Effect of Sensitization Time on Intergranular Corrosion of 347H Stainless Steel

In this paper, 347H stainless steel of three situation of solution, primary and secondary stabilizing with 0, 12, 24, 48 and 72 h accelerated sensitization heat treatment. The change of sensitization degree with time was studied by metallographic test, double-loop electrochemical potentiodynamic reactivation (DL-EPR) test, intergranular corrosion test and microscopic morphology observation. The result shows that the sensitization degree of the solution material increases rapidly and reaches the maximum value after 12h sensitization heat treatment. After that, it still belonged to severe sensitization situation, but the index gradually decreased. After stabilizing heat treatment, the sensitization degree of the material is lower than the situation of solution. After heat treatment for 48h, the material located on “possible sensitization” range, and the sensitization degree of the secondary stabilized material was always lower than is of the primary. It indicates that the sensitization of materials can not be completely inhibited by stabilizing heat treatment, and other anti-corrosion measures should be considered.

Effects of Stabilizing Heat Treatment on Properties of Simulated Microstructures in the Heat-Affected Zone of 347H Stainless Steel

In this paper, two kinds of heat affected zone (HAZ) simulation structures of 347H stainless steel, which are coarse grain zone (CGZ) and unmixed zone (UZ), were prepared by thermal simulator. The material properties of toughness, reheat crack susceptibility and intergranular corrosion susceptibility of the two kinds of HAZ simulation structures were studied by impact test, high temperature tensile test, electrochemical potentiodynamic reactivation (EPR) test and micro morphology test. The result shows that CGHAZ had better toughness. But after the stabilizing heat treatment, it was weakened while that of the UZ was enhanced. The reheat crack susceptibility of the CGZ and UZ both increases after stabilization heat treatment, and the tendency of the UZ are more obvious. Stabilizing heat treatment has a certain effect on the prevention of sensitization process, which can improve the intergranular corrosion resistance of the material. Stabilizing heat treatment is double-edged to 347H HAZ, and it needs combined with the specific situation to used.

Synthetic BiOBr/Bi2S3/CdS Crystalline Material and Its Degradation of Dye under Visible Light

Constructing heterojunction has attracted widespread concerns in photocatalysis research. BiOBr/Bi2S3/CdS composite material with a sea urchin shape was directly obtained by first synthesizing BiOBr microspheres. The morphology, structure and composition of the composite material were characterized by XRD, EDX, SEM and XPS. Dye degradation experiments showed that 83.3% of methylene blue removal was achieved after 2 h of visible light irradiation. The reaction rate under optimal conditions was 0.014 min−1 and the photocatalytic degradation process follows a pseudo-first-order kinetic model. Based on the EPR test results, the main active species involved in the reaction were •O2− and h+. The conduction band and valence band edge potential calculations confirmed the key role of CdS in the production of •O2−.

Friction Stir Welding: Mitigating the Susceptibility to Intergranular Corrosion of Alloy 625

In this work, friction stir welding (FSW) was employed to alloy 625 grade I (soft annealed) sheets. Therefore, solid-state based welding was undertaken with a tool rotational speed of 200 rpm and welding speed of 1 mm/s. Microstructural features were analyzed by light optical and scanning electron microscopy (SEM). Moreover, microhardness measurements were performed. The susceptibility to intergranular corrosion was verified by the double-loop electrochemical potentiokinetic reactivation (DL-EPR) test. Complementary, intergranular corrosion was evaluated by ASTM G28 Method A. FSW promoted grain refinement, increased microhardness, and reduction in the degree of sensitization. Finally, the mean corrosion rate observed in the ASTM G28A test was 0.4406 mm/year, which suggests a good weld quality.

