What Does Urea Mean?
Urea is a nitrogen-containing organic chemical compound with the chemical formula CO(NH2)2 in which two -NH2 groups join with a carbonyl (C=O) functional group.
Urea has common uses in fertilizers and feed supplements. It is also a starting material for the production of plastics and drugs. It can cause corrosion to steel and agricultural equipment.
Urea is a chemical compound that serves an important role in the metabolizing of nitrogen-containing compounds in animals and tends to be the main nitrogen-containing substance in urine. It is an odorless, colorless, solid that is highly soluble in water and is not dangerous to inhale or come in contact with in small doses. It is useful in the body as it does not become either acidic or alkaline when dissolved in water.
Many processes utilize urea, notably nitrogen excretion. Although it is a valuable resource for the body, its properties cause corrosion when created synthetically.
Corrosionpedia Explains Urea
Urea is a nitrogenous organic compound containing a carbonyl group and two amine groups. It is a crystalline substance that melts at 132.7°C (271°F) and decomposes before boiling. Besides fertilizers and feed supplements, urea is also used to manufacture plastics, glues, toilet cleaners, detergents, pesticides and fungicides. Urea is also an increasingly common additive for diesel fuel for NOx control.
Synthetic urea comes from a two-step reaction consisting of carbamate formation and urea conversion. During urea synthesis, parts of the process contain significant concentrations of ammonium carbamate. At production temperature, approximately 160°C (320°F) and higher, this carbamate is notorious for corroding most materials. Ammonium carbamate’s aggressive corrosiveness even corrodes the graded stainless steel specifically designed for these purposes.
In a urea plant, stainless steel requires an injection of passivation air into the process stream to maintain a passive corrosion resistant layer. This layer will not completely prevent corrosion, however. Therefore a good alternative to stainless steel is zirconium, which does not require any passivation air to remain resistant to corrosion even in the high process temperatures found in a urea plant.
The science as to why urea is so corrosive when in contact with stainless steel, especially when hot, is not very widely studied, and there are a variety of reasons why it may react with steel in this manner.
The lack of study in this area is likely due to its limited demand in the industry and the specific conditions and temperature required to cause corrosion in the first place. At lower temperatures, urea is considered non-corrosive for stainless steel.
Urea has a wide field of areas where it may cause damage, such as in the steel industry and agriculture. At high temperatures, urea begins to attack both aluminum and galvanized steel.
With the development of ultra-low carbon stainless steel smelting technology, the purity of the steel lends itself more to urea production. The chemical composition, microstructure and corrosion resistance of urea-grade stainless steel are very specific to fully inhibit the ferritic content in steel and ensure a full austenitic structure. The removal of intermetallic phase precipitations improves the corrosion resistance and selective corrosion ability of the steel.
All urea-grade stainless steels are normally subjected to a Huey test and a selective corrosion test. Additionally, there is also a metallographic inspection; no sigma phase and metal inclusions are permitted. General stainless steel, such as 316L, doesn't meet these conditions.
310MoLN (S31050/725LN /1.4466/2Re69) and 316L mod (724L/316LN) are the most widely used urea-grade stainless steels. 310MoLN is urea-grade, ultra-low carbon, austenitic stainless steel with a density of 7.9g/cm. Its corrosion resistance is similar to that of 904L steel and results in excellent corrosion resistance, especially in ammonium carbamate and nitric acid. The 316L modified stainless steel has low carbon content, extra-low silicon content and substantially higher molybdenum contents. 316L modified has improved corrosion resistance in urea-carbonate environments.