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The surging demand for advanced fluorine corrosion-resistant materials underscores their significance in ensuring operational safety and reliability across various industries. This study investigates the corrosion behavior of the FeCoNiCrMo high-entropy alloy (HEA) via a series of 28-day immersion tests in hydrofluoric acid (HF) solutions. The results demonstrate the FeCoNiCrMo HEA's superior corrosion-resistant performance in HF environments, exhibiting remarkably low corrosion rates of 0.179 mm/y, 0.276 mm/y, and 0.352 mm/y in 20 vol%, 30 vol%, and 40 vol% HF solutions, respectively. Comprehensive phase and microstructural characterizations were conducted on samples exposed to the 40 vol% HF solution to elucidate the corrosion mechanisms. The study revealed that localized pitting corrosion preferentially initiates within the interdendritic regions of the HEA matrix upon HF exposure. During the intermediate stage, micro-galvanic corrosion occurs between the dendritic arms and interdendritic regions, leading to the formation of a uniform and compact corrosion product film on the alloy surface. This film, enriched with Mo, Cr, and O, provides temporary protection. However, as corrosion progresses, the partial detachment of particulate corrosion products compromises the integrity of the film, resulting in increased dissolution within the interdendritic regions and the formation of irregular corrosion grooves in the later stage. These insights significantly enhance the understanding of the corrosion mechanisms of FeCoNiCrMo HEA in HF environments and provide valuable guidance for developing innovative protective materials designed for fluorine-rich engineering applications.

The influence of B and Sb on the microstructure and corrosion behavior of T6-treated Al–Si–Mg alloys has been specifically studied. It is found that the addition of 0.03 wt% B to Al–Si–Mg alloys results in the lowest corrosion rate (0.02451 mm/y) when immersed in 3.5 wt% NaCl solution for 3 days. This improvement can be attributed to the transformation of coarse dendritic α-Al grains into refined equiaxed grains, which is beneficial for the formation of uniform and compact passive film. In contrast, the Al–Si–Mg alloy with addition of 0.2 wt% Sb shows the highest corrosion rate (0.05795 mm/y), which can be attributed to two reasons. First, the addition of Sb contributes to the transformation of rod-like eutectic Si particles into a spherical morphology, increasing the number of micro-galvanic corrosion sites and thus accelerating the corrosion process. Second, the generated Mg3Sb2 phase, being more noble than the eutectic Si phase, strengthens the micro-galvanic corrosion.

Effects of Mo and C addition on the microstructural evolution and corrosion resistance in chloride solution of FeNiCrAl duplex alloy were investigated. The results showed that α phase stabilizing element Mo increased the α/γ volume ratio and γ phase stabilizing element C decreased its ratio in FeNiCrAl alloy, and the chemical composition of α and γ phases was changed by Mo and C addition, which affected the corrosion resistance of FeNiCrAl alloy. Pitting corrosion mainly occurred in α phase around NbC precipitated at α/γ interface due to the micro-galvanic corrosion (MGC) effect between α (anode), γ (cathode) and NbC (cathode) phases. The pitting corrosion resistance was improved by Mo and C addition, resulting from that Mo and C enhanced the passivity of passive films and decreased the MGC effect between α, γ and NbC phases.

The findings of this study provide valuable insights into the electrochemical characteristics of high-chromium white cast irons (HCCIs) and their suitability as impellers in flue-gas desulfurizer (FGD) slurry pumps of coal-fired power plants. By evaluating the corrosion and wear resistance of HCCI specimens with varying Cr contents, it was confirmed that the corrosion mechanism of HCCI involves micro-galvanic corrosion between the matrix and iron–chromium and carbon intermetallic compounds. Furthermore, the study revealed that HCCI with 32 wt% Cr content exhibited superior corrosion resistance, owing to the full-ferrite matrix with complete Cr dissolution that facilitated the enrichment of Cr(OH)3 and Cr2O3 on the HCCI surface. The results demonstrate that the addition of 32 wt% Cr to HCCI enhances its passivation ability and improves its ability to resist both micro-galvanic corrosion and erosion–cavitation. Finally, the performance of the impeller fabricated with HCCI containing 32 wt% Cr was validated in a real environment for 16 months with a FGD slurry pump, and it was found to be in excellent condition, further highlighting the effectiveness of HCCI for use as impellers in FGD slurry pumps. [ABSTRACT FROM AUTHOR]

