Cr-depleted Zone Research Articles

Pitting corrosion resistance of tempered N08825 alloy in the inner layer of bimetallic composite pipe

Pitting corrosion resistance of N08825 alloy in the inner layer of bimetallic composite pipe SA210A/N08825 tempered at 570∼720 ℃ was investigated in NH4Cl solution by scanning electron microscopy (SEM), X-ray diffraction (XRD), and electrochemical measurement techniques. The M23C6 precipitates, observed at the grain boundaries of inner layer alloy 825, exhibit a temperature-dependent behavior during tempering at 570∼720 ℃. The quantity of M23C6 precipitates and the pitting corrosion resistance change with the tempering temperature. When the tempering temperature increases from 570 to 660 ℃, the quantity of M23C6 precipitates increases, with the pitting potential (Ep) decreasing from about 499 ± 8.6–354 ± 6.7 mVSCE, the passive current density (ip) increases from about 1.9 ± 0.2–2.45 ± 0.3 μA cm−2, and the average number of pits on the specimen surface rises from 3 to 7. Conversely, when the tempering temperature increases from 660 to 720 ℃, the quantity of M23C6 precipitates decreases, leading to the Ep increases from about 354 ± 6.7–394 ± 14.6 mVSCE, the ip decreases from about 2.45 ± 0.3–2.34 ± 0.4 μA cm−2, and the average number of pits on the specimen surface decreases from 7 to 6. There is a strong relationship between the M23C6 precipitates and the pitting corrosion resistance. As the quantity of M23C6 precipitates increases, it results in the expansion of Cr-depleted zones on the specimen surface, the rise of passive film defects, and a subsequent reduction in the pitting corrosion resistance of inner layer alloy 825.

Effect of grain size on high-temperature corrosion performance of laser cladding inconel 625 coating

As the calorific value of waste-to-energy incineration continues to rise, the corrosion damage to boilers becomes increasingly severe. The impact of the grain size on the high-temperature corrosion resistance of Inconel 625 coating was investigated. The results indicated that the corrosion performance of the coating in 1:1 NaCl–KCl salts at 700 °C for 120 h was highly dependent on the grain size of the coating. The coating fabricated with a laser power of 1800 W and an overlapping distance of 1.5 mm had an average grain size of 13.7 μm and highest hardness, exhibiting the best corrosion performance, with a corrosion weight loss of 1.4 mg/cm2. Coatings processed under different conditions all formed a three-layered structure, with a NiO outermost layer, a Cr2O3 intermediate layer, and an innermost Cr-depletion zone. Coatings with finer grains formed a continuous and dense protective oxide layer of NiO and Cr2O3 due to the rapid diffusion of alloy elements, effectively preventing the invasion of corrosive media and protecting the substrate from corrosion.

The effect of tempering temperature on microstructure and corrosion resistance of M390 powder metallurgical martensitic stainless steel

The corrosion resistance of M390 powder metallurgical martensitic stainless steel with different tempering temperatures was investigated by potentiodynamic polarization measurements, salt spray tests, and microstructural analyses utilizing scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The tempering temperature had no significant effect on the size and volume fraction of carbides. The corrosion resistance of M390 steel gradually deteriorated with increasing tempering temperature, and a loss passivation (LOP) effect was observed when tempered at 450 °C, 500 °C, and 550 °C. Transmission electron microscopy (TEM) analysis showed that the width of the Cr-depleted zones around the undissolved M7C3 carbides increased with increasing tempering temperature, while the Cr content in these zones decreased, which was the main reason for the deterioration of corrosion resistance. This study offers valuable insights into optimizing the tempering process to improve the corrosion resistance of M390 steel for practical applications.

