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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.
Abstract: Influence of changes in microstructure caused due to welding on microbiologically influenced corrosion of a duplex stainless steel was studied by exposing the weldment and parent metal to chloride medium containing sulfate-reducing bacteria (SRB). Identically prepared coupons (same area and surface finish) exposed to sterile medium were used as the control. Etching-type attack was observed in the presence of SRB, which was predominant in the heat-affected zone (HAZ) of the weldment. The anodic polarization studies indicated an increase in current density for coupon exposed to SRB-containing medium as compared to that obtained for coupon exposed to sterile medium. The scanning electron microscopy (SEM) observations after anodic polarization revealed that the attack was preferentially in the ferrite phase of HAZ of the weldment, whereas it was restricted to the austenite phase of the parent metal.
Pub.: 11 Sep '08, Pinned: 19 Apr '17
Abstract: The influence of microstructural evolution on selective corrosion in duplex stainless steel flux-cored arc welding joint was investigated by the modified double loop electrochemical potentiokinetic reactivation in acidified chloride. Due to the lower pitting resistance equivalent number, the secondary austenite was preferentially attacked over the ferrite and primary austenite phases, and inclusions acted as nucleation sites for micro-pits in the weld metal. Localised corrosion was generated in the secondary austenite and Cr-depleted zone around the Cr2N and sigma phases in the heat-affected zone. The weld metal exhibited the smallest susceptibility to localised corrosion because of the least weak phases.
Pub.: 06 Mar '17, Pinned: 19 Apr '17
Abstract: Due to the difference in reheating effects depending on the heat input of subsequent weld passes, the microstructure of the weld metal varies between acicular type austenite and a mixture of polygonal type and grain boundary mixed austenite. These microstructural changes may affect the corrosion properties of duplex stainless steel welds. This result indicates that the pitting resistance of the weld can be strongly influenced by the morphology of the secondary austenite phase. In particular, the ferrite phase adjacent to the acicular type austenite phase shows a lower Pitting Resistance Equivalent (PRE) value of 25.3, due to its lower chromium and molybdenum contents, whereas the secondary austenite phase maintains a higher PRE value of more than 38. Therefore, it can be inferred that the pitting corrosion is mainly due to the formation of ferrite phase with a much lower PRE value.
Pub.: 20 Dec '12, Pinned: 19 Apr '17
Abstract: Microbiologically Influenced Corrosion (MIC) is a serious problem in many industries because it causes huge economic losses. Due to its excellent resistance to chemical corrosion, 2707 hyper duplex stainless steel (2707 HDSS) has been used in the marine environment. However, its resistance to MIC was not experimentally proven. In this study, the MIC behavior of 2707 HDSS caused by the marine aerobe Pseudomonas aeruginosa was investigated. Electrochemical analyses demonstrated a positive shift in the corrosion potential and an increase in the corrosion current density in the presence of the P. aeruginosa biofilm in the 2216E medium. X-ray photoelectron spectroscopy (XPS) analysis results showed a decrease in Cr content on the coupon surface beneath the biofilm. The pit imaging analysis showed that the P. aeruginosa biofilm caused a largest pit depth of 0.69 μm in 14 days of incubation. Although this was quite small, it indicated that 2707 HDSS was not completely immune to MIC by the P. aeruginosa biofilm.
Pub.: 06 Feb '16, Pinned: 19 Apr '17
Abstract: The effect of solution annealing temperature ranging from 950 to 1200 °C on the microstructure and corrosion performance of duplex stainless steel (DSS) 2204 were investigated. The proportion of the ferrite phase increased while the austenite phase decreased and the ferrite stabilizing elements diluted in the ferrite phase with the increase of annealing temperature. The critical pitting temperature (CPT) of specimens annealed at 1000 °C was higher than those annealed at 950 °C, whereas further increasing the annealing temperature to 1200 °C decreased the CPT. The pitting initiation sites were observed in the austenite phase, at the boundary of ferrite/austenite phase and inside the ferrite phase for specimens annealed at 950, 1000 °C and exceeding 1100 °C, respectively. The evolution trend of the CPT and the pit initiation site were analyzed by the pitting resistance equivalent number.
Pub.: 11 Apr '16, Pinned: 19 Apr '17
Abstract: This work focuses on the effect of welding parameters on corrosion behavior of welded duplex stainless steel (DSS) and super duplex stainless steel (SDSS). The effect of welding parameters, such as heat input, inter-pass temperature, cooling rate, shielding/back purging gas, on corrosion behavior was studied. DSS and SDSS pipes were welded with Gas Tungsten Arc Welding (GTAW) process. After welding, the test samples were non-destructively tested to ensure no defects and test samples were prepared for microstructural examinations and ferrite content measurements. The root region had complex microstructure because of the repetitive heating of the zone during different weld layers. It was observed that at low heat input desirable microstructure was formed. The test samples were subjected to corrosion tests, i.e. ASTM G48 test for the determination of pitting corrosion rate, potentiodynamic polarization tests, and potentiostatic tests to verify susceptibility of the alloys to corrosion attack. DSS weldments had CPT in between 23 °C to 27 °C and SDSS weldments had CPT between 37 °C to 41 °C in potentiostatic measurements. The corrosion test results were correlated to the microstructures of the weldments. The pitting resistance of individual phases was studied and the effect of secondary austenite on corrosion attack was also observed.
Pub.: 19 Feb '16, Pinned: 19 Apr '17
Abstract: This study aimed at evaluating the pitting corrosion of duplex stainless steel (2205) welded by filler metal (2209) after exposure to increased service temperatures. In this work selected welded samples were aged at different temperatures (650 °C, 850 °C, 950 °C and 1050 °C), in order to simulate heat exposure during processing or service stages. The results showed that the pitting corrosion rate increased in chloride environments with increasing aging temperature till 850 °C; afterward pitting corrosion rate started to decrease and the joints restored their original pitting resistance at 1050 °C; and after that increasing the aging to above 1050 °C, pitting resistance was believed to decrease again, as a result from decreasing γ/δ. A correlation between different welding processes and aging times was also conducted.
Pub.: 11 Oct '16, Pinned: 19 Apr '17
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
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