Can the drop evaporation test evaluate the stress corrosion cracking susceptibility of the welded joints of duplex and super duplex stainless steels?

Abstract

Duplex and super duplex stainless steels (DSS and SDSS) are susceptible to stress corrosion cracking (SCC) in marine environments. Influencing factors are their microstructure and the values of the service temperature and tensile stresses. Drop evaporation testing (DET) using synthetic seawater was used to assess the SCC susceptibility of DSS and SDSS welded joints at temperatures from 70 °C to 110 °C and tensile stresses from 70% to 100% of the yield stress. Three welding energy levels (0.5, 1.0 and 3.5 kJ/mm) were used to produce different microstructures of the heat affect zone (HAZ) and weld metal (WM) in terms of the ratio between austenite/ferrite phases and formation of deleterious phases. All specimens tested at 90 °C and 100 °C did not feature any evidence of SCC, nevertheless they showed localized and preferential corrosion of the ferrite phase in the base metal (away from the HAZ and WM). All specimens tested at 110 °C, however, were susceptible to SCC in the base metal (away from the HAZ and WM). The SCCs of the SDSS specimens propagated preferentially along the α/γ interfaces, while for DSS specimens the SCCs propagated preferentially in transgranular mode. Failure analysis methodology indicated that the conventional DET was not capable to assess the effect of thermal exposure on the SCC susceptibility of the welded joints as DET produced undesirable temperature and pH gradients along the surface of the SCC test pieces. Moreover, the formation of a volcano-like salt deposit on the surface of the specimens tested at 110 °C promoted crevice corrosion, leading to localized corrosion with preferential corrosion of the ferrite phase and the austenite/ferrite interfaces, and, finally, to the nucleation and growth of SCCs in the base metal (away from the dripping zone and welding joints). The susceptibility of the welded joints to SCC could not be assessed by the conventional DET.