Corrosion of Piping in HP Urea / Carbamate Service

[ashfaqanwer]

In Fertilizer Industry, corrosion in Urea Plant is the biggest problem Inspection Engineers have to face. Urea itself and its intermediate products like Ammonium Carbamate are too corrosive even for normal grade Stainless Steel material like 304, 316. Most commonly used material in such applications where temperature and pressure conditions are also severe, SS316L modified (urea grade) or 25Cr.22Ni.2Mo material is used. SS316L modified contains higher Ni content which makes it more austenitic than normal 316L grade and eventually offers more corrosion resistance. Preferred welding material for pipe welding is also 25Cr.22Ni.2Mo (ER/E-310Mo) with good controls during welding operation especially checking of ferrite content and good purging during welding of root pass.

In one of the case, where piping material used was 316L normal grade and welded with ER/E-316L, high rate of corrosion was noticed especially in welds.
Attached are some of the pictures showing the extent of corrosion.

[mechaengr]

Asfhaq...... In case of SS-316L MOD slightly higer nitrogen contents act as austinite stabilizer which protects against corrosion against highly corrosive services like carbamate....... Also during welding temperature should also be taken into account, specially in the case when weld buildup is to be done...... Also if machining of Urea grade component is required on that surface which is to be in direct contact of the service, excessive machining should be avoided otherwise there would be a requirement of solution annealing of the surface..........

[ashfaqanwer]

It's Nickel along with low carbon and silicon not the nitrogen which makes 316L Modified fully austenitic.