Cr-depleted Region Research Articles

Accelerated formation of M2C carbides by proton irradiation inhibits molten salt corrosion in Ni-based alloy

The effect of displacement damage on the corrosion behavior in a molten FLiBe salt environment was investigated. The element distribution and microstructure around the grain boundaries (GBs) after corrosion were characterized. The results show a decrease in corrosion thickness with increasing irradiation dose and the presence of intergranular corrosion. Nanoscales M2C carbides were observed to be distributed, with a denser and thicker distribution in samples with higher irradiation doses. Their distribution depth is related to the Cr depletion region, inhibiting Cr diffusion toward the GBs and surface. Furthermore, the nucleation mechanism of M2C carbides along the GBs and in irradiated regions was revealed, attributed to the combined effects of thermal influences, element preferential dissolution due to corrosion, and irradiation-induced segregation.

Microstructures and Corrosion Behaviors of Non-Equiatomic Al0.32CrFeTi0.73(Ni1.50−xMox)(x = 0, 0.23) High-Entropy Alloy Coatings Prepared by the High-Velocity Oxygen Fuel Method

The non-equiatomic Al0.32CrFeTi0.73(Ni1.50−xMox) (x = 0, 0.23) high-entropy alloy (HEA) coatings were prepared by the high-velocity oxygen fuel (HVOF) method. The microstructures and corrosion behaviors of the HVOF-prepared coatings were investigated. The corrosion behaviors were characterized by polarization, EIS and Mott-Schottky tests under a 3.5 wt.% sodium chloride aqueous solution open to air at room temperature. The Al0.32CrFeTi0.73Ni1.50 coating is a simple BCC single-phase solid solution structure compared with the corresponding poly-phase composite bulk. The structure of the Al0.32CrFeTi0.73Ni1.27Mo0.23 coating, combined with the introduction of the Mo element, means that the (Cr,Mo)-rich sigma phase precipitates out of the BCC solid solution matrix phase, thus forming Cr-depleted regions around the sigma phases. The solid solution of large atomic-size Mo element causes the lattice expansion of the BCC solid solution matrix phase. Micro-hole and micro-crack defects are formed on the surface of both coatings. The growth of both coatings’ passivation films is spontaneous. Both passivation films are stable and Cr2O3-rich, P-type, single-layer structures. The Al0.32CrFeTi0.73Ni1.50 coating has better corrosion resistance and much less pitting susceptibility than the corresponding bulk. The corrosion type of the Mo-free coating is mainly pitting, occurring in the coating’s surface defects. The Al0.32CrFeTi0.73Ni1.27Mo0.23 coating with the introduction of Mo element increases pitting susceptibility and deteriorates corrosion resistance compared with the Mo-free Al0.32CrFeTi0.73Ni1.50 coating. The corrosion type of the Mo-bearing coating is mainly pitting, occurring in the coating’s surface defects and Cr-depleted regions.

Ultrafine-grained stainless steel with wear-resistant nanoborides decorating grain boundaries produced by laser powder bed fusion

Stainless steel composites combine corrosion resistance and toughness of the metallic matrix with wear resistance granted by hard reinforcing nanoparticles. Given their near-net-shape manufacturing characteristics, additive manufacturing techniques are attractive to produce these hard and wear-resistant classes of materials with low machinability. In this work, from a SAF 2205 duplex stainless steel with boron addition, laser powder bed fusion (LBPF) was employed to produce an ultrafine-grained (∼1 µm), dense (porosity ∼0.1 %), and crack-free ferritic-induced matrix composite with Cr2B-nanoborides reinforcing the grain boundaries (GBs) without formation of Cr-depleted regions. The composite showed significant higher hardness (up to 456 HV0.5) and wear resistance (4.4 x 10−5 mm3 N−1 m−1) in sliding condition compared to a hot-rolled (225 HV0.5 and 2.9 x 10−3 mm3 N−1 m−1) and a LPBF-produced SAF 2205 (314 HV0.5 and 3.3 x 10−4 mm3 N−1 m−1).