Mg-6Zn-2X(Fe/Cu/Ni) alloys were prepared through semi-continuous casting, with the aim of identifying a degradable magnesium (Mg) alloy suitable for use in fracturing balls. A comparative analysis was conducted to assess the impacts of adding Cu and Ni, which result in finer grains and the formation of galvanic corrosion sites. Scanner electronic microscopy examination revealed that precipitated phases concentrated at grain boundaries, forming a semi-continuous network structure that facilitated corrosion penetration in Mg-6Zn-2Cu and Mg-6Zn-2Ni alloys. Pitting corrosion was observed in Mg-6Zn-2Fe, while galvanic corrosion was identified as the primary mechanism in Mg-6Zn-2Cu and Mg-6Zn-2Ni alloys. Among the tests, the Mg-6Zn-2Ni alloy exhibited the highest corrosion rate (approximately 932.9 mm/a) due to its significant potential difference. Mechanical testing showed that Mg-6Zn-2Ni alloy possessed suitable ultimate compressive strength, making it a potential candidate material for degradable fracturing balls, effectively addressing the challenges of balancing strength and degradation rate in fracturing applications. [ABSTRACT FROM AUTHOR]

In the present work, the corrosion behavior of the B4C/pure Al composite in a Cl− containing environment is investigated. Thereby, the B4C/pure Al composites have been subjected to the different heat treatments to highlight the impact of the corrosion behavior at the different interfaces. The results of current sensing atomic force microscope (CSAFM) and scanning Kelvin probe microscopy (SKPFM) show that the dissolution of the Al matrix is relevant to the micro-galvanic corrosion between anodic Al matrix and cathodic B4C particles. The as quenched (AQ) composites with a higher volta potential difference (VPD) show inferior anticorrosion property than that of the as produced (AP) composites and the as annealed (AA) composites. By combining X-Ray diffraction (XRD) and geometric phase analysis (GPA), the higher residual stress and obvious interfacial strain between the B4C particles and the Al matrix of the AP samples is detected, which result in increasing the driving force of micro-galvanic corrosion.

The influence of Al–Mn intermetallic on corrosion resistance of AM50 alloy was investigated. TEM-SAED was utilized to identify microstructure and composition of the corrosion products on Al–Mn intermetallic. The electrochemistry results revealed that the corrosion rate of pure Mg was lower than AM50 due to thermodynamic driving force for micro-galvanic corrosion by Al–Mn with matrix. Although the Volta potential difference reduce because of accumulation of corrosion products, the cathodic activity of AM50 alloy increases with exposure time. The progress of corrosion could not be inhibited since the loose and porous corrosion products provided an area for containing and storing the corrosion medium.

Understanding the corrosion behavior of Mg alloys in the soil environments is crucial to preventing catastrophic accidents in their facilities due to the soil corrosion. The Mg–2Nd alloys were exposed to the soil after cooling in different ways under the same heat treatment conditions. The corrosion mechanisms of Mg–2Nd alloy in the soil are mainly affected by corrosion micro-batteries and macro-batteries. The more precipitates, the more severe micro-galvanic corrosion are formed. Meanwhile, the corrosion product films are damaged by micro-galvanic corrosion, which have a serious impact on the differential concentration of oxygen and differential infiltration of rainwater corrosion. The Mg–2Nd alloy buried in the soil with high water content and low oxygen permeability in corrosive macro-batteries will be corroded into anodes, thereby accelerating the corrosion rate of Mg–2Nd alloy in this part. Furthermore, the formation of corrosion product MgCO3 reduces the electrochemical activity of the sample surface, thereby reducing the corrosion rate of Mg–2Nd alloy in the soil. Therefore, the Mg–2Nd alloy after water-cooling has the best corrosion behavior in the soil environment.