Electrochemical characterization of passive film and corrosion in Co-rich high entropy alloys in Ringer's solution

This study investigates the corrosion characteristics of Co30Cr20Ni20Fe20Mo10 (Co30HEA), Co40Cr20Ni15Fe15Mo10 (Co40HEA), and Co50Cr20Ni10Fe10Mo10 (Co50HEA), designed as Co-rich high entropy alloys, compared to 316 L stainless steel and L605. Co30HEA has a single FCC phase, whereas the other two alloys exhibit a dual phase of FCC + (Cr, Mo) σ phase. The presence of high Cr and Mo content in the σ phase results in Cr depletion zones. Co30HEA shows the highest corrosion resistance in Ringer's solution, which diminishes with increasing Co content. Its passive film exhibits ideal capacitive behavior at low frequencies, indicating strong passivation. The films, primarily composed of Cr oxides and hydroxides, vary in thickness, with Co30HEA having a film about twice as thick as the others. All three alloys demonstrate pitting corrosion; however, Co30HEA shows the smallest pits mainly at grain boundaries, while the others show pits near the σ phase due to unstable passive films in the Cr and Mo-depleted zones, acting as anodic sites for galvanic corrosion.

The influence of alloying elements segregation on heterogeneous nucleation of TiN on Al2O3 and its local corrosion resistance

A comprehensive study utilizing first-principles calculations has examined the heterogeneous nucleation of TiN on Al2O3 in steel and the impact of alloying elements on this process. Additionally, the study delved into how this nucleation affects local corrosion activity. The findings indicate that alloying elements such as Cr, Mn, Mo, and V tend to concentrate at the Al2O3 (0001)/TiN (001) interface, enhancing the interface's stability, thereby favoring the heterogeneous nucleation of TiN on Al2O3. Consequently, the nucleation leads to the formation of interfaces, namely Al2O3 (0001)/TiN (001) and TiN (100)/bcc-Fe (100), which are less prone to adsorbing chloride ions, replacing the Al2O3(0001)/bcc-Fe (110) interface that can stably adsorb chloride ions, consequently reducing the adsorption of chloride ions at grain boundaries. Moreover, it is difficult for Cr atoms to incorporate into the TiN structure, while Ti atoms readily replace Cr in CrN. TiN exhibits a lower formation energy compared to CrN, and the precipitation of TiN mitigates the formation of Cr-depleted zones, thereby inhibiting local corrosion.

Investigation of sigma phase generated by simulated welding heat treatment on corrosion behavior of 2205 hydraulic control pipeline steel

Duplex stainless steel (DSS) hydraulic control pipelines readily form sigma phase during the heat treatment process, reducing corrosion resistance and leading to pipeline leakage and failure. This study employs various analysis methods to investigate the effect of sigma phase on the corrosion behavior of 2205 hydraulic control pipeline steel, focusing on the electronic characterization of passive film and pitting corrosion behavior. Results revealed that the sigma phase content reaches a maximum value of 13 % at 850 °C within the heat treatment temperature range of 750–950 °C. The voltaic potential of the sigma phase is approximately 47 mV higher than the matrix, but the Cr-depleted zone around the sigma phase is about 85 mV lower than the matrix. The passive film of specimens containing sigma phase exhibits higher carrier densities and lower content of Cr(OH)3 and Cr2O3 compared to as-received specimens. The morphologies of the film show obvious groove defects, which provide a corrosion channel to the matrix. Moreover, specimens containing sigma phase exhibit higher intergranular corrosion susceptibility and lower pitting corrosion resistance. Pits preferentially nucleate and develop in the Cr-depleted zone around the sigma phase, with their quantity and size positively correlated with the sigma phase content.