Intergranular corrosion of Ni-30Cr in high-temperature hydrogenated water after removing surface passivating film

High-resolution transmission electron microscopy and atom probe tomography are used to characterize the initial passivation and subsequent intergranular corrosion of degraded grain boundaries in a model Ni-30Cr alloy exposed to 360 °C hydrogenated water. Upon initial exposure for 1000 h, the alloy surface directly above the grain boundary forms a thin passivating film of Cr2O3, protecting the underlying grain boundary from intergranular corrosion. However, the metal grain boundary experiences severe Cr depletion and grain boundary migration during this initial exposure. To understand how Cr depletion affects further corrosion, the local protective film was sputtered away using a glancing angle focused ion beam. Upon further exposure, the surface fails to repassivate, and intergranular corrosion is observed through the Cr-depleted region. Through this combination of high-resolution microscopy and localized passive film removal, we show that, although high-Cr alloys are resistant to intergranular attack and stress corrosion cracking, degradation-induced changes in the underlying metal at grain boundaries make the material more susceptible once the initial passive film is breached.

Role of MnS in the intergranular corrosion and depassivation of sensitized Type 304 stainless steel

During polarization in 1 M NaCl (pH 5.4), intergranular corrosion of sensitized re-sulfurized Type 304 stainless steel occurred after pit initiation at MnS inclusions. Intergranular corrosion occurred when the pitting corrosion reached a Cr-depleted region along the sensitized grain boundaries. During depassivation under open-circuit conditions in 1 M NaCl (pH 1.0), MnS dissolution occurred as the first step, and the second step was the initiation of intergranular corrosion. Finally, dissolution of the steel matrix occurred. The role of MnS in the initiation of intergranular corrosion and depassivation of sensitized steel is discussed.

Effect of annealing temperature on the microstructure evolution and corrosion behavior of Carbon-interstitial FeMnCoCrNi high-entropy alloys

The microstructure evolution, corrosion resistance, and passive-film properties of an interstitial FeMnCoCrNi high-entropy alloys annealed at different temperature were investigated. Deformation twins, Strip-shape and ellipsoidal M23C6 carbides are remained within grains and precipitated along the grain boundaries of the HEAs annealed at 500 and 700 °C. The 900 °C annealed i-Cantor alloys are fully recrystallized without deformation twins and precipitated with round M23C6 carbides. The corrosion resistance of the cold-rolled HEAs was deteriorated by annealing at 500 and 700 °C, but improved by annealing at 900 °C, which decreases the dislocation densities and depresses the Cr-depleted regions in i-Cantor alloys.

Precipitate composition regulation and corrosion resistance improvement of boron-containing S31254 steels through a pre-aging treatment

Boron (B) at the grain boundary (GB) influences the segregation of Cr and Mo in S31254 super austenitic stainless steels, which inhibits the formation of precipitates. In this work, to make full use of B at the GB, a pre-aging treatment schedule at 300 ℃ for 50 and 150 min was proposed. After pre-aging treatment and high-temperature aging at 950 ℃, B-containing S31254 was used to investigate the effect of heat treatment on the precipitates. Results verified that pre-aging treatment for 50 and 150 min could promote the segregation of B at GB, and the nucleation of precipitates was inhibited during high-temperature aging. Consequently, the B-containing and Mo-rich precipitates were formed at 950 ℃. Especially, pre-aging treatment for 50 min showed the optimum effect on regulating precipitate compositions. B-containing and Mo-rich phase reduced the Cr-depleted region near the precipitates, which could remarkably improve corrosion resistance. Pre-aging treatment and high-temperature aging can regulate precipitate compositions, which can in turn improve corrosion resistance.

Unveiling the pitting corrosion mechanism of borated stainless steel in the wet storage environment of spent nuclear fuels

Borated stainless steels (BSSs) with high neutron absorbability are promising alloys for the wet storage of spent nuclear fuel. However, consensus on the underlying mechanisms for the effects of B content in BSS and H3BO3 in wet storage environment on the pitting corrosion resistance of BSSs is still lacking. To determine the mechanisms, we designed three hypoeutectic BSSs containing 0.6, 1.2 and 1.8 wt.% B and systematically investigated their electrochemical behavior in solutions with various Cl−/H3BO3 ratios through experimental and computational studies. Results revealed that the pitting corrosion resistance of BSS was dependent on the synergistic effect of the pit initiation induced by localized Cr depletion and the pit stable growth caused by microgalvanic effect, causing the pitting corrosion preferentially occurred at the (Cr, Fe)2B/matrix interface. A higher B content increased the density and width while reduced the Cr contents of the Cr-depleted regions, enhancing the susceptibility to pitting initiation. Meanwhile, the strengthened micro-galvanic effect between the (Cr, Fe)2B phase and γ matrix further facilitated the stable pit growth. For the H3BO3 effect, the first-principles calculations indicated that the competitive adsorption of B(OH)4− to Cl− inhibited pitting initiation in solutions with lower Cl−/H3BO3 ratios. However, as the Cl−/H3BO3 increased, the inhibition effect of the H3BO3 was weakened, and the acidizing effect became the dominant factor deteriorating the corrosion property. Ultimately, based on the Cr depletion-microgalvanic synergetic mechanism, the corrosion resistance of the BSS was significantly improved by tailoring the size and distribution of the (Cr, Fe)2B phase.

