Materials used in Urea Plants

1. SS Materials issues in Urea plats

3. Amino formic acid can form at low pressure zone (3)-
destroy passive film on SS & corrodes
Urea
Ammonium
carbonate

5. Influence of oxygen partial pressure on
corrosion reaction
Reaction(3) is
amino formic acid-
can form ammonia
and CO2 at low
pressure itself. This
acid is responsible
for high corrosion
rate urea MoC. (1)
ammonia
carbamate

6. Field SS Corrosion Summary
a. Stainless steels (UG) can corrode at HAZ and ferrite
rich regions; c.
b. Welding start and finish points are defect prone and
corrode
c. Temper bead type welding procedure improves
d. High ferrite forms if welding is done at high heat
input (J)
e. Urea reactor’s SS lining and tray support bracket lugs
corrodes more
f. Top dome SS petal type weld lining also corrodes &
may even leaks
g. In carbamate condenser tube to tube sheet weld
area may corrode. TS lining also get affected

7. Lab Test results & Conclusions
• HPHT expt. with NH3 & CO2 as Bi-molecular reaction
indicated the possibility of forming amino-formic
acid.
• Electrochemical reaction rates are influenced by O2
and the type of passive films on metal surface.
• Thus any region which possesses weak passive films-
--tend to corrode at high rate selectively.
• Passive film formed on Delta ferrite region of SS weld
is comparatively weak than the base and can
corrode at rates of 2-3mm/year or more.
• Contd.-----

8. • However, ferrite present in duplex stainless
steels is likely to behave differently.
• Delta ferrite in weld forms at high temperature
and would contain lot of Si, P and other
trump/impurity elements act as ‘scavenger’ so
the passive film would be much weaker than
that formed on ferrite phase in DSS.
• Urea grade 316L SS is refined and contain much
less Si, P etc., and form least amount delta
ferrite and thus has low corrosion rate.