Intergranular Oxidation Research Articles

Unraveling the SCC behavior and enhanced creep strength mechanism of AFA alloy in supercritical CO2

The oxide scale of 20Cr25Ni2.5Al AFA alloy formed under the coupling of stress and supercritical carbon dioxide (sCO2, 650 ℃ / 15MPa) consisted of Fe3O4, Cr2O3, preferential intergranular oxidation zones, but lacked a thin Al2O3 protective layer. The poor oxidation resistance resulted in significant CO2 penetration into the matrix and subsequent formation of Cr2O3, thereby hindering the initiation of Cr - rich σ phase beneath the oxide scale. This competition between oxidation and precipitation effectively reduced crack initiation and delayed the connection between SCC and creep cracks, thus significantly prolonging the creep lifetime in sCO2 compared to that in air.

Understanding preferential intergranular oxidation in alloy X-750 in 480 °C CO-CO2 environments at the nanoscale

Alloy X-750 was exposed to a 480 °C CO-CO2 reducing gas mixture at an oxygen partial pressure below the NiO dissociation pressure. Post-exposure analytical transmission electron microscopy (ATEM) analysis confirmed the occurrence of preferential intergranular oxidation (PIO) and internal oxidation which resulted in the formation of Ni nodules. Thus, reducing CO-CO2 mixtures can promote PIO in Alloy X-750, similar to Alloy 600 exposed to similar oxygen partial pressure but in H2-steam environment. In Alloy X-750, intergranular oxidation of Ti and Al were observed to occur prior to Cr oxidation, while the presence of intergranular Cr carbides hinders PIO penetration depth.

Tensile and cracking behavior of thin-walled 316Ti austenitic steel after exposure to 550 °C lead-bismuth eutectic with different oxygen concentration

The corrosion evolution of 316Ti steel in 550°C lead-bismuth eutectic (LBE) with oxygen concentrations (Co) of 5×10-6 wt.% and 5×10-8 wt.% and their impact on the tensile behavior were investigated in this study. The steel primarily underwent oxidation at Co of 5×10-6 wt.%, slightly influencing the tensile strength and elongation of specimen within 1000 hours exposure. In contrast, the structural integrity and tensile strength of specimen were significantly deteriorated at Co of 5×10-8 wt.% where the steel mainly experienced dissolution corrosion. The cracking behaviors of oxide layer and ferrite layer, and their influences on the failure of thin-walled specimen were discussed. Special concerns should be raised on the detrimental effect of localized corrosion, such as intergranular oxidation, LBE-wetted precipitate bands and dissolution pits, since cracks are expected to initiate easily in these zones and propagate into steel matrix under tension, potentially leading to the premature failure of thin-walled components.

Sealing Ni-Cr/Ni-Al alloys with borosilicate glass: Bonding strength, sealing interface, and fracture behavior

Encapsulation of the cold junction of sheathed thermocouples by incorporating glass-to-metal seal, which substitutes for the conventionally used organic sealants, is expected to improve the reliability of thermocouples. In this study, the sealing of borosilicate glass with thermocouple wire materials, i.e., Ni-Cr and Ni-Al alloys, was investigated. Three types of specimens were prepared for both alloys: nonoxidized, mildly preoxidized, and deeply preoxidized. High-temperature sealing with borosilicate glass was conducted for each specimen. The results revealed that grain boundaries were preferentially oxidized in both alloys and that preoxidation significantly increased the bonding strength between the alloys and borosilicate glass. However, the preoxidation conditions corresponding to the optimal bonding strength differed between the two alloys, which necessitates tailored pretreatment of the metal surface. For the Ni-Cr alloy, the sealing interfaces indicated that the deeply preoxidized specimens achieved the optimal bonding strength because of the synergistic effect of the glass saturation mechanism and mechanical interlocking mechanism, whereas the mildly preoxidized specimens were affected by unexpected chemical reactions and the consequent microscopic defects at the sealing interface. For the Ni-Al alloy, the deeply preoxidized specimens exhibited inferior bonding strengths compared with the mildly preoxidized specimens, which was attributed to their overmature intergranular oxidation, in which the grain boundaries were eroded by molten glass during sealing and trapped the glass after cooling. The glass in the grain boundaries was stressed by the cooling contraction of the metals and became prone to cracking. This study reveals the microscopic bonding mechanisms for the sealing of glass to Ni-based alloys and the causes of failure, providing a new perspective on the regulation and optimization of the sealing interface.