Achieving a superior combination of tensile properties and corrosion resistance in AISI420 martensitic stainless steel by low-temperature tempering

The mechanical and electrochemical properties of AISI420 martensitic stainless steels (MSS) tempered at different temperatures (200, 400, and 600 ℃) are systematically investigated. The increased tempering temperature leads to the decreased strength, while the elongation remains relatively stable. Notably, the MSS tempered at 200 ℃ exhibits a superior strength-ductility synergy, featuring a strength of 1.65 GPa, and elongation of 10%. Moreover, increasing tempering temperature reduces the electrochemical properties and pitting corrosion resistance. In the tempered samples, pitting corrosion initiates near the inclusions and carbides, with the Cr-rich carbides and Cr-depleted regions serving as channels that promote the propagation of corrosion.

New sights on intergranular corrosion resistance mechanism of type 304 austenitic stainless steel by adjusting carbon contents

IGC mechanisms and/or corresponding IGC resistance mechanisms of 304 and 304L ASSs by adjusting carbon contents were systematically investigated by microstructure characterization, electrochemical and IGC tests. The results reveal that a maximum IGC degree of 304 ASS is observed at an aging temperature of 660 °C for 10 h based on a continuous precipitation of Cr-carbides along grain boundaries. The roles of precipitated Cr-carbides are concluded as two aspects, involving an occurrence of Cr-depleted regions and a destruction of film stability along grain boundaries. However, this IGC behavior is not linearly relevant with precipitation behavior of Cr-carbides, and a co-segregation mechanism of Cr and C atoms before a nucleation of carbides plays a decisive role in this current study. In comparison, 304L ASS (by adjusting carbon contents) exhibits a superior IGC resistance after a long-term aging treatment, which is mainly attributed to simultaneously restricting a precipitation of Cr-carbides and ensuring a continuous supply of Cr atoms toward Cr-depleted regions.

Effect of WC-10Co on microstructure and properties of medium-entropy alloy coatings via electron beam cladding

In this study, non equiatomic FeCrNiCoMoW MEA composite coatings were successfully prepared by electron beam cladding Fe-based amorphous powders. The effects of WC-10Co amounts (0–30 wt%) on the microstructure, mechanical properties, and electrochemical property of the MEA composite coatings were investigated. The added WC-10Co decomposed, and the original amorphous alloy was completely crystallised. The composite coatings primarily comprised the face-centred cubic (FCC) matrix, M23C6 (M = Cr, Mo, Fe, or W) and WC particles. The average hardness of the composite coatings gradually increased with increased WC-10Co amounts from 5.66 ± 1.09 GPa to 8.02 ± 2.54 GPa. The composite coating with 30 wt%WC-10Co showed optimal hardness and wear resistance, which were approximately 4 and 2 times that of the substrate, respectively. The wear mechanism changed from severe adhesive wear for 0 wt% WC-10Co composite coating to abrasive and slight adhesive wear for 10–30 wt% WC-10Co composite coatings. The 10 wt% WC-10Co coating exhibited the best corrosion resistance in 3.5 wt% NaCl solution. The number of corrosion pits decreased with the addition of WC-10Co increased, and corrosion occurred in the Cr-depleted regions. Solid solution, dislocation, WC particles and M23C6 nanoprecipitates were responsible for the improvement in mechanical properties and corrosion resistance of the coatings.