Influence of ageing treatment time on microstructural evolution, sensitization and self-healing of Inconel 600

Samples of Inconel 600 were thermally aged at 739 °C from 1 up to 300 h followed by water quenching. The changes in microstructure were characterized and its effect on sensitization was evaluated by subjecting the heat-treated samples to double-loop electrochemical potentiokinetic reactivation (DL-EPR) tests. The evolution of the microstructure was analysed in terms of grain size, carbide formation and chromium diffusion by optical and scanning electron microscopy. The results showed that the formation of chromium carbides is rapid at this temperature, increasing the density of carbides at the grain boundaries in the early stages of the ageing treatment. The formation of Cr7C3 and Cr23C6 along the grain boundaries and within the austenitic matrix modified the corrosion behaviour as seen in the measurements of the degree of sensitization (DOS). A decrement in DOS was seen with ageing time up to 200 h, due to the redistribution of chromium, induced by the equilibrium between chromium at grain boundaries and within the matrix. Electron backscattering diffraction suggested that self-healing of the alloy may be due to the mechanism of near-boundary gradient zones, where the increased number of dislocations leads to precipitation of carbides by promoting diffusion and thereby decreasing the Cr-depleted zones.

Corrosion of Ni-based alloy coatings prepared by laser cladding in high-temperature chloride environment

Due to the complex incineration environment of waste incineration power generation boilers generates large amounts of acidic gases (such as Cl2 and HCl) and alkali metal chlorides (such as NaCl and KCl), leading to boiler corrosion failure. Herein, a new type of NiCrAlTi alloy coatings was deposited on 304 stainless steel surfaces by laser cladding technology. High-temperature corrosion experiments were then carried out at 650 °C for 64 h and the results were compared with those of traditional Inconel625 alloy coating. The high-temperature chloride corrosion mechanism and the corrosion resistance of alloy coatings were examined. The results revealed the decomposition of chlorides, such as HCl, NaCl, and KCl into Cl2 at high temperatures as the source of corrosion of high-temperature chlorides. The released C12 reacted with the metals present in the coating to form nonprotective metal chlorides. The low melting point and high vapor pressure of metal chlorides allowed their transition into a gaseous state to diffuse outward and oxidize into Cl2. In turn, the released Cl2 reacted with the metal to induce corrosion. During the corrosion process of NiCrAlTi alloy coating, a Cr depletion zone was induced, and a continuous Al2O3/TiO2/Cr2O3 three-layer protective film was formed on the coating surface, conducive to better alleviating the corrosion caused by the reaction between Cl2 passing through the film and the metal substrate. Under the conditions of this experiment, the corrosion resistance of NiCrAlTi alloy coating is 1.8 times that of the common material 304 stainless steel, and about 1.2 times that of Incon625 alloy coating. With the extension of the experiment, its corrosion resistance will be better. Overall, rich experimental data and theoretical guidance were provided for further research and development on high-temperature chloride corrosion-resistant alloys or coating materials for high-efficiency cost-effectiveness boilers.

Understanding the effect of decreasing C contents and increasing solid-solution time on intergranular corrosion resistance of 304 austenitic stainless steel

The purpose of this study is to investigate the intergranular corrosion resistance mechanism of 304 austenitic stainless steel treated by decreasing C contents and increasing the solid-solution treatment time. The findings indicate the effect of decreasing C contents and increasing the solid-solution treatment time effectively limits the intergranular corrosion behavior of the steel within an acceptable range (degree of sensitization, DOS values < 1). On the one hand, reducing the carbon content in the 304 austenitic stainless steel leads to insufficient levels of carbon that is necessary to reach the critical value for Cr-carbide nucleation, which decreases the occurrence of Cr-depleted zones. In comparison, the intergranular corrosion tendency of 304 austenitic stainless steel decreases at higher solid-solution temperatures and for longer treatment times, and its main purpose is to reduce the occurrence of large-sized Cr-carbide and its pinning effect on the grain boundaries. Moreover, the reduction in grain boundaries due to the grain growth further constrains the intergranular corrosion behavior of 304 austenitic stainless steel, which provides strong support for further optimizing the process conditions and reducing the failure caused by intergranular corrosion.

