It is a well known fact that in highly oxidising environments Mo-alloyed stainless steels perform worse then comparable Mo-free alloys. Thus, when Mo-free X2CrNiN 23 4 duplex stainless steel was introduced, it was recognised as a promising alternative construction material to the AISI 304L and AISI 310L stainless types. Extensive research was carried out regarding the corrosion resistance of this new alloy in the welded condition. This research comprised both laboratory testing under simulated service conditions (alternating condensation and evaporation), as well as exposing welded test-coupons to actual process conditions in situ. In these tests X2CrNiN 23 4 outperformed 304L and 310L. However, weld metal and HAZ as well as the cross cut ends could show relatively severe attack. This resulted in the testing of a tube piece comprising several different weld filler metals – matching, type 310L, type 309L and type 304L – in the neutralisation section of a urea plant. All welds performed satisfactorily, indicating the safety and reliability of welded X2CrNiN 23 4 sections under these particular process conditions.

In the same time in a nitric acid plant a sacrificial pipe-piece on top of a cooler-condenser was constructed in X2CrNiN 23 4. In this pipe nitric acid is sprayed onto the pipe wall in order to prevent corrosion in the cooler-condenser underneath. This resulted in severe corrosion of AISI 304L and AISI 310L material. The X2CrNiN 23 4 performed much better, but even so it suffered significant corrosion at the welds, and at cross cut ends.

Based on supplier information, in-house experience and the test results X2CrNiN 23 4 has been selected for several important applications.

In the off-gas handling in the neutralisation section of a urea plant 304L showed borderline corrosion behaviour, and X1NiCrMoCu 31 27 4 suffered from stress induced intergranular cracking. Since 1994 X2CrNiN 23 4 has been used, and has since then outlasted the X1NiCrMoCu 31 27 4 parts, without showing any corrosion problems.

In 1994 two condensers in the same neutralisation section were fitted with X2CrNiN 23 4 tubes. Formerly AISI 304L tubes showed chloride stress corrosion cracking (cooling water side) and AISI 316L tubes suffered general attack due to condensing NOx. The duplex tubes were welded in AISI 304L tubesheets using 304L type filler metal. Since that time both condensers have been in service without any tube-related failures.

Finally, in 1997 a new cooler-condenser for a nitric acid plant has been fitted with X2CrNiN 23 4 tubes. These are expected to provide excellent performance, both on the process side as on the cooling water side. Again the tubes were welded in AISI 304L tubesheets using 304L type filler metal.

Summarising, the experience with X2CrNiN 23 4 in laboratory and service conditions shows that this material is a good alternative for commonly used austenitics in highly oxidising environments. However stringent quality assurance remains crucial.