Grain size effect on corrosion behavior of Inconel 625 film against molten MgCl2-NaCl-KCl salt

This study evaluated the grain size effect of Inconel 625 film against molten chloride salt. It is found that larger grain size Inconel 625 exhibits better corrosion resistance by reducing Cr depletion. ~ 2 nm surface NiO layer of Inconel 625 is incapable of compensating the negative effects from more initiation corrosion sites. Mo is the least likely leached element and its enrichment occurs in Cr-depleted regions as a result of Mo diffusion. Mg and Cl are two most seen salt elements in all corroded samples, while K and Na only attack Inconel 625 with the smallest grain size.

Effect of laser energy volume density on wear resistance and corrosion resistance of 30Cr15MoY alloy steel coating prepared by laser direct metal deposition

For laser repairing the wear and corrosion failure parts, the effect of laser energy volume density (EVD) on the wear resistance and the corrosion resistance of 30Cr15MoY alloy steel coating prepared by the laser direct metal deposition (DLMD) was investigated. The 30Cr15MoY alloy steel coating composed of martensite, retained austenite and grain boundary precipitates was fabricated, exhibiting good quality without cracks and pores. A good metallurgical bonding between the 30Cr15MoY alloy steel coating and the substrate formed, the element dilution from substrate to coating was slight. The wear failure of coatings is a combination of abrasive wear, adhesive wear and oxidation wear, the sample has the lowest wear rate (6.49 × 10−6 mm3/N·mm) and the highest micro-hardness (768 HV0.2) when the EVD is 128 J/mm3. The wear resistance of the 30Cr15MoY alloy steel coating is mainly related to the solid solution content, the grain boundary type and the phase composition. The main corrosion failure of the DLMD 30Cr15MoY alloy steel coating is the pitting corrosion, and the sample with EVD of 128 J/mm3 displays the best corrosion resistance (Icorr: 0.43 × 10−6 A·cm2, Ecorr: −217.40 mV). The content of grain boundary precipitates is the main determining factor of the corrosion resistance, and the Cr-depleted regions surrounding the grain boundary precipitates are the preferential sites of the pitting corrosion. Besides, the corrosion resistance is also related to the solid-solution content of C and Cr and the grain boundary type. It is expected that this study could provide new material and wear and corrosion resistance theories for laser repairing short stress line rolling mill and other mechanical parts.

Assessment of the correlation between M23C6 precipitates and pitting corrosion resistance of 0Cr13 martensitic stainless steel

The effect of M23C6 precipitates on the pitting behavior of 0Cr13 martensitic stainless steel (MSS) was investigated. The results revealed that the precipitates in the 0Cr13 MSS with different tempering temperatures were all M23C6 with a face-centered cubic (FCC) structure. The Cr-depleted regions had approximately 6 mV Volta potential lower than the steel matrix. The increasing Cr contents in the Cr-depleted regions hampered the effect of Cr deficiency in the weak passive film. The low width and density of Cr-depleted regions reduced the intensity of pits on steel surface. The pitting corrosion resistance was enhanced as the tempering temperature increased.

Corrosion resistance of UNS S41426 stainless steel in acid media and its relation to chemical partition and austenite fraction

The present work evaluates the impact of tempering treatment on austenite reversion and corrosion resistance of UNS S41426 in 0.5 M H2SO4 solution. Higher austenite fraction was related to higher tempering temperature, engendering lower hardness degrees. Despite a reduction in corrosion rates in immersion tests, Ni-rich austenite decreased corrosion resistance in more oxidizing environments. Cr-depleted regions created a secondary anodic peak in polarization tests. Chemical partitioning is presented to be the main feature that influences corrosion in supermartensitic stainless steel. Finally, tempering at 650 °C presented the best results as it had ten times less corrosion rate than as-quenched state.

Microstructure and Corrosion Studies on Different Zones of Super Duplex Stainless Steel UNS S32750 Weldment

The influences of microstructure and elemental distribution on pitting corrosion resistance and passive film in different regions of super duplex stainless steel welded joint were investigated. The banded microstructure disappeared after welding and transformed to the coarse equiaxed ferrite grains in the heat-affected zone. Three types of austenitic microstructure formed in the weld centre, only two types occurred in the weld cap and weld root. It is found that the elements Ni and N were enriched in austenite, while Cr and Mo were concentrated in the ferrite. The Cr2N was found precipitating in some specific regions with a lower austenite content, which led to the relatively poorer resistance to pitting corrosion in the heat-affected zone, weld root, and weld cap. The ferrite observed in each studied region of the joint was prone to experience selective corrosion due to its lower pitting resistance equivalent number. The Cr-depletion region around the Cr2N was disappeared as the original location of pitting. The electrochemical impedance spectroscopy results also showed that there is no difference exists in all of the regions of the joint in the passive film structure and regular microstructure was helpful to the passive film.

