CO2 and water are corrosive at low temperature. Materials in CO2 removal systems, where temperatures are in the range of 100 to 250°F, have long been a problem in synthesis gas purification. Operators who have seen part of their CO2 strippers disappear or whose copper liquor regenerator has acquired a moth-eaten appearance, can testify that CO2 in water is corrosive.
Urea reactors have the prerequisites for CO2 corrosion at 200°F. but temperature, one of the prime factors in corrosion, is not 200°F. It is as high as 400°F, in a mixture of CO2, water, ammonia, carbonate, carbamate and urea.
Even with a high excess of ammonia at these temperatures the urea reactor mass will dissolve many corrosion resistant materials.
Fortunately, the problem, which may have seemed difficult or impossible in the early stages, has now been resolved. There are several lining systems, which do allow urea reactors to give satisfactory life. Many materials have been used. Lead, silver, titanium, zirconium and stainless steel each has it own story in urea reactor service. Each has its own problems in process application, fabrication and cost.