High-throughput first-principles study on the effect of Ni segregation on the formation of Cr-depleted zone and corrosion resistance of S32205 duplex stainless steels

Recent studies have found that in the absence of Cr carbide precipitation, co-segregation of Cr and other elements at the interface between α-Fe and γ-Fe can also lead to the formation of Cr-depleted zones, leading to intergranular corrosion. The study employed three-dimensional atomic probes, focused ion beams, and high-throughput first-principles calculation techniques to investigate the impact of Ni atoms on the α-Fe/γ-Fe interface of UNS S32205 duplex stainless steels. The goal was to analyze the formation of Cr-depleted zones and interface corrosion resistance. The results indicate that both Ni and Cr atoms are segregated at the interface between α-Fe and γ-Fe phases; The strong interaction between Ni and Cr atoms hinders Cr segregation at the interface and slows down the formation of Cr-depleted zones; co-segregation of Ni and Cr can improve the stability of the α-Fe/γ-Fe phase interface, reduce the generation of microcracks, and reduce the stability of Cl atom adsorption in the cracks. This establishes the theoretical basis for further suppressing the formation of Cr-depleted zones and preventing intergranular corrosion.

Mechanism of improving corrosion resistance in heat affected zone of duplex stainless steel by tantalum addition

The effect of tantalum addition on corrosion resistance in HAZ of duplex stainless steel was investigated by pitting corrosion test and material characterizations (SEM, TEM, APT). The Ta addition improved corrosion resistance of HAZ simulating welding at low heat input and rapid cooling rate since Ta segregated in the Cr nitrides and suppressed formation of Cr-depleted zone. The Ta addition also improved corrosion resistance in HAZ subjected to isothermal aging heat treatment assuming welding at high heat input and low cooling rate because Ta promotes the formation of χ phase and delays the formation of σ phase and Cr-depleted zone.

Effect of aging temperature on corrosion resistance of 316L stainless steel in simulated proton exchange membrane fuel cell cathode environment

The effects of aging temperature on the corrosion behavior of 316 L stainless steel in a simulated cathodic environment of a proton exchange membrane fuel cell (PEMFC) were investigated by potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS), double-loop electrochemical potential response (DL-EPR). The results show that the corrosion resistance of 316 L is significantly degraded after aging at 750–900 °C for 1 h in the simulated cathodic environment of PEMFC. A strong sensitivity to intergranular corrosion takes place after aging at 850 °C, and chromium rich M23C6 carbides precipitate on grain boundaries. The precipitation of M23C6 leads to discontinuity of the passivation film near the grain boundaries, resulting in Cr-depleted zones at the grain boundaries, increasing intergranular corrosion, and decreasing corrosion resistance. When the aging temperature exceeds 950 °C, grain boundary precipitation of M23C6 does not occur and the corrosion resistance is improved. At an aging temperature of 1050 °C, a dense Cr2O3 film is formed on the surface of the alloy during the polarization test, giving it the best corrosion resistance in the simulated cathodic environment of PEMFC.

On the creep behaviour of HP40Nb alloy with high-temperature carburisation damage

The effect of carburisation damage on the creep behaviours of HP40Nb alloy was investigated. The carbon concentration profile and microstructural evolution were characterised for the carburised specimens with and without creep, respectively. The variations of creep properties and fracture mode in carburisation treatment were also revealed. The results indicate that massive Cr-rich secondary carbides are precipitated in the matrix after carburising for 5 h, leading to the prolongation of creep lifetime. The further carburisation promotes the coarsening of primary and secondary Cr-carbides and weakens the creep strength of HP40Nb alloy. In creep process, the outward diffusion of Cr atoms contributes to the formation of Cr2O3 oxide layer on the surface and the emergence of Cr-depleted zone on the subsurface, while the inward diffusion of C atoms causes the rapid rise and gradual fall in the carbon concentration for all creep-carburisation specimens. Additionally, the creep fracture mode changes from brittle fracture to ductile fracture with the prolonging carburising time and the increasing creep stress, and the continuous carburisation results in the transformation of creep fracture mechanism towards the viscoplastic cavity growth over the applied stress range.

