A study has been made under urea service conditions of the corrosion behavior of the following classes of stainless steels (SSs): AISI 316 urea grade (UG), 25Cr-22Ni-2MoN super-austenitic (SA), 21-25Cr6-7Ni and 1.5-3Mo duplex, and 21-25Cr extra low interstitial (ELI) ferritic Nb and Nb + Ti stabilized. Electrochemical measurements were conducted under simulated urea synthesis service conditions [NH3:(H2O + urea) = 0.6 wt%, with H2O:urea = 2.2, T = 195 C, and P = 20 MPa]. The Huey test, which is used widely as the acceptance test for UG materials, was also applied. Two dissolution peaks were observed in the potentiodynamic curve: the first (–800 to –700 mV vs Ag-AgCl) corresponded to the active potential region; and the second (–500 to –300 mV) was in the passive region of the curve. The free corrosion potential falls in the first or second peak, depending on the steel grade and oxygen partial pressure. Chromium has a markedly beneficial effect under both active and passive conditions. Nickel has a detrimental effect under active conditions. A correlation exists between the height of the second peak and the corrosion rate (CR) in the Huey test solution. New urea synthesis processes based on lower oxygen partial pressures call for low-Ni SSs.

Subject
Ferritic steel, Materials, Anode potential, Correlation, Synthesis, Anodic dissolution, Penetration, Corrosion behavior, Steel, Oxygen, Corrosion mechanisms, Urea, Annealing
National Association of Corrosion Engineers
1987
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