Interlaboratory Testing to Assess Repeatability and Reproducibility of the Double Loop Electrochemical Potentiokinetic Reactivation (DL-EPR) Tests for Type 304H Stainless Steels

The sensitization of stainless steels may decrease their corrosion resistance in industrial applications. Traditional immersion tests exist to determine the degree of sensitization (DOS) of the stainless steels. However, electrochemical methods may be preferred because they are less expensive and faster to perform. The fast and robust double loop electrochemical potentiokinetic reactivation (DL-EPR) test has been introduced to the corrosion community some decades ago but an interlaboratory testing study was necessary to assess the repeatability and reproducibility of the DOS results. This work reports on a recent study where 11 laboratories returned results that show a high degree of confidence in the data obtained by DL-EPR.

Evaluation of the DOS by DL−EPR of UNSM Processed Inconel 718

In this work, influence of ultrasonic nanocrystal surface modification (UNSM) on the degree of sensitization (DOS) in Inconel 718 has been studied and correlated with the resulting microstructure. The UNSM processed samples decreased their grain size from 11.9 µm to 7.75 µm, increasing the surface of grain boundaries, and thus enhancing the area where δ phase and niobium carbides precipitate. The effect of the UNSM process on the DOS of Inconel 718 was studied by the double loop electrochemical potentiokinetic reactivation (DL−EPR) test. The DL−EPR showed that for UNSM processed samples with no thermal treatment, the DOS increased up to 59.6%, while for UNSM treated samples that were post-annealed at 1000 °C for 10 min and water quenched the DOS decreased down to 40.9%. The increase of grain boundaries surface area and triple junctions after the UNSM process enables the formation of twice the amount of δ phase compared to the as-received Inconel 718 bulk sample. The area fraction of the grain boundary covered by δ phase was of 9.87% in the UNSM region while in the bulk the area fraction was 4.09%. In summary, it was found that after UNSM process, the annealing at 1000 °C for 10 min and water quenching promoted the transformation of γ″ to form δ phase on the grain boundaries, which reduces the intergranular corrosion susceptibility.

The Intergranular Corrosion Susceptibility of Metastable Austenitic Cr–Mn–Ni–N–Cu High-Strength Stainless Steel under Various Heat Treatments

The intergranular corrosion (IGC) behavior of a new metastable austenitic Cr–Mn–Ni–N–Cu high-strength stainless steel under various heat treatments was studied. The samples were solution treated at 1050 °C for 30 min and then aged at 600 to 900 °C for 10 to 300 min, respectively. The IGC susceptibility of aged samples was investigated using a double-loop electrochemical potentiokinetic reactivation (DL-EPR) test in a solution of 0.1 M H2SO4 and 0.002 M KSCN and the 10% oxalic acid etch. The surface morphologies of samples were characterized using optical microscopy and the scanning electron microscopy after electrochemical tests. Two time-temperature-sensitization diagrams were plotted based on the DL-EPR test and oxalic acid etching. No IGC and precipitate were observed for samples aged at 600 °C and 900 °C. For samples aged at 650 °C to 750 °C, the IGC susceptibility and the amount of precipitate both increased with the extended aging time. For samples aged at 800 °C and 850 °C, the amount of precipitate increased as the aging time was prolonged. However, only the sample aged at 800 °C for 60 min showed slight intergranular corrosion in the DL-EPR test. The IGC of the Cr–Mn–Ni–N–Cu austenitic stainless steel originated from the precipitation of Cr23C6 and Cr2N at the grain boundaries. The chromium-depleted zones near grain boundaries stood as the corrosion nucleation sites, but the dissolution of the weak area followed a consistent crystallographic orientation along each grain boundary.

Assessment of Thermal Aging of Austenitic Stainless Steel Weld Metal by Using the Double Loop Electrochemical Potentiokinetic Reactivation Technique

Double loop electrochemical potentiokinetic reactivation (DL-EPR) was applied to evaluate thermal aging of 308L stainless steel weld metal. It was found that the activation and reactivation peaks of DL-EPR curve were induced by dissolution of austenite and δ-ferrite, respectively. Before saturation of hardness, the linear relationship between reactivation ratio and hardness could be used for assessing the thermal aging-induced hardening. In the following thermal aging process, the reactivation ratio is applicable to assess the occurrence of the saturation of thermal aging-induced hardening. The results demonstrated that the DL-EPR test is applicable to assess the evolution of thermal aging.