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I am a PhD Researcher that focuses on characterising the microstructural behaviour of superalloys.

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Discover the ongoing research to reduce the effects of corrosion on duplex stainless steels!

In 10 seconds? The requirement of the oil and gas industry for metallic materials that can withstand more extreme conditions - i.e. more corrosive and greater pressures - lead to the development of the duplex stainless steel family (duplex, lean, super, and hyper).

All four developments are susceptible to corrosion. However, the greater concern is pitting corrosion, which can result in catastrophic failure if it goes unrecognised. Pitting corrosion is a very challenging issue for material scientists because this particular form of localised corrosion is very difficult to predict. Engineers would much rather a material to suffer from general corrosion because for one, it is easier to detect.

Don't believe it? Read the pinned articles to understand the severity of pitting corrosion and methods of reducing the likelihood that a duplex stainless steel will suffer from this failure mechanism.

What is duplex stainless steel? Normally, stainless steels contain one phase within its microstructure i.e. ferritic, austenitic, martensitic etc. However, duplex stainless steel contain two phases as its name suggests – austenite and ferrite. The phase composition of austenite:ferrite is commonly 50:50; however this composition can be altered depending on the materials application. Finally, similar to all stainless steels it possesses a passive film.

How does pitting corrosion occur? For localised pitting corrosion to occur in the duplex stainless steel, the protective passive film will have to have been damaged (chemically or mechanically) or possibly the film has a slight imperfection from manufacture process. The severity of these small pits can be unknown because the formation of these pits can take many different forms - it is possible for a cavity to stretch along under the passive film.

8 ITEMS PINNED

Influence of microstructure and elemental partitioning on pitting corrosion resistance of duplex stainless steel welding joints

Abstract: The influences of microstructure and elemental partitioning on pitting corrosion resistance of duplex stainless steel joints welded by gas tungsten arc welding (GTAW) and flux-cored arc welding (FCAW) with different shielding gas compositions were studied by optical microscopy, electron backscatter diffraction, scanning electron microscopy, transmission electron microscopy, energy dispersive spectroscopy, electron probe microanalysis, and potentiostatic and potentiodynamic polarization methods The adding 2% N2 in shielding gas facilitated primary austenite formation in GTAW weld metal (WM) and suppressed Cr2N precipitation in GTAW weld root. In the HAZ, the banded microstructure disappeared while the coarse ferrite grains maintained same orientation as the banded ferrite in the BM. In the WM, the ferrite had one single orientation throughout a grain, whereas several families of austenite appeared. The austenite both in BM and WM enriched in Ni and nitrogen, while Cr and Mo were concentrated in the ferrite and thus no element showed clear dendritic distribution in the WM (ER2209 and E2209T1). In addition, the secondary austenite had higher Ni content but lower Cr and Mo content than the primary austenite. The N2-supplemented shielding gas promoted nitrogen solid-solution in the primary and secondary austenite. Furthermore, the secondary austenite had relatively lower pitting resistance equivalent number (PREN) than the ferrite and primary austenite, thereby resulting in its preferential corrosion. The Cr2N precipitation led to relatively poor resistance to pitting corrosion in three HAZs and pure Ar shielding GTAW weld root. The N2-supplemented shielding gas improved pitting corrosion resistance of GTAW joint by increasing PREN of secondary austenite and suppressing Cr2N precipitation. In addition, the FCAW WM had much poorer resistance to pitting corrosion than the GTAW WM due to many O-Ti-Si-Mn inclusions. In the BM, since the austenite with lower PREN compared to the ferrite, the pitting corrosion occurred at the ferrite and austenite interface or within the austenite.

Pub.: 14 Oct '16, Pinned: 19 Apr '17