Atomic-scale mechanism of effect of co-separation of elements at interface of ferrite and austenite on Cr-depleted zone of duplex stainless steel

Three-dimensional atom-probe tomography and first-principles calculation combined with density functional theory were used to study the effect of the co-segregation of different elements formed during the solidification process of S32205 duplex stainless steel on the Cr-depleted zone at the interface between ferrite and austenite. It was found that the co-segregation of different elements formed during the solidification process of duplex stainless steel can also form Cr-depleted zone at the interface between ferrite and austenite. Moreover, Mo, Si, B, C and P atoms promote co-segregation with Cr atoms, which promotes the formation of Cr-depleted zone at the interface between ferrite and austenite in duplex stainless steel. Mo and Si strongly promote the segregation of Cr at the interface between ferrite and austenite, thereby promoting the formation of Cr-depleted zone. B, C and P elements also promote the segregation of Cr element at the interface between ferrite and austenite and the formation of Cr-depleted zone, but their effect is weaker than that of Mo and Si elements. These conclusions provide a new theoretical basis for improving the intergranular corrosion performance of duplex stainless steel.

Low-Potential Zone at the Interface of Precipitate and Austenite Affecting Intergranular Corrosion Sensitivity in UNS N08028 Nickel–Iron–Chromium Alloy

The precipitates and the intergranular corrosion behavior of a UNS N08028 nickel–iron–chromium alloy sensitized at different temperatures were studied by employing transmission electron microscopy, Kelvin probe force microscopy and other methods. It was found that sigma precipitates appeared at the grain boundaries of the alloy being sensitized. There was a Cr-depleted zone and a low-potential zone around these precipitates. The potential difference between the sigma precipitates and the low-potential zone was 102 mV, and this increased with the growth of the sigma precipitates. At this potential difference, the migration of the vacancies in the passive film accelerated significantly, and then the protectiveness of the passive film decreased. The intergranular corrosion mechanism of the steel has also be discussed.

Effect of ferrite and grain boundary characteristics on corrosion properties of thermal simulated 316 L heat affected zone

The microstructure and corrosion resistance evolution of 316 L heat-affected zone were studied by simulated welding thermal technology. The different thermal simulation parameters affected the ferrite content and grain boundary characteristics. The ferrite content and grain boundary characteristics in thermal simulation samples were studied by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Combining the electrochemical test, it was observed that the Cr-depleted zone existed at the austenite/ferrite interface and the high angle grain boundary (HAGB) of the austenite interface usually became the nucleation point of pitting corrosion.

Study on the characteristics of functionally graded materials from Ni-20cr to Ti-6Al-4V via directed energy deposition

Functionally graded materials (FGM) have been developed by gradually combining two or more materials and are used to improve the properties of metallic materials, such as heat resistance, corrosion resistance, and hardness. FGM can be effectively fabricated using additive manufacturing. In this study, FGM were fabricated using a direct energy deposition method with Ni-20Cr and Ti-6Al-4V metal powders. The initial layer contents were 100% Ni-20Cr and 0% Ti-6Al-4V, and the final layer contents was 100% Ti-6Al-4V and 0% Ni-20Cr, for a total of 11 layers stacked sequentially. Scanning electron microscopy, energy-dispersive X-ray spectroscopy, and phase diagram analyses were performed to examine the microstructure, composition, microhardness, and crack generation. Intergranular cracks were observed owing to the Cr depletion zone in the Ti alloy contents 40 ∼ 60% region. Cr carbides, such as Cr23C6, are responsible for the Cr depletion zone. A calculated phase diagram was used to investigate the Cr carbide precipitation temperature, the deposition conditions were suggested to prevent intergranular cracks and the effect was verified through a verification experiment. These experimental results can be used to develop other stable FGMs.