Determining the microstructure effects on the stress corrosion cracking initiation behavior of laser powder-bed-fusion printed 304L stainless steel in high-temperature hydrogenated water

This study investigated the microstructure effects on the stress corrosion cracking (SCC) initiation behavior of laser powder-bed-fusion (L-PBF) printed 304 L stainless steel in high-temperature hydrogenated water. The dislocation cells facilitate Cr transportation, thereby mitigating intergranular oxidation and SCC initiation. Compared to the warm-rolled dislocation cells, as-printed dislocation cells result in reduced strain localization, which is beneficial for suppressing SCC initiation. This is because the as-printed dislocation cells are stable and composed of abundant screw dislocations. Micro-inclusions can lead to nodular corrosion, thus increasing the depth of intergranular oxidation and promoting SCC initiation.

Nano-micrometer scale characterization of PWSCC crack tips in the transition zone of 52M overlay and the implication to intergranular cracking

This study examines the microstructural characteristics of the primary water stress corrosion cracking (PWSCC) crack tip in the transition zone (TZ) of the 52 M overlay and its implications for intergranular cracking. In the TZ, the Cr content at grain boundaries near the fusion boundary (FB) is lower than those farther from the FB. Additionally, residual strain at grain boundaries near the FB is higher than that farther away, with the fine-grained zone around the FB showing higher local deformation than the surrounding columnar grains. Stress corrosion cracking (SCC) crack growth rate (CGR) test results indicate that the TZ showed certain SCC sensitivity, and the closer to the FB, the lower the SCC resistance. Analysis of the SCC crack tip found that the distinctive composition distribution of the 52 M overlay TZ, characterized by low Cr and high Fe near the FB, is prone to intergranular oxidation, thereby reducing SCC resistance. Conversely, higher Cr and lower Fe content at grain boundaries in the TZ farther from the FB form dense, Cr-rich oxides ahead of the crack tip that slow SCC crack growth, resulting in the diffusion of oxidation along the dislocation structure into the grains and forming a fibrous oxidation zone.

Assessing the influence of welding-induced mechanics on oxidation and stress corrosion cracking in an Alloy 600-Alloy 152 M weldment under simulated PWR primary water

The impact of welding-induced mechanics on the oxidation and stress corrosion cracking (SCC) behavior of Alloy 600 in a pressurized water reactor (PWR) primary water was investigated using an Alloy 600-Alloy 152 M weldment. The microstructural analysis found a 0.05 mm-wide composition transition zone from welding. The Alloy 600 heat-affected zone (HAZ) grain size matches the Alloy 600 base (600B). Deformation hardening initially decreases and stabilizes from the fusion boundary (FB) to the 600B Comparing the microstructural characteristics of the specimen from the HAZ at about 0.5 mm from the FB with specimen 600B shows identical composition, but the former exhibits higher Vickers hardness (HV), kernel average misorientation (KAM), dislocation density, and residual stresses. Results from both short- and long-term oxidation tests indicate that the specimen from the HAZ exhibits a greater thickness of the inner oxide film, a higher number of local oxidation sites, and an increased maximum depth of intergranular (IG) oxidation compared to the 600B specimen. Multiple sets of SCC test results demonstrate that the crack growth rate (CGR) in the specimens from the HAZ is higher than that in the base, with the former showing longer IG cracks and a greater proportion of IG cracking.

