Microstructure and Corrosion Studies on Different Zones of Super Duplex Stainless Steel UNS S32750 Weldment

Abstract

The influences of microstructure and elemental distribution on pitting corrosion resistance and passive film in different regions of a super duplex stainless steel welded joint were investigated. The banded microstructure disappeared after welding and transformed to the coarse equiaxed ferrite grains in the heat-affected zone. Three types of austenitic microstructure formed in the weld center, only two types occurred in the weld cap and weld root. It is found that the elements Ni and N were enriched in austenite, while Cr and Mo were concentrated in the ferrite. The Cr 2 N was found precipitating in some specific regions with a lower austenite content, which led to the relatively poorer resistance to pitting corrosion in the heat-affected zone, weld root, and weld cap. The ferrite observed in each studied region of the joint was prone to experiencing selective corrosion due to its lower pitting resistance equivalent number. The content of Cr in the region around the Cr 2 N disappeared, so that Cr-depletion was formed as the original location of pitting. The electrochemical impedance spectroscopy results also showed that no difference exists in the passive film structure in all of the regions of the joint, and regular banded microstructure was helpful to the passive film.

Study of the Passivation Film on S32750 Super-Duplex Stainless Steel Exposed in a Simulated Marine Atmosphere

“…The passivity and corrosion of ferritic and austenitic stainless steels are highly interesting and widely studied [11][12][13][14][15]. Raman measurement confirms the presence of the austenite and ferrite phases on the passivation layer created on the surface of 2205 duplex stainless steel [13].…”

“…In chloride solution, pitting potential increased and passive current density decreased with the addition of HCO 3 − and SO 4 2− [14]. The passive film structure of UNS S32750 was a single layer, the microstructure of austenite contents and morphologies had no influence on the passive film structure [15]. The XPS analyser has become popular for studying passive films.…”

Microstructural Evolution and Its Influence on Low‐Temperature Toughness of Simulated Heat‐Affected Zone in Super Duplex Stainless Steel Welded Joint

“…In addition, the Cr 2 N precipitation forms a Cr-depleted zone in the ferrite surrounding, which ultimately reduces resistance to localized corrosion. [10,11] Furthermore, when the HAZ in this metastable condition is reheated, as in multipass welding, the most apparent change is secondary austenite (γ 2 ) formation. [12][13][14] During the ferritization process, the austenite formed by the solid-state phase transition of ferrite (δ!…”

“…In addition, the Cr 2 N precipitation forms a Cr‐depleted zone in the ferrite surrounding, which ultimately reduces resistance to localized corrosion. [ 10,11 ] Furthermore, when the HAZ in this metastable condition is reheated, as in multipass welding, the most apparent change is secondary austenite (γ 2 ) formation. [ 12–14 ] During the ferritization process, the austenite formed by the solid‐state phase transition of ferrite (δ→ γ 1 ) or directly solidified from the molten metal during solidification (L → γ 1 and L + δ → γ 1 ) is called as primary austenite (γ 1 ), while the austenite nucleated and grew by subsequent reheating action is usually called as γ 2 .…”