Effect of solution annealing temperature on the localised corrosion behaviour of a modified super austenitic steel produced in an open-air atmosphere

Two annealing heat treatments were investigated regarding localised corrosion behaviour on Nb and Mn-bearing super austenitic stainless steel. The material was designed based on physical metallurgical principles supported by Thermo-Calc modelling and was produced in an open-air atmosphere based on pre-sorted SS scrap plus ferroalloys elements. The production method aims for metal waste valorisation considering small foundries without an atmosphere-controlled process. Secondary phases at annealing temperatures were experimentally analysed by different microstructural characterisation methods and correlated by the Thermo-Calc modelling. In addition, electrochemical techniques and SKPFM were used to study the relationship between observed phases and localised corrosion performance. As the main results, a good correlation was observed between Thermo-Calc calculations and the microstructural characterisation. In particular, M23C6 carbides, σ phase, and Nb-MX were identified for the sample treated at 1120 °C, while for the sample treated at 1180 °C, σ phase and Nb-MX were identified, no M23C6 were detected. As regards localised corrosion behaviour, the M23C6 carbides and σ phase for sample 1120 °C generated Cr and Mo depletion zones and diminished the corrosion resistance in corrosive aqueous solutions. In the case of sample 1180 °C, the σ-phase with a smaller size and volume fraction than sample 1120 °C was observed, leading to a more uniform Cr and Mo distribution through the microstructure, obtaining high corrosion resistance and showing promising corrosion behaviour, similar to commercial SS 254smo considering its production in open-air.

Effects of B and Ce Grain Boundary Segregation on Precipitates in Super Austenitic Stainless Steel

In order to reduce the segregation of Cr and Mo and inhibit the precipitates, we added a small amount of B and Ce to traditional S31254 steel. Using an air-cooling and low-temperature diffusion treatment, the purpose was to control B and Ce at the grain boundary. The heat-treatment process could prompt co-segregation of B, precipitate-forming elements, and Ce at the grain boundary at 950 °C. After aging at 950 °C for different amounts of time, the diffusion treatment had an obvious inhibitory effect on the precipitates that caused them to become discontinuous, fine, and serrated. The B-containing serrated precipitates were only rich in Mo, while Cr was homogeneously distributed in the probed volume. A uniform distribution of Cr reduced the Cr-depleted zone in the area adjacent to the phase interface. Ce was observed to be segregated at the grain boundary. This showed that Ce could inhibit the formation of precipitates at the grain boundary. The serrated precipitates had an obvious resistance to intergranular corrosion.

Investigation on intergranular corrosion susceptibility of a nitrogen-containing austenitic stainless steel QN2109 by optimized DL-EPR tests

PurposeThe purpose of this study is to investigate the intergranular corrosion (IGC) susceptibility of a nitrogen-containing austenitic stainless steel QN2109. The intergranular corrosion (IGC) susceptibility of a nitrogen-containing austenitic stainless steel QN2109 was investigated.Design/methodology/approachThe double-loop electrochemical potentiodynamic reactivation (DL-EPR) tests were carried out. Scanning electron microscopy and atomic force microscopy were used to characterize the microstructure.FindingsThe optimized test condition for QN2109 was 1 M H2SO4 + 0.01 M NH4SCN at 40°C. The nose temperature of the temperature–time–sensitization (TTS) curve of QN2109 plot was approximately 750°C. Moreover, the IGC susceptibility started to appear at approximately 120 min. The Cr-depletion zone of QN2109 was generated by the formation of M23C6 rather than by the addition of nitrogen. The depth–width ratio of the grain boundaries after the DL-EPR tests decreased as the aging temperature increased. The degree of Cr depletion and size of the Cr-depletion zone at the grain boundary were reflected by the degree of sensitization and depth–width ratio, respectively.Originality/valueThe optimized test condition for DL-EPR tests of a nitrogen-containing austenitic stainless steel QN2109 was investigated. The TTS curve of QN2109 was first plotted to avoid IGC failure. The morphology of the Cr-depletion zone was reflected by the depth–width ratio.