Influence of dissolved hydrogen and IG Cr carbide on the oxidation behavior of Alloy 600 in hydrogenated primary water

The internal oxidation (IO) and preferential intergranular oxidation (PIO) behavior of Alloy 600 depending on the dissolved hydrogen (DH) content and the IG Cr carbide in hydrogenated primary water were characterized in detail using analytical electron microscopy techniques. The oxidation layer was unstable when the DH concentration was such that Ni was in the vicinity of Ni/NiO equilibrium and it could easily be peeled off. Hence, the grain boundaries of the bare metal were attacked. PIO occurred and Cr-rich oxide identified as Cr2O3 was formed at the oxidized grain boundary. NiO emerged when the DH concentration was such that Ni was in an oxidizing state, whereas Ni enrichment occurred inside the oxidized grain boundary when the DH concentration was such that Ni was in a reducing state with respective to Ni/NiO equilibrium. The IG Cr carbide strongly affected the PIO behavior by means of the consumption of oxygen penetrating into the grain boundary. The depth of the IO layer decreased as the DH concentration increased. The different oxidation behaviors depending on the DH content and IG Cr carbide are believed to affect the PWSCC resistance of Alloy 600 significantly.

Oxidation mechanism and microstructure evolution of invar alloy with high temperature annealing process

The high temperature oxidation behavior of Fe–36Ni Invar alloy in the forged billet stage and hot rolled rod stage in the compressed air atmosphere has been systematically studied. The results show that the oxide layer can be divided into the external oxide layer composed of Fe2O3, Fe3O4 and NiO, and the inner transition oxide layer composed of FeO, Fe2O3, Fe3O4 and NiO. In addition, hot rolled rod samples treated by solution and rolling have higher oxidation resistance. The mechanism of O diffusion during oxidation is analyzed, and it is found that the O atom adsorbed on the surface of the substrate diffuses into the grain along the grain boundary. The grain boundary is the optimal path for O diffusion to the interior of the material, which results in the degree of intergranular oxidation of the alloy being greater than the degree of intragranular oxidation. In summary, the oxidation resistance of Invar alloy can be improved by proper solution treatment and avoiding the formation of grain boundary cracks during production.

Corrosion of FeCrAl-1 Nb alloy in deaerated water at 290 ℃ and 7 MPa: The oxidation behavior of grain boundary

An investigation into the grain boundary corrosion behavior of a FeCrAl-1 Nb alloy at 290 ℃ and 7 MPa deaerated solution comprising lithium hydroxide and boric acid was conducted. After 745 hours of corrosion exposure, observations revealed the existence of voids along grain boundary, and preferential oxidation with the transformation from spinel oxide to amorphous oxide at the grain boundary was detected. 18O corrosion labeling results suggested that the corrosion of the alloy exhibited uniformity within a brief timeframe. Further experiments are necessary to investigate the potential impact of intergranular voids and oxidation on long-term corrosion.

Achieving excellent oxidation resistance in a Mg alloy by grain boundary adhesion and in-situ phase transformation

The oxidation resistance of Mg–Zn–Zr (ZK60) alloys with the addition of minor rare-earth element (RE) oxides (CeO2, La2O3) was investigated in 420 °C in air and pure O2 environments. It was revealed that the oxidation behavior of ZK60, ZK60-La2O3 and ZK60-CeO2 is different. In comparison to typical intergranular oxidation in ZK60 alloy, owing to diverse phase transformations between La and Mg, the La–Mg phase is continuously distributed in the grain boundaries, and furthermore, a dense and protective layer consisting La–Mg intermetallic phase is coated on the surface, which can effectively inhibit the oxidation of the Mg substrate, exhibiting excellent oxidation resistance. However, the presence of discontinuously distributed Ce-rich phases at the grain boundaries and the chain reaction during high-temperature exposure are the reasons for the deterioration of the oxidation resistance in ZK60-CeO2. It is anticipated that our findings will inspire the development of next-generation low-cost and excellent high-temperature performance Mg alloys.

