UREMIUM29 is a super duplex stainless steel specifically designed for use in the most demanding high-pressure sections of urea plants , such as the stripper . This vessel is characterised by very harsh operating conditions due to the combination of high temperature and pressure and a highly corrosive environment with ammonium carbamate . UREMIUM29 has shown excellent resistance to this corrosive medium . High mechanical strength is another important advantage of this material compared to other austenitic materials . Its superior performance make it an excellent candidate for use in various urea plant applications .

By B Calleja & A López , Tubacex Innovación AIE , Spain

The corrosion of stainless steel is a recognised problem in the highpressure section of urea plants . The stripper is the most critical equipment in the urea process . The function of a stripper is to remove unconverted ammonium carbamate from the urea solution . Working pressure and temperature range within 140-200 bar and 180-210 ° C , respectively . High temperature , high pressure and the presence of ammonium carbamate make the conditions in the urea stripper extremely harsh . Preventing corrosion in the stripper is crucial to guarantee urea plants ’ safe and stable operation . TUBACEX supplies tubes , pipes and special products in several corrosion-resistant materials for urea plants : high Cr-high Ni-Mo austenitic stainless steels and various types of duplex stainless steels . An example of manufactured stripper tubes is presented in Figure 1 . UREMIUM29 ( UNS S32906 ) is an austeno-ferritic stainless steel with 29 % chromium , with very high resistance to carbamate-induced corrosion , recommended for the tubes of highpressure equipment .

Excellent corrosion performance UREMIUM29 and other steel grades that can be applied in different sections of a urea plant were tested in an experimental setup specially designed to simulate the highly corrosive conditions of a urea stripper . During the tests , oxygen was eliminated from the experimental setup due to the current tendency to decrease O 2 levels to reduce costs and increase plant efficiency and safety . Therefore , the atmosphere generated inside the autoclave was even more severe than in real strippers , as most active urea plants work with oxygen injection . The main objectives of this study were to identify the basic mechanisms controlling corrosion in urea synthesis

Figure 1 . Stripper tubes produced by TUBACEX .

process and to establish the relationship of these mechanisms with the chemical composition and the microstructure of the tested steels . Five different materials were investigated :

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UNS S31050 : 25Cr-22Ni-2Mo austenitic stainless steel .

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UNS S31803 : 22Cr-5Ni-3Mo duplex stainless steel .

Table 1 . Corrosion rate and maximum depth of attack after autoclave tests .

UREMIUM29 clearly shows the lowest corrosion rate and attack .

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