Problem No. 36: Change in colour of urea product

Stainless steel owes its corrosion resistance to the presence of a protective chromium oxide layer on the surface. As long as this layer remains intact, the metal corrodes at a very low rate. If the passive layer in stainless steel is damaged, active corrosion starts in aggressive corrosive environments. Stainless steels exposed to carbamate containing solutions in the urea synthesis section can be kept in a passive state by adding a minimum amount of oxygen. If the oxygen content drops below this minimum, active corrosion starts, which can be identified by a shiny silver colour on the surface (see picture). Adding oxygen and maintaining a sufficiently high oxygen content in the various process streams are prerequisites for preventing excessive corrosion of the equipment and piping. Once active corrosion occurs, the corrosion rate is extremely high (>50 mm/ year) and will not stop even if more oxygen is supplied to the process.