Electrochemical Study of Stainless Steels in High Temperature Urea Solutions

The corrosion of stainless steels in high temperature and high pressure urea solutions was studied by electrochemical measurements together with immersion tests to elucidate the corrosion mechanism and to rationalize the corrosion prevention methods. It was demonstrated that the corrosion of stainless steel was either active region dissolution, caused by the lack of dissolved oxygen, or dissolution near the transpassive region in process fluids with sufficient oxygen. High Cr/low Ni (or Ni-free)/Mo stainless steels were readily passivated. Stainless steels with high Cr content and/or with homogeneous distribution of Cr had superior resistance to corrosion near the transpassive region. As the temperature rose, the passivation current increased. At the same time, dissolution of oxygen into the simulated process fluids was more pronounced; therefore, high temperature was not necessarily unfavorable for passivation. Ammonia content in the process fluids strongly affected the passivation current; the current increased as the NH3:H2O molar ratio fell. By comparing the environmental variables in urea plants with the obtained data, guidelines for suitable corrosion prevention methods were developed.