Insights into the fatigue damage mechanism of T91 steel in liquid lead-bismuth eutectic at 150–550 °C

The corrosion fatigue damage mechanism of T91 steel in liquid lead-bismuth eutectic (LBE) at 150–550 °C was investigated. The results showed that the corrosion behavior is dependent on temperature. The properties of the oxide scale may affect the fatigue crack initiation and embrittlement susceptibility. The crack propagated along the prior or deformed sub- grain boundaries (GBs) at 150 °C and 350 °C, which was characterized by quasi-cleavage cracking. The intergranular oxidation and Pb/Bi permeation into the deformed GBs significantly degraded the fatigue properties at 550 °C. The temperature dependence of fatigue damage mechanism was discussed.

The enhanced high-temperature oxidation resistance of additively manufactured GH4169 by adding small amounts of 304L

Enhancing the high-temperature oxidation resistance of nickel-based alloys is critical for expanding their industrial applications. Herein, we report a novel strategy for enhancing the high-temperature oxidation resistance of the GH4169 nickel-based alloy by adding small amounts of 304L stainless steel using selective laser melting technology. The oxidation property of GH4169 with various 304L additions (2 wt.%, 4 wt.%, and 6 wt.%) at 900 °C for 45 h was investigated. Results show that the addition of 304L significantly improves the oxidation resistance of GH4169. The thickness of the oxide film and the intergranular oxidation depth of GH4169 with 6 wt.% 304L added are reduced by 61% and 69%, respectively. The addition of 304L increases Cr and decreases Ni in the matrix, which facilitates the formation of a more compact oxide film. The Nb in Laves phase will be increased with 304L addition, which effectively inhibits oxygen penetration and metallic element diffusion.

Preferential intergranular oxidation as a potential degradation mechanism for Alloy X-750 CANDU spacers

Oxidation experiments on Alloy X-750 were performed in reducing CO-CO2 mixtures to assess the possibility of preferential intergranular oxidation (PIO) in relation to CANDU garter-spring spacers. The testing environments simulate hypothetical off-chemistry scenarios, promoting the oxidation of the alloying elements, but not Ni. Surface and cross-section microscopy corroborated by Auger electron spectroscopy confirmed the occurrence of internal and intergranular oxidation in Alloy X-750, accompanied by Ni nodule expulsion to the surface. Results are qualitatively similar to observations in Alloy 600 exposed to 480°C hydrogenated steam, used as an accelerated environment for primary water stress corrosion cracking (PWSCC) testing.

Evading the intermediate-temperature embrittlement of L12-strengthened high entropy alloys by alleviating intergranular oxidation

L12-strengthened high entropy alloys (HEAs) usually suffer from severe intermediate-temperature embrittlement. In this study, we revealed that the intermediate-temperature embrittlement of a (Ni2Co2FeCr)92Nb4Al4 L12-strengthened HEA originates from the severe oxidation of intergranular D019 precipitates which weakens the grain-boundary cohesion. With the minor addition of 1.5 at% Si, we successfully achieved a brittle-to-ductile transition in the alloy via alleviating intergranular oxidation. The alloyed Si preferentially partitioned into the intergranular precipitates, significantly improved their oxidation resistance and pinning effects on the grain boundaries, which effectively mitigated environmentally assisted grain-boundary damage and thus suppressed the crack propagation upon deformation.

High-Temperature Steam and Atmospheric Oxidation Characteristic of a Heat-Resistant SP2215 Steel

The high-temperature oxidation performance of SP2215 has become an important issue when they were used as superheaters and reheaters exposed to two different high-temperature environments. In this study, the oxidation behavior of SP2215 steel was investigated under steam and an atmosphere of 650–800 °C for 240 h. The microstructural and chemical characterization of the samples were performed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), a glow discharge optical emission spectrometer (GD-OES), and atomic force microscope (AFM). The kinetic curves of oxidation revealed excellent oxidation resistance under both environments, but significant different oxidation characteristics, oxide film composition, and structure were obvious. In the steam experiment, selective intergranular oxidation was evident at relatively low temperatures, which was attributed to the preference absorption of supercritical water molecules at the grain boundary. Conversely, a double-layer structure of outer Fe2O3 and a small amount of Fe3O4 and inner Cr2O3 was formed uniformly at 800 °C. In the high-temperature atmosphere experiment, a protective chromium film was dominant at 650–700 °C, and a loose multicomponent oxide film was formed at 750–800 °C, primarily consisting of Cr2O3, spinel FeCr2O4, and CuO.

