ammonia odor. Other buyers when excess ammonia levels in the corrosion spot on one side. The jar worry about odor complaints from bulk UAN are less than 100 ppm was sealed and left in the sun for 7 neighbors if ammonia levels are too to begin with, the corrosivity of the days (Figure 3) and during that time high during application. In addition, small amount of UAN remaining there was no sign of new corrosion. very high ammonia levels could be a on the tank walls can increase The jar was then opened to the air for safety hazard for personnel loading dramatically. about 3 hours and resealed and left and unloading UAN. Furthermore, Surface corrosion overnight. The following morning, it some herbicides and other tank mix was noted that severe corrosion had As UAN levels in storage tanks products are sensitive to having too developed on the coupon (Figure 4). drop during the season, this same much ammonia in the UAN. With However, this generalized surface corrosive “thin film” is formed on either preference, corrosion can be corrosion was not very damaging the vertical wall as fresh air comes controlled as long as the ammonia and was easily wiped off with a paper
  • 3. 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.” Figure 4. Severe corrosion developed on coupon after loss of ammonia. The chemistry that occurs with under in steel piping. Blowing them deposit corrosion is complicated clear with air just flashes off and a rigorous description is beyond any remaining ammonia buffer, the scope of this article. In brief, leaving thin films of acidic corrosion sludge prevents the normal UAN on the pipe wall. Instead, flow of ions in the UAN solution and consider a flush with aqua results in the formation of “pockets 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 Figure 5. Surface corrosion initially hold or transport UAN. This is results in minimal damage. particularly important for trucks UAN should assume a more active and rail cars because it helps role managing UAN corrosion. There prevent the loss of excess are mechanical, operational, and ammonia. Dealers who carelessly chemical means to do so, and all leave this equipment open to three areas should be considered at the atmosphere increase the Figure 3. Sealed jar left in sun for 7 days. any UAN handling facility. cost that all dealers pay for UAN Mechanical transport and storage. of corrosive ions” under the sludge • Drain and clean UAN storage • Install a recirculation header in on the tank floor. The buildup of tanks at least every two years. storage tanks. Corrosion sludge corrosion sludge can also prevent This will remove the surface is bad, but sludge that does corrosion inhibitors from accessing corrosion sludge that leads to the not move is even worse, as this the steel under the sludge. The net result is the deep pitting that usually more damaging “under deposit occurs on UAN tank bottoms. It is not or pitting type corrosion.” unusual to see UAN pits as deep as • Never leave small puddles or a quarter inch develop in as little as “heels” of UAN in storage tanks 5 years when preventive measures for long periods of time. As in the are not taken. Examples of UAN rail car example cited above, this sludge buildup and the resulting UAN can easily be stripped of pitting corrosion are shown below in the buffering ammonia and this Figures 6, 7, 8 and 9. results in acidity in the UAN and Managing UAN corrosion severe corrosion. Fluid fertilizer dealers who handle • Avoid leaving puddles of UAN Figure 6. UAN tank sludge.
  • 4. allows for the buildup of “pockets tank sludge to keep moving • Consider installing a tank liner of corrosive ions” under the across the tank bottom. This or applying a protective coating. corrosion sludge. Many UAN helps to greatly reduce pitting Evaluate these options carefully, producers now have adopted corrosion. as these components can also a best practice of installing an • Some new tank builders are now fail, potentially resulting in an inlet recirculation pipe across recommending installation of a even larger problem. the entire diameter of their tank sloped tank bottom with a center Operational bottom. This header pipe has outlet. This design is great for • Check the pH of incoming several outlet spouts that point corrosion prevention, as sludge is UAN. Measurement of pH is a toward the floor at a 45o angle. constantly removed from the tank good indication of how much Recirculation of the UAN allows so buildup is always minimal. excess ammonia is contained in the UAN. Most producers ship product with a pH above 7.0. A pH as low as 6.5 is quite acceptable, but much below that there is a risk of stressing all the corrosion management systems. Discuss with your supplier a desire to receive product with a pH no lower than 6.5. UAN purchasers can check the pH of incoming UAN with something as inexpensive as pH paper tape (less than $50), or consider the purchase of a pH meter (less than $500). This is highly recommended if you are purchasing imported product, which has been known to lose Figure 7. Pitting under UAN tank sludge. Figure 9. Pitting under UAN rail car sludge. excess ammonia during the long transport time. • If available, ask the supplier to include excess ammonia levels on the Certificate of Analysis for each load. Excess ammonia should be no less than 0.010 weight percent (100 ppm), and preferably should be at least 0.020 to 0.050 weight percent (200-500 ppm). • Another important quality of the UAN is known as the AN/ Urea Ratio. This is a simple mathematical calculation of the Figure 8. UAN rail car sludge.