The AZ series of Mg alloys have become promising in several industrial fields thanks to its potential microstructure refinement and the β-Mg17Al12 eutectic that controls the mechanical behaviour. Simultaneously, the rapid degradation characterizing Mg alloys makes the investigation of their corrosion behaviour necessary. The present work considers high-pressure die cast (HPDC) AZ91 alloy to evaluate its corrosion behaviour in 1M NaCl solution and investigates how different friction stir process parameters can affect the corrosion responses. No studies analyse the effects induced by the friction stir processed zone, reached using high rotational speeds (>2000 rpm), on the unprocessed HPDC AZ91 alloy. In addition, the morphological analysis of the corroded surfaces having a friction stir processed zone, in which the grain refinement was not obtained, is not present in the literature yet. Microstructural features were investigated by optical microscope and X-ray diffraction analysis before and after the friction stir process. These were subsequently correlated to the corrosion responses after the immersion tests. The results show that HPDC samples with a very smooth surface have the best corrosion resistance with a corrosion rate lower than 3 mm/year, evaluated through the weight loss, compared to the rougher ones. Both the amount of β-Mg17Al12 eutectic and the wt.% Al in the α-Mg matrix, as well as the surface roughness, influence the corrosion behaviour of friction stir processed samples. The best corrosion resistance was obtained with an HPDC alloy processed at 2500 rpm and 50 mm/min. [ABSTRACT FROM AUTHOR]

Inferior absolute strength and dissolution properties are the main bottlenecks for the widespread application of dissolvable magnesium alloys in complex working environments for unconventional oil and gas resources. Here, a novel functional peak-aged Mg-9.5Gd-2.7Y-0.9Zn-0.8Cu-0.4Ni (wt.%) alloy for fracturing tools is reported, and it possesses an ultimate tensile strength of 457.6 MPa, ultimate compressive strength of 620.7 MPa and dissolution rate of ∼43.7 mg·cm−2·h−1 in 3 wt.% KCl solutions at 93 °C. The excellent strength of the aged-alloy is primarily attributed to the combination of grain refinement, long-period stacking ordered (LPSO) strengthening, and precipitation strengthening induced by stacking fault and β’ phase, among which the precipitation strengthening is dominant. Further investigations confirm that the corrosion is triggered from the micro-galvanic coupling between the Mg matrix and the cathodic lamellar and block LPSO phases. Strip-shaped corrosion pits along with LPSO phases are subsequently formed, significantly accelerating corrosion. The β’ precipitates can effectively improve the strength without compromising the dissolution rate because of their nanoscale size. This study provides an excellent material selection for dissolvable fracturing tools and presents a strategy by which a synergistic combination of strength and dissolution rate is achieved via peak-aging treatment.

A model specimen with a single boundary of the α/β phase simulating Mg-Al alloys was successfully fabricated by spark plasma sintering. A small electrode area of α phase or β phase was prepared using the model specimen, and the OCPs (open-circuit potentials) of each phase and a small electrode area containing the α/β phase boundary in 0.1 M NaCl at pH 8.0 were compared: the β phase exhibited a higher potential, and the α phase showed a lower potential. The OCP of the small area containing the α/β phase boundary was the intermediate value of these phases. In a small area containing α/β phase boundary, discoloration and gas bubbles were observed on the α phase, but no bubble generation was detected on the β phase. The gas bubbles were initially generated on the α phase near the β phase, but as the discoloration (corrosion) of the α phase approached the β phase, the bubbles were generated on the β phase. In micro-galvanic corrosion of the α and β phases, the β phase did not always function as the preferred cathode. The α phase partially corroded (or discolored) and became the anodes, so that the surrounding areas were most likely to be the cathodes. When corroded areas (anodes) in the α phase approached the β phase, the β phase would become cathodes. In addition to the micro-galvanic corrosion mechanism, the role of Al in corrosion resistance at the α/β phase boundary was determined by surface analysis.