Intergranular oxidation behaviour of Ni-Cr-Al model alloys in 480 °C hydrogenated steam

The oxidation of Ni-Cr-Al model alloys was investigated after exposure to a 480 °C hydrogenated steam environment. Low angle grain boundaries (LAGBs) and the studied coincidence site lattice grain boundaries (CSLBs) showed intergranular oxidation, while high angle grain boundaries (HAGBs) were protected by an external oxide. The intergranular oxide consisted of a central Al-rich oxide, surrounded by a Cr-rich oxide. Al contribution in intergranular and external oxidation was enhanced in Ni-Cr-Al alloys compared to Ni-Al alloys. Thus, the third element effect is applicable at low homologous temperature along short-circuit diffusion pathways, in this case grain boundaries.

The effects of grain boundary structure and bulk Cr content on grain boundary degradation behavior of Ni-Cr binary alloys in high temperature CO2

The effects of grain boundary (GB) structure and bulk Cr content (from 15 wt.% to 30 wt.%) on GB oxidation behavior of Ni-Cr binary alloys were investigated in 650°C/1atm CO2 gas. The degradation behaviors of GBs, which varies greatly (including preferential intergranular oxidation, general intergranular oxidation and GB passivation), is dictated by the available Cr source which depends on the bulk Cr content and the Cr diffusion flux to the reaction front. The Cr diffusivity along GB is dependent on GB structure and decreases in the order of random high angle GB (RHAB), low angle GB (LAGB), incoherent twin boundary (ITB) and coherent twin boundary (CTB). Interestingly, other coincident site lattice boundary (CSLB (Σ>3)) shows a Cr diffusivity between ITB and CTB. More importantly, the threshold bulk Cr content for achieving GB passivation is also closely related to GB structure. To be specific, LAGB completely passivates when the bulk Cr content is between 20 wt.% and 25 wt.%, while ITB completely passivates at a bulk Cr content of 25 wt.% and CSLB (Σ>3) completely passivates when the bulk Cr content is between 25 wt.% and 30 wt.%. In contrast, CTB is subject to general intergranular oxidation while RHAB shows passivation regardless of the bulk Cr content.

New Insights into Internal Oxidation of Alloy 690 and Model Alloys in Hydrogenated Steam

The oxidation behavior of Alloy 690 and Ni-xCr-10Fe model alloys exposed to 480°C hydrogenated steam, in the nickel metal stability region, was studied using a variety of techniques, including time-resolved scanning electron microscopy, transmission electron microscopy, time-of-flight secondary ion mass spectrometry, and electron energy loss spectroscopy. The alloy underwent internal oxidation intragranularly, resulting in the expulsion of metallic Ni nodules to the surface. A compact Cr-rich “healing” layer developed over time at the intragranular internal oxidation front near grain boundaries and significantly retards further internal oxidation. Protective external Cr-rich oxide formed at and very near the grain boundaries, hindering intergranular oxidation. In view of the fact that the protective oxide forms micrometers away from the grain boundaries, one cannot simply say that Cr diffuses up the boundary and forms an oxide that spreads laterally—internal stress relief must play a role, just as it does within the grains, only with different mechanisms at play.

Microstructural and micromechanical characterization of intergranular oxidation in Fe-15Cr alloy

Fe-Cr alloy has great potential as an interconnect material for SOFCs at temperatures up to 800 °C, with Cr providing excellent oxidation resistance through the formation of a protective chromium oxide layer. However, internal brittle oxides formed at grain boundaries (GBs) can lead to severe cracking and cause material failure. Here, we report the formation of internal GBs oxide in an Fe-15 wt%Cr alloy. We carried out microstructural and chemical characterization of the intergranular oxide and performed micromechanical testing and FEM simulation to study the fracture behavior at the GBs. Our findings provide insights for improving the oxidation resistance in the material design.