  • 5. weight percent of ammonium As detailed above, purchasers inhibitors actually combine with nitrate divided by the weight should monitor the product they the corrosion products when percent of urea. In UAN 32, this receive and ask questions about corrosion is initiated and stop ratio is typically something like the quality of the UAN being corrosion by “sealing off” the 45%/35% = 1.28 AN/Urea Ratio. purchased. If suppliers can’t tell corrosion cell. Both types have The AN/Urea Ratio is important buyers about the quality of the advantages and disadvantages because it greatly affects the UAN being offered, they may not and when applied correctly temperature at which UAN starts be monitoring it themselves. either type will greatly reduce to salt out. UAN 32 typically Chemical UAN corrosion. Some inhibitor has a salt-out temperature of formulations offer additional • Fluid fertilizer dealers should about 30o F. But this is only true features such as sludge ask what type and how much at the optimal AN/Urea ratio dispersants, metal chelants, corrosion inhibitor is being used range of about 1.20 to 1.40. At anti-foam additives and tracer in the UAN they purchase. It ratios above 1.6 or below 1.0, compounds. A sales engineer, might be possible to request UAN 32 could start to salt out educated in UAN corrosion the treatment rate and brand of at temperatures above 45o F or inhibitors, can help you decide corrosion inhibitor be included on higher. This is important because which type is best for your UAN. the Certificate of Analysis. salt-out means lost nitrogen content. In addition, both AN and • If purchasing UAN from multiple Phil Bureman is Nalco’s urea salts form very aggressive suppliers or sites, it is possible technical consultant for corrosion cells, so they are not to end up with UAN containing Chemicals and Biofuels and a good thing to have sitting at multiple inhibitors in your storage lives in Olathe, KS; pebureman@ the bottom of a tank (it’s like tanks. Although compatibility nalco.com and Dr. Craig Myers leaving rock salt on the driveway among different inhibitors is not is a research scientist at Nalco’s at home). Lastly, both AN and an issue, these different inhibitors headquarters in Naperville, IL; urea salts have a negative heat (now diluted) may not be as cmyers1@nalco.com of solution. This means that large effective together as they were by amounts of heat are needed to themselves at full strength. get these salts back into solution. • Fluid fertilizer dealers should This may not be a problem when consider purchasing and purchasing UAN in the spring, applying their own corrosion when ambient temperatures are inhibitor programs. Corrosion already well above 50o F, and the inhibitor chemicals are UAN is to be applied right away. inexpensive, compared to the But the AN/Urea Ratio should cost of UAN. Corrosion inhibitor be considered if one plans to programs typically cost $0.60- store UAN 32 during cold winter $0.70 per ton of treated UAN months. There is no easy way for when purchasing in the small UAN purchasers to measure this amounts likely to be needed by parameter, but local suppliers fluid fertilizer dealers. This cost should be able to give customers could be much less if the inhibitor some typical values, or possibly choice is matched to the one include this measurement on used by the UAN supplier. Most the Certificate of Analysis, if UAN corrosion inhibitors are easy requested. to apply and safe to use. There • Try to store only UAN 32. are some products, however, Contrary to what many believe, which themselves are quite acidic UAN 28 is more corrosive than and are inherently hazardous UAN 32. Consider purchasing to handle and store. Be sure and storing only UAN 32, and to discuss this question with then if needed, dilute it with water potential inhibitor vendors. to 28% just prior to application. • There are two basic types of UAN • Lastly, purchasing quality corrosion inhibitors. “Filmer” UAN from a trusted source is type inhibitors form a protective probably worth a higher price. film on steel. “Passivator” type