ABSTRACT
Corrosion of stainless steels is a known problem in the high-pressure section of urea plants. A particularly demanding part of the plant, which often has the highest passive corrosion rates, is the high-pressure stripper. The reason is that the corrosion is strongly temperature dependent. This paper presents the development program of UNS S83071, a new improved duplex stainless steel customized for this environment. The key for the successful development was alloy screening in ammonium carbamate solution corresponding to stripper conditions. It was shown that there is a clear difference between screening in the ammonium carbamate environment compared to screening in the standardized corrosion tests that were used during the development of UNS S31050 and UNS S32906, two stainless steels that are commonly used in the urea industry today. The new material, which will shortly be available for use in urea plants combines good mechanical properties with improved corrosion resistance and enables extended life time of the high-pressure stripper.
INTRODUCTION
Urea is produced by the reaction of ammonia and carbon dioxide at high pressure (140 bar) and temperature (183 °C). The first step in the synthesis is the formation of ammonium carbamate, which is the intermediate step in the urea process. Ammonium carbamate then forms urea and water in a second step. The second reaction is not complete; so, the non-converted carbamate needs to be separated from the urea solution.1
The high-pressure synthesis section is critical from a mechanical point of view due to the high pressure and temperature; and from a corrosion point of view due to the high corrosiveness of the ammonium carbamate. It was early discovered that it was possible to use austenitic stainless steels in the synthesis section if oxygen or air was added to the process for passivation. The use of stainless steels enabled industrial production of urea. A large process improvement was the CO2 stripping process together with an improved austenitic stainless steel X2CrNiMo 25-22-2 (UNS S31050) developed to withstand the high temperatures in the stripping process.
