Mechanism of Localized Corrosion and Phase Transformation of Tube-to-Tube Sheet Welds of Hyper Duplex Stainless Steel in Acidified Chloride Environments

Abstract

The mechanism of localized corrosion and phase transformation of tube-to-tube sheet welds of hyper duplex stainless steel (HDSS) were investigated in acidified chloride environments. The HDSS tube-to-tube sheet welded with an Ar shielding gas without a filler metal, which increases the ferrite content, Cr 2 N and pitting resistance equivalent number (PREN) difference between the two phases, demonstrated the lower corrosion resistance than that with Ar shielding gas with a filler metal. The pitting corrosion of the weld metal (WM) in the HDSS tube-to-tube sheet welded using a pure Ar shielding gas with a filler metal occurred at the dendrite core (DC) that Cr and Mo were depleted and at the region adjacent to Mu phase that Mo and W were depleted. The localized corrosion was selectively initiated at the ¡-phase because the PREN value of the ¡-phase was much smaller than that of the £-phase, irrespective of the chemical compositions of the shielding, and the areas of the WM, heat affected zone (HAZ) and base metal (BM) (except for the WM in the HDSS tube-to-tube sheet welded using Ar with a filler metal).

Correlative EBSD and SKPFM characterisation of microstructure development to assist determination of corrosion propensity in grade 2205 duplex stainless steel

“…The formation of Cr-nitrides is usually associated with increased ferrite contents in duplex stainless steels [11,36,61,62], and large fractions of nitrides have, therefore, been observed in welds and heat-affected zones [11,34]. The formation of Cr 2 N has been reported to either occur during cooling, when ferrite is supersaturated with nitrogen leading to the formation of elongated and discretely-shaped intra-granular precipitates with 0001 h i Cr 2 N 011 d h i k orientation relationship, or during isothermal ageing in the 700-900°C temperature range where inter-granular Cr 2 N precipitates are formed on dd-and dc-boundaries [16].…”

“…Both Cr-nitrides are typically enriched in Cr, N, Fe, and Mo, but Cr 2 N contains more Cr than CrN, whilst the opposite holds true for the N content [36,62]. The extent of elemental depletion zones around these precipitates usually develops as a function of element enrichment within the precipitate and the volumetric size.…”

Microbiologically Influenced Corrosion of 2707 Hyper-Duplex Stainless Steel by Marine Pseudomonas aeruginosa Biofilm

“…The corrosion resistance of DSS is determined by the ratio of the α-phase and γ-phase and by the Cr, Mo, and W depleted regions that are adjacent to the secondary phases 8 9 10 . HDSS contains high levels of Cr, Mo and N 11 , resulting in its excellent corrosion resistance and a high value (45–50) of pitting resistance equivalent number (PREN), which is calculated from wt.% Cr + 3.3 (wt.% Mo + 0.5 wt.% W) + 16 wt.% N 12 .…”

Nitrogen Addition to the Shielding Gas for Welding Hyper-duplex Stainless Steel

“…A statistical analysis using Student's t-test, with 95% confidence, showed that there was no variation between the BM and WM regions. The homogeneity in the microhardness values is another indicator of the absence of any deleterious second phase precipitation [12]. The plot in Figure 7 shows the austenite volume fractions obtained for each of the welded joints conditions and the BM.…”

“…Welding processes usually involve sufficient heat input to cause microstructural changes in steels which are rich in alloying elements. Kim et al [12] performed GTAW welding tests with Ar shielding gas in a hyper-duplex steel getting equal volume fractions of ferrite and austenite for welding with a filler metal, however the joint without a filler metal, presented 80% of ferrite, thus exhibiting a low corrosion resistance. In a more recent study [13], the authors have investigated the possibility to improve the weld joint without filler metal by the addition of 2% N2 in the shielding gas.…”