Failure analysis of gas-gas heater tubes for a flue gas desulfurization system

This paper presents the failure analysis of a gas-gas heater (GGH) tube in a coal-fired flue gas desulfurization (FGD) system. The GGH was operated at a temperature below those of the hydrochloric acid dew point as well as sulfuric acid dew point. Leaks were detected in the U-bend of the failed GGH tube. Corrosion products and ashes were attached on the surface of the tube, and uniform and localized corrosion were present throughout the surface. Corrosion products were mainly shown in reddish brown, orange and black color which were identified as hematite (Fe2O3), magnetite (Fe3O4), ferric sulfate (Fe2(SO4)3), ferrous sulfate heptahydrate (FeSO4·7H2O) and ferrous carbonate (FeCO3). In addition, S and Cl were observed in the corrosion products. This is a clear evidence that both sulfuric acid dew point corrosion and hydrochloric acid dew point corrosion occurred simultaneously. Further, when Cu and Cr are alloyed together, Cu segregates Cr into the grain boundary, forming a Cr depletion region, and localized corrosion occurs in this area. In the case of the fin-tube joint corrosion, the galvanic corrosion occurred at the filler metal because the filler metal acts as an anode with a middle potential.

Effect of Pre-passivation on the Corrosion Behavior of PH13-8Mo Stainless Steel in Industrial—Marine Atmospheric Environment

The corrosion behavior and mechanism of PH13-8Mo stainless steel with and without nitric acid passivated film were studied through 5 years of outdoor exposure tests in industry-marine atmospheric environments in Qingdao, by means of morphological observation, weight-loss measurement, X-ray photoelectron spectroscopy (XPS) and Scanning Kelvin Probe (SKP). The results showed that the pre-passivation certainly inhibits the corrosion, but exhibits an obvious limitation of protection period for PH13-8Mo stainless steel in industrial-marine atmosphere. Compared with bare PH13-8Mo steel, the number and depth of pitting of pre-passivated steel were reduced, and the average corrosion rate decreased by 14.7%. The reasons of improved corrosion resistance included the higher ratio of Cr/Fe and oxide/hydroxide, and the more positive and homogeneous Kelvin voltage potential for the surface of outdoor exposed pre-passivated steel. Meanwhile, the pre-passivation film on the Cr-depleted regions caused by the Cr-rich precipitated phase was preferentially destructed in the atmosphere with Cl-, which decreased the protection period.

Pitting corrosion resistance of sensitized type 2205 duplex stainless steel in hot concentrated seawater

Pitting corrosion resistance of 2205 duplex stainless steel aged at 850 °C was investigated in the hot concentrated seawater (i.e., the simulated low-temperature multi-effect distillation environment) by optical microscopy (OM), scanning electronic microscopy (SEM), and electrochemical measurement techniques. The precipitation of the σ phase leads to sensitization in the specimens during the aging treatment. The degree of sensitization (DOS) presents a maximum value with the change of σ phase content. The pitting corrosion resistance of the specimens degrades markedly in the presence of very weak sensitization. The pitting potential decreases almost linearly with the logarithm of DOS. The pit propagation mainly takes place along the Cr-depleted region around the σ phase. The repassivation ability of the specimens changes insignificantly with the aging time.

Effect of Al content on high temperature oxidation resistance of AlxCoCrCuFeNi high entropy alloys (x=0, 0.5, 1, 1.5, 2)

The oxidation behavior of AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5, 2) high entropy alloys are discussed in this work. Five kinds of HEAs with nominal compositions of are prepared by vacuum arc melting and subjected to isothermal oxidation at 1000°C for 100 h in static air. The results indicate that three typical CuNi-rich, AlNi-rich and matrix phases emerge after oxidation. In all oxidized HEAs there is a tendency to generate the Cr depletion or Al depletion region directly beneath the oxidation layer. With the increase of Al content, the oxidation scale mainly consists of alumina instead of Cr2O3 and the parabolic rate constant of Al2CoCrCuFeNi HEA reaches the minimum value, which clarifies the superior high temperature resistance.