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Pitting Corrosion Resistance Of UNS S32202 Welds: Bulk Microstructure Vs Surface Behaviour

Pitting Corrosion Resistance Of UNS S32202 Welds: Bulk Microstructure Vs Surface Behaviour
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Duplex stainless steels (DSS) are an attractive alternative to conventional austenitic 300 series. They are becoming more and more present in industrial applications requiring high mechanical properties combined with good corrosion resistance.  UNS S32202 is a lean duplex grade designed to guarantee corrosion resistance superior to that of 304L in most environments and even equivalent to 316L in NaCl environment at room temperature. Its yield strength is twice as high as 304L and 316L allowing thickness and weight reduction in structural components. With low nickel content (2.5%) and no molybdenum addition, the impact of raw material price fluctuation is reduced. It makes UNS S32202 suitable for a high number of applications including public transportation, building & construction, watersystems, liquid storage and pulp&paper industry. 

Product Number: 51322-17913-SG
Author: Veronica SCHEIBER, Pauline HUGUENIN, Jérôme BRIDEL, Florent KRAJCARZ, Vincent VIGNAL
Publication Date: 2022
Industries: Corrosion Monitoring and Control , Coatings
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Duplex stainless steel (DSS) provides a good alternative to austenitic stainless steel to build engineering designs thanks to excellent mechanical properties and corrosion resistance associated to their biphasic microstructure. Their microstructure changes during the welding. The influence of microstructure and elemental distribution on pitting corrosion resistance of different zones of the lean DSS UNS S32202 welded by Gas Tungsten Arc Welding (GTAW) was analyzed. The welded transversal cross section surface after polishing and the welded surface after pickling were studied. Comparing both surfaces, the welded surface is more resistant to pitting corrosion than welded transversal cross section surface, according to the electrical charge and the total mass loss. The High Temperature Heat Affected Zone (HTHAZ) surface after pickling has no pitting thanks to the presence of the superficial layer (DSS with a specific microstructure). However, the HTHAZ transversal cross section has the lowest pitting resistance. Its pit initiation sites could be associated to Cr2N. Pit initiation sites in the molten zone (MZ) could be linked to oxide particles and Cr2N for both surfaces. In all pits (in MZ and HTHAZ, for both surfaces) ferrite dissolves preferentially (austenite remains intact). Austenite is more resistant to pitting due to its higher nitrogen content than in ferrite.   

Duplex stainless steel (DSS) provides a good alternative to austenitic stainless steel to build engineering designs thanks to excellent mechanical properties and corrosion resistance associated to their biphasic microstructure. Their microstructure changes during the welding. The influence of microstructure and elemental distribution on pitting corrosion resistance of different zones of the lean DSS UNS S32202 welded by Gas Tungsten Arc Welding (GTAW) was analyzed. The welded transversal cross section surface after polishing and the welded surface after pickling were studied. Comparing both surfaces, the welded surface is more resistant to pitting corrosion than welded transversal cross section surface, according to the electrical charge and the total mass loss. The High Temperature Heat Affected Zone (HTHAZ) surface after pickling has no pitting thanks to the presence of the superficial layer (DSS with a specific microstructure). However, the HTHAZ transversal cross section has the lowest pitting resistance. Its pit initiation sites could be associated to Cr2N. Pit initiation sites in the molten zone (MZ) could be linked to oxide particles and Cr2N for both surfaces. In all pits (in MZ and HTHAZ, for both surfaces) ferrite dissolves preferentially (austenite remains intact). Austenite is more resistant to pitting due to its higher nitrogen content than in ferrite.   

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