Urea is made by the reaction of ammonia and carbon dioxide under a pressure of 150-220 bar, in a reactor at temperatures in the range 170-200°C. The corrosive conditions are severe in all high pressure parts where the intermediate component ammonium carbamate is present. Several factors govern the corrosion of stainless steel in carbamate solutions.

Corrosivity is influenced by the amount of carbamate, but also by the ammonia/carbon dioxide ratio. Besides temperature, very important is the oxygen content. If there is no oxygen present, stain less steels will corrode actively at high rates, whereas only small amounts of oxygen are sufficient to keep them in the passive state. The amount of oxygen necessary is determined by the type of steel being used. To control corrosion rates, oxygen is added continuously to the feed. The effect of dissolved oxygen on the corrosion rate of different stainless alloys is illustrated in table 1. The table confirms the better performance of UNS No 31050 (25/22/2), which needs less oxygen to passivate and shows less loss of material in both the active and passive states. In operation, the oxygen addition must be much higher to ensure that there is a sufficient amount of oxygen in all parts of the plant.

Table 1. Effect of dissolved oxygen content on the corrosion rate of stainless steel alloys in urea
solution.

 

O2, ppm	Corrosion rate,	mm/year
	UNS No 31603	UNS No 31050
0,5	30-40 active 1	10 active 1
1	30-40 active 1	10 active 2
3	30-40 active 1	0,03 passive 2
5	0,12 passive 2	0,03 passive 2
7,3	0,12 passive 2	0,03 passive 2