The presence of a second phase in the Mg–8Al–4Ca (at. %) alloy plays a significant role on both its corrosion behavior and the chemical conversion coating processes. Using scanning Kelvin probe force microscopy (SKPFM), a lower Volta-potential of the second phase present on the surface has been measured. The β-Al-Ca phase has a higher electrochemical activity than the α-Mg matrix and may act as the micro-galvanic anode in a local electrochemical corrosion process. Transmission electron microscopy (TEM) examinations reveal that the β-Al-Ca phase is more susceptible to corrosion than the α-Mg matrix in an aqueous solution, and its higher activity and higher corrosion rate accelerate the hydrogen evolution rate on the α-Mg matrix in the cerium (Ce) conversion coating process. It's also been discovered that by immersing the bare Mg–Al–Ca alloy in deionized (DI) water, the β-Al-Ca phase, exposed on the surface, can be dissolved and converted in situ into aluminum hydroxide (Al(OH)3), and the Ce conversion coating can be deposited via replacement reactions in the subsequent conversion coating process. A thicker Ce coating with smaller blisters has then been produced on the DI-treated Ce-coated Mg specimen; it indeed improves the corrosion resistance.

The Mg-xGd-1Er-1Zn-0.6Zr alloys with Gd contents of 7%(mass fraction), 9% and 11% were prepared by gravity casting method.The microstructure of the alloys was studied by means of optical microscope, scanning electron microscope and X-ray diffractometer.The corrosion behavior of the alloys were evaluated by means of open circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy measurements in 3.5%NaCl solution.The results show that when Gd content increases from 7% to 11%, the peak time of open circuit potential decreases from 1609 s to 851 s, the charge transfer resistance decreases from 588.50 Ω to 31.9 Ω, the corrosion current density increases from 2.21×10-5 A/cm2 to 3.97×10-5 A/cm2, indicating that the corrosion resistance of the alloys decreases with the increase of Gd content.It is attributed to the combined operation of the micro-galvanic corrosion effect as well as corrosion barrier effect of second phase.When the Gd content increases from 7% to 11%, the volume fraction of (Mg, Zn)3(Gd, Er) phase increases from 1.9% to 5.2%, and changes from discontinuous distribution to semi-continuous distribution along grain boundaries, the volume fraction of the lamellar-shape LPSO phase increases from 11.7% to 26.7% and penetrates into grains.The increase in the volume fraction of the (Mg, Zn)3(Gd, Er) phase and the lamellar-shape LPSO phase results in the decrease of corrosion resistance, however, a large number of fine lamellar-shape LPSO phases is able to prevent the corrosion from spreading and slow down the growth of corrosion rate of the alloy with 11%Gd content in 8-24 h.

Abstract Background Although several studies on the Mg-Zn-Ca system have focused on alloy compositions that are restricted to solid solutions, the influence of the solid solution component of Ca on Mg-Zn alloys is unknown. Therefore, to broaden its utility in orthopedic applications, studies on the influence of the addition of Ca on the microstructural, mechanical, and corrosion properties of Mg-Zn alloys should be conducted. In this study, an in-depth investigation of the effect of Ca on the mechanical and bio-corrosion characteristics of the Mg-Zn alloy was performed for the optimization of a clinically approved Mg alloy system comprising Ca and Zn. Methods The Mg alloy was fabricated by gravitational melting of high purity Mg, Ca, and Zn metal grains under an Ar gas environment. The surface and cross-section were observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to analyze their crystallographic structures. Electrochemical and immersion tests in Hank’s balanced salt solution were used to analyze their corrosion resistance. Tensile testing was performed with universal testing equipment to investigate the impact of Ca addition. The examination of cytotoxicity for biometric determination was in line with the ISO10993 standard. Results In this study, the 0.1% Ca alloy had significantly retarded grain growth due to the formation of the tiny and well-dispersed Ca2Mg6Zn3 phase. In addition, the yield strength and elongation of the 0.1% Ca alloy were more than 50% greater than the 2% Zn alloy. The limited cell viability of the 0.3% Ca alloy could be attributed to its high corrosion rate, whereas the 0.1% Ca alloy demonstrated cell viability of greater than 80% during the entire experimental period. Conclusion The effect of the addition of Ca on the microstructure, mechanical, and corrosion characteristics of Mg-Zn alloys was analyzed in this work. The findings imply that the Mg-Zn alloy system could be optimized by adding a small amount of Ca, improving mechanical properties while maintaining corrosion rate, thus opening the door to a wide range of applications in orthopedic surgery.

