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In this study, the relationship between spinodal decomposition and the formation of Ni-rich clusters and G-phase in the ferrite on hardening and pitting corrosion of two thermally aged duplex stainless steels (DSSs) at 475 °C was investigated. Results indicate that, for 2205 DSS, pitting corrosion behavior is influenced by the presence and size of G-phase precipitates for longer aging times, but this contribution is masked by the advanced stage of spinodal decomposition in the ferritic structure. On the other hand, for 2101 DSS, the formation of Cr-richer nitrides impairs pitting corrosion resistance more than spinodal decomposition.

Recent findings reveal that Cr nitride precipitation during lean duplex stainless steel (LDSS) 2101 aging at 475 °C is more detrimental to its corrosion resistance than Cr depletion caused by spinodal decomposition. Thus, the purpose of this study is to understand the role of Cr depletion caused both by evolution of spinodal decomposition and precipitation of other Cr-rich phases in pitting corrosion of LDSS 2404 aged at 475 °C. Our findings indicate that, although Cr carbide and nitride precipitation occur at 475 °C in LDSS 2404, the main cause of Cr depletion and loss of pitting resistance is spinodal decomposition.

The aim of this study concerns the effect of multi-pass shielded metal arc welding (SMAW) on the corrosion behavior of CW6MC cast nickel alloy. Using optical and SEM techniques the welded joint is analyzed. Vickers microhardness mapping and potentiodynamic polarization in NaCl and H2SO4 solutions are also evaluated. Both the Laves phase and NbC-type carbides are identified in the base metal (BM) and weld metal (WM) regions. The main microstructural difference observed between these regions is the morphology aspect and fineness of the dendritic arrays. The welding process promotes the finer columnar grains formation with refined intermetallic particles in the WM than equiaxed grains of the BM, which in turn results in higher microhardness values in the former region. However, no substantial changes were observed in the corrosion behavior between the BM and WM regions, considering both acid and saline media. Nevertheless, during the multi-pass SMAW process, some non-metallic micrometric inclusions (Mo and S-rich regions) can be constituted to occur in the WM region. This is associated with a significant drop in the corrosion performance of this region when the electrochemical tests are evaluated.

The microstructural, mechanical and corrosion behavior of the surface of an AA6181-T4 aluminum alloy overlap welded onto Ti-6Al-4V titanium alloy by friction spot welding (FSpW) was analyzed based on optical microscopy (OM), scanning electron microscopy (SEM), microhardness and potentiodynamic polarization techniques. The results indicated that the FSpW process modified the microstructure of the AA6181-T4 aluminum alloy, as indicated mainly by the size, type and quantity of precipitates in various welding regions. The microhardness tests showed similar hardness values in the stir zone (SZ) and base metal (BM), which was ascribed to the breakdown and homogenization of precipitates in the SZ. On the other hand, the heat affected zone (HAZ) showed the lowest hardness, which was attributed to the coalescence of Mg2Si precipitates caused by the thermal cycle of the welding process. The potentiodynamic polarization tests indicated that the SZ showed the highest pitting potential due to the refined microstructure in this zone. SEM images recorded after potentiodynamic polarization testing indicated that preferential sites for pitting nucleation were regions adjacent to the Al (Fe, Si, Mn, Mg) precipitates, and the mildest corrosive attack was found in the SZ. Keywords: Friction spot welding, FSpW, Dissimilar joint, aluminum alloys, Hardness, Pitting corrosion

The effect of topologically close-packed (TCP) phase formation on the electrochemical corrosion behavior of the UNS N26455 (Ni–16Cr–16Mo) superalloy was evaluated using different electrochemical techniques. We verified that this superalloy is resistant to highly corrosive environments but its corrosion potential decreases and the anodic current density increases as the TCP phase concentration increases in the microstructure. Its anodic behavior indicates that there is a selective dissolution process associated with this secondary phase, which is completely dissolved before repassivation occurs. Although the repassivation potential remains nearly unchanged, the degraded points configure defects that may negatively affect the material’s performance during service.

Among shape memory materials, iron based shape memory alloys are of special interest. Its relative low cost encourages mass scale production, such as concrete reinforcement. Metallurgical design of new compositions is abundant, and corrosion resistance must always be a concern. In the present work, the influence of cerium in the corrosion properties of a Fe-14Mn-4Si-9Cr-4Ni-xCe (x = 0.0, 0.18, 0.42, 0.96) was determined by anodic potentiodynamic polarization and EIS in simulated pore solution, with 0.6 M NaCl. Results revealed that cerium is responsible for an improvement in corrosion properties, although there is a threshold where this element begins to be detrimental.

Titanium alloys are suitable for construction of the Coriolis mass flow meter due to their high strength to weight ratio, high thermal stability and excellent corrosion resistance. However, environments with high hydrogen content can be very harmful to titanium and its alloys, since this element may cause hydride formation and leads to hydrogen embrittlement. In this context, this paper reports on the analysis of a grade 9 titanium tube of a mass flow meter, which was performed using cyclohexanol (68%), water (2%) and Raney nickel catalyst (30%). After the active period, the flow meter was deactivated and left on standby for one year, and then reassembled on a production line, whereupon it failed prematurely. The failure was analyzed by OM, SEM, EDS, Vickers microhardness and XRD, whose results indicated that the cause was hydride embrittlement due to the chemical interaction between Raney nickel and the titanium tube during the deactivated period.

The present study assessed the influence of refill friction stir spot welding (RFSSW) process on the microstructure and electrochemical corrosion behavior of the AA5754-H22 alloy surface of an AA5754-H22/Ti-6Al-4 V overlapped joint. The results demonstrate that the RFSSW process promotes substantial microstrutural changes along the welded joint, which, in turn, reflect directly on the mechanical properties and corrosion behavior of distinct welding regions. The potentiodynamic polarization tests and electrochemical impedance spectroscopy measurements indicate that the stir zone region has better corrosion resistance than the other RFSS welded regions [heat affected zone and base metal] due to higher homogeneity and microstructure refinement. The achieved results are evidence that the RFSSW process is an interesting alternative for the welding of overlapping dissimilar joints.

The relationship between alloy chemical composition and oxidation behavior of austenitic Fe-Mn-Si-Cr-Ni shape memory stainless steels (SMSSs) during exposure at 800 °C in air was investigated. Although high Si and Cr additions are beneficial for both the shape memory effect and oxidation resistance, the increase of these ferrite stabilizer elements in Fe-Mn-Si-Cr-Ni SMSSs makes the oxidation-induced ferrite stable at the early stages of the oxidation exposure, which in turn allows a faster diffusion of Mn, controlling the kinetics of oxidation. Our results suggested that the austenite stability should be maintained during the oxidation process for higher performance of these materials.