The traditional alloys
Over the last 30 years, two austenitic stainless grades have been dominant in urea plants with stripper technology. The lower alloyed version is a modified ASTM 316L (UNS No 31603), referred to as 316L MOD or 316L Urea Grade. It is modified to meet the requirement of a maximum 0.6 % ferrite and a maximum corrosion rate in the Huey test of 0.60 mm/year. Both are needed to provide the low corrosion rate for this type of alloy. The commercial version of 316L does not meet these requirements, as the impurity level is far too high.
For the more corrosive areas, a higher alloyed material is needed. It is referred to as 25/22/2, which stands for the chromium, nickel and molybdenum contents. With a chromium content raised from 17 to 22%, and increased nickel content from 14 to 22%, corrosion resistance was further improved compared with 316L Urea Grade. Also, 25/22/2 alloy has an international designation UNS S 31050.
|Stainless Steel Sandvik tubes under fabrication
However, for use in the urea process, the impurities like sulphur, phosphorus and carbon have to be kept low, and as S 31050 allows rather high impurity levels, it is not sufficient to specify the alloy by using only the UNS designation. As Sandvik was involved from the beginning in developing the 25/22/2 type, and thereafter made improvements to the composition, they believe that they produce the best 25/2212 version.