The corrosion behavior and corrosion resistance of WE43 and AZ80 immersed in NaCl and Na2SO4 solutions were investigated, respectively. Two alloys were immersed in 0.6 M NaCl and Na2SO4 solution to observe the corrosion morphologies. Hydrogen evolution and weight loss experiments were conducted to obtain the corrosion rates. Electrochemical tests were used to characterized detailed corrosion situation. The results show that, when immersed in Na2SO4 solution, WE43 alloy shows a unique micro-galvanic corrosion behavior. On the other hand, the corrosion rate of WE43 in Na2SO4 solution is much faster than that in NaCl solution, which is the direct opposite of AZ80 and most magnesium alloys. The protection of the surface film maybe the key factor to the unexpected phenomena. [ABSTRACT FROM AUTHOR]

Inferior absolute strength and dissolution properties are the main bottlenecks for the widespread application of dissolvable magnesium alloys in complex working environments for unconventional oil and gas resources. Here, a novel functional peak-aged Mg-9.5Gd-2.7Y-0.9Zn-0.8Cu-0.4Ni (wt.%) alloy for fracturing tools is reported, and it possesses an ultimate tensile strength of 457.6 MPa, ultimate compressive strength of 620.7 MPa and dissolution rate of ∼43.7 mg·cm−2·h−1 in 3 wt.% KCl solutions at 93 °C. The excellent strength of the aged-alloy is primarily attributed to the combination of grain refinement, long-period stacking ordered (LPSO) strengthening, and precipitation strengthening induced by stacking fault and β' phase, among which the precipitation strengthening is dominant. Further investigations confirm that the corrosion is triggered from the micro-galvanic coupling between the Mg matrix and the cathodic lamellar and block LPSO phases. Strip-shaped corrosion pits along with LPSO phases are subsequently formed, significantly accelerating corrosion. The β' precipitates can effectively improve the strength without compromising the dissolution rate because of their nanoscale size. This study provides an excellent material selection for dissolvable fracturing tools and presents a strategy by which a synergistic combination of strength and dissolution rate is achieved via peak-aging treatment. [ABSTRACT FROM AUTHOR]

A model specimen with a single boundary of the α/β phase simulating Mg-Al alloys was successfully fabricated by spark plasma sintering. A small electrode area of α phase or β phase was prepared using the model specimen, and the OCPs (open-circuit potentials) of each phase and a small electrode area containing the α/β phase boundary in 0.1 M NaCl at pH 8.0 were compared: the β phase exhibited a higher potential, and the α phase showed a lower potential. The OCP of the small area containing the α/β phase boundary was the intermediate value of these phases. In a small area containing α/β phase boundary, discoloration and gas bubbles were observed on the α phase, but no bubble generation was detected on the β phase. The gas bubbles were initially generated on the α phase near the β phase, but as the discoloration (corrosion) of the α phase approached the β phase, the bubbles were generated on the β phase. In micro-galvanic corrosion of the α and β phases, the β phase did not always function as the preferred cathode. The α phase partially corroded (or discolored) and became the anodes, so that the surrounding areas were most likely to be the cathodes. When corroded areas (anodes) in the α phase approached the β phase, the β phase would become cathodes. In addition to the micro-galvanic corrosion mechanism, the role of Al in corrosion resistance at the α/β phase boundary was determined by surface analysis. [ABSTRACT FROM AUTHOR]

In this study, the reason for the non-basal-plane-rich area more corrosion resistant than the basal-plane area on pure Mg in NaCl electrolyte was demonstrated based on quasi-in-situ EBSD observations. Due to the higher concentration of dissimilar crystallography orientations in the basal-plane-rich area, the gradient textures and 'micro-texture' galvanic couples were promoted. This exacerbates the progression of localized corrosion in basal-plane-rich area, ultimately deteriorating the overall corrosion performance of Mg. Randomizing the gradient texture diminishes the tendency towards micro-galvanic corrosion and enhances the corrosion resistance of Mg. These findings shed light on yet another facet of complex mechanism of Mg corrosion. • Quasi-in-situ EBSD observations were used to investigate the influence of texture on Mg corrosion. • The reason for higher corrosion resistance of non-basal-plane-rich area than basal-plane-rich area in Mg was identified. • A 'micro-texture' galvanic corrosion mechanism induced by gradient textures was proposed and rationalized. • The model elucidating localized corrosion mechanism affected by texture were established. [ABSTRACT FROM AUTHOR]

Dilute Mg–Al–Mn–Ca–Gd alloy with extraordinary corrosion resistance (P H = 0.21 mm/year) comparable to that of high-purity Mg was prepared. The Gd addition promoted the second phase transformation from the Al–Mn phase to the Al 8 Mn 4 Gd and Al 2 Gd phases with decreased potential difference with Mg matrix, thus weakening the micro-galvanic corrosion. The homogeneous distribution of nano-sized second phases effectively inhibited localised corrosion, contributing to the integrity of surface film. Furthermore, Gd alloying promoted the formation of compact corrosion products mixed with Gd 2 O 3 and embedded with the nano-sized Al 2 Gd phase, resulting in a stable and protective surface film. [Display omitted] • Dilute Mg alloy with corrosion resistance comparable to high-purity Mg is prepared. • Gd promotes the transformation of Al-Mn phase to less noble Al 8 Mn 4 Gd and Al 2 Gd phase. • The uniform distribution of dense nano-sized phase inhibits localised corrosion. • The surface film containing Gd 2 O 3 and nano-sized Al 2 Gd is more protective to matrix. [ABSTRACT FROM AUTHOR]

The AZ series of Mg alloys have become promising in several industrial fields thanks to its potential microstructure refinement and the β-Mg17Al12 eutectic that controls the mechanical behaviour. Simultaneously, the rapid degradation characterizing Mg alloys makes the investigation of their corrosion behaviour necessary. The present work considers high-pressure die cast (HPDC) AZ91 alloy to evaluate its corrosion behaviour in 1M NaCl solution and investigates how different friction stir process parameters can affect the corrosion responses. No studies analyse the effects induced by the friction stir processed zone, reached using high rotational speeds (>2000 rpm), on the unprocessed HPDC AZ91 alloy. In addition, the morphological analysis of the corroded surfaces having a friction stir processed zone, in which the grain refinement was not obtained, is not present in the literature yet. Microstructural features were investigated by optical microscope and X-ray diffraction analysis before and after the friction stir process. These were subsequently correlated to the corrosion responses after the immersion tests. The results show that HPDC samples with a very smooth surface have the best corrosion resistance with a corrosion rate lower than 3 mm/year, evaluated through the weight loss, compared to the rougher ones. Both the amount of β-Mg17Al12 eutectic and the wt.% Al in the α-Mg matrix, as well as the surface roughness, influence the corrosion behaviour of friction stir processed samples. The best corrosion resistance was obtained with an HPDC alloy processed at 2500 rpm and 50 mm/min.

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The initial corrosion process of dual phase (DP) steel produced by the continuous annealing process (CAP) in NaCl solution was systematically studied in this paper. The microstructure, nobility of the phases, and micro-galvanic corrosion behavior were characterized by scanning electron microscope (SEM), electron backscattered diffraction (EBSD) technique, scanning Kelvin probe force (SKPFM), and electrochemical impedance spectroscopy (EIS). Results indicate that there are two types of martensite, namely fresh martensite (FM) and tempered martensite (TM), embedded in the ferrite matrix. The two types of martensite are surrounded by dislocations and connect to each other as in a chain-like network at the ferrite grain boundary. Both FM and TM can act as a micro-cathode to promote the anodic dissolution of adjacent ferrite matrix since FM is up to 15 mV noble to the ferrite matrix and TM is to 30 mV noble. The initial corrosion process of DP steel can be divided into two stages. The first stage involves an increasing corrosion rate of the steel due to the successive emergence of cathodic TM and FM, while the corrosion rate decreases in the second stage because of the separation of martensite.

The limited wide applicability of commercial Mg alloys is mainly attributed to the poor corrosion resistance. Addition of alloying elements is the simplest and effective method to improve the corrosion properties. Based on the low-cost alloy composition design, the corrosion behavior of commercial Mg-3Al-1Zn (AZ31) alloy bearing minor Ca or Sn element was characterized by scanning Kelvin probe force microscopy, hydrogen evolution, electrochemical measurements, and corrosion morphology analysis. Results revealed that the potential difference of Al2Ca/α-Mg and Mg2Sn/α-Mg was (230 ± 19) mV and (80 ± 6) mV, respectively, much lower than that of Al8Mn5/α-Mg (430 ± 31) mV in AZ31 alloy, which illustrated that AZ31-0.2Sn alloy performed the best corrosion resistance, followed by AZ31-0.2Ca, while AZ31 alloy exhibited the worst corrosion resistance. Moreover, Sn dissolved into matrix obviously increased the potential of α-Mg and participated in the formation of dense SnO2 film at the interface of matrix, while Ca element was enriched in the corrosion product layer, resulting in the corrosion product layer of AZ31-0.2Ca/Sn alloys more compact, stable, and protective than AZ31 alloy. Therefore, AZ31 alloy bearing 0.2wt% Ca or Sn element exhibited excellent balanced properties, which is potential to be applied in commercial more comprehensively. [ABSTRACT FROM AUTHOR]

In the present work, the corrosion behavior of a twin‐roll cast WZ73 magnesium alloy is investigated by potentiodynamic polarization and climate chamber tests in different sodium chloride solutions. The corrosion parameters Ecorr and jcorr are extracted from the polarization curves, and the critical concentration for the initiation of pitting corrosion is thus identified. The critical concentration for the initiation of pitting is confirmed by means of climate chamber tests. As a consequence, the WZ73 magnesium alloy is found to be highly susceptible to pitting corrosion in sodium chloride solutions of concentrations higher than 10−4 mol/L. Moreover, when the NaCl concentration increases, the corrosion mechanism changes from pitting to filiform corrosion, which is attributed to micro‐galvanic effects. At low NaCl concentrations, the corrosion attack remains localized. When the NaCl concentration increases, pitting occurs and, in addition to the pits that form, micro‐galvanic corrosion leads to filiform corrosion attack. This is caused by the electrochemical potential of the long period stacking ordered phase, which is higher than that of the α‐Mg matrix. Consequently, the corrosion attack is focused on the phase boundary between the α‐matrix and LPSO phases. [ABSTRACT FROM AUTHOR]

Effect of the second phase in the micro-galvanic corrosion of a commercial Mg alloy containing rare earth elements, cast WE43 alloy, was investigated in 0.6 M NaCl solution and 0.6 M Na 2 SO 4 solution by scanning electron microscopy (SEM) observations, scanning Kelvin probe force microscopy (SKPFM) analysis, hydrogen evolution, weight loss measurement, and electrochemical techniques. It is confirmed that the second phase of cast WE43 alloy is more active than Mg matrix and exhibits an anodic role in the micro-galvanic corrosion with α-Mg matrix as cathode and dissolves preferentially in Na 2 SO 4 solution, in contrast to the situation in NaCl solution. The corrosion rate of cast WE43 alloy in Na 2 SO 4 solution is much higher than that in NaCl solution, which is different from the conventional wisdom and could be attributed to the different role of the second phase in the micro-galvanic corrosion in two solutions. [ABSTRACT FROM AUTHOR]

Abstract In the present work, the corrosion behavior of a twin‐roll cast WZ73 magnesium alloy is investigated by potentiodynamic polarization and climate chamber tests in different sodium chloride solutions. The corrosion parameters Ecorr and jcorr are extracted from the polarization curves, and the critical concentration for the initiation of pitting corrosion is thus identified. The critical concentration for the initiation of pitting is confirmed by means of climate chamber tests. As a consequence, the WZ73 magnesium alloy is found to be highly susceptible to pitting corrosion in sodium chloride solutions of concentrations higher than 10−4 mol/L. Moreover, when the NaCl concentration increases, the corrosion mechanism changes from pitting to filiform corrosion, which is attributed to micro‐galvanic effects. At low NaCl concentrations, the corrosion attack remains localized. When the NaCl concentration increases, pitting occurs and, in addition to the pits that form, micro‐galvanic corrosion leads to filiform corrosion attack. This is caused by the electrochemical potential of the long period stacking ordered phase, which is higher than that of the α‐Mg matrix. Consequently, the corrosion attack is focused on the phase boundary between the α‐matrix and LPSO phases.

This study aims to process Mg alloys using rotary swaging technique to enhance their corrosion resistance and mechanical properties. By optimizing the rotary swaging technology, precise control over the Mg alloy structure was achieved, manifesting as the refinement of grains, the generation of high-density dislocations, the decrease of potential difference between second phase and Mg matrix, and a more uniform distribution of second phase. Microstructural analysis and mechanical tests confirmed the beneficial effects of grains refinement and high-density dislocations on mechanical properties of Mg alloys. Further corrosion performance tests demonstrated that the grain refinement and weak micro-galvanic corrosion caused by low potential difference between second phase and Mg matrix, and a more uniform distribution of second phase effectively improve the corrosion resistance of Mg alloys. Through rotary swaging, the corrosion rate of Mg-8Gd-3Y-0.4Zr (GW83) alloy decreased from 7 to 0.4 mm/y and the tensile stress increased from 210 to 435 MPa synergistically. This research offers a new approach for optimizing the processing of Mg alloys, demonstrating feasibility and potential application prospects in both mechanical and corrosion performance. • The corrosion and mechanical properties of Mg-8Gd-3Y-0.4Zr alloy were improved synergistically. • The RGW83 alloy has a lower corrosion rate than CGW83 alloy and EGW83 alloy. • The potential difference decreased from 250 mV to 150 mV and an anodic second phase with a −40 mV potential emerged. • The uniform second phase is the key to more uniform and weaker micro-galvanic corrosion. [ABSTRACT FROM AUTHOR]

Effects of microstructural evolutions on electrochemical corrosion properties and passive film characteristics of precipitation-strengthened (Ni 2 Co 2 FeCr) 92 Al 4 Nb 4 high-entropy alloys (HEAs) were investigated in a saline environment. Compared with single-phase FCC alloys, the appearance of nano-sized L1 2 particles increases the metastable pitting activities and slightly reduces the pitting potential, while the lamellar D0 19 phase further deteriorates the localized corrosion resistance. The chemical composition and oxide state of passive films are closely related to the different precipitation scenarios. The corrosion morphologies of the single-phase and L1 2 -strengthened alloys are dominated by pitting, whereas the intergranular corrosion (IGC) and intragranular dealloying-like corrosion occur in the D0 19 -strengthened alloy. [Display omitted] • Effects of the L1 2 phase and D0 19 phase on the electrochemical corrosion behavior of HEAs were revealed. • The characteristics of surface passive films are closely related to the precipitation scenarios. • The structure-corrosion-mechanics relationship of HEAs was established. • A bilayer passive film with a Fe-rich outer layer and a Cr-rich inner layer was directly observed in the L1 2 -strengthened HEA. • IGC occurs in the D0 19 -strengthened HEA due to the localized micro-galvanic corrosion. [ABSTRACT FROM AUTHOR]

The corrosion behavior and corrosion resistance of WE43 and AZ80 immersed in NaCl and Na2SO4 solutions were investigated, respectively. Two alloys were immersed in 0.6 M NaCl and Na2SO4 solution to observe the corrosion morphologies. Hydrogen evolution and weight loss experiments were conducted to obtain the corrosion rates. Electrochemical tests were used to characterized detailed corrosion situation. The