Study on the microstructural changes and impact on the pitting corrosion behaviour of 2507 duplex stainless steel weld regions

The critical pitting temperature (CPT) test (ASTM G-150) is an electrochemical test generally used to evaluate the resistance of stainless steels to pitting corrosion, based on the concept of determining the potential - independent critical pitting temperature. Although this test has been done on the base metals of most stainless steels, few studies have been conducted on the weld regions, particularly the duplex stainless steels. In the present study, CPT and conventional potentiodynamic scanning tests were conducted on the various weld passes, namely root, filler and cap. Results were compared with the base material as a reference. The microstructure of the various weld regions, were characterised using optical microscopy, scanning electron microscopy and EDX analysis. The extent of degradation and the mechanisms responsible for pitting corrosion in terms of the various microstructural features within the different weld regions were examined. Variations in the microstructure was observed between the different weld regions in terms of ferrite/austenite ratio, grain shape and size and the formation of undesirable secondary phases such as chromium nitride and sigma phase. The CPT measurements revealed that the lowest pitting resistance was observed for the cap then the root pass; maximum corrosion resistance being observed for the base metal. Possible correlations between the microstructural changes and the pitting corrosion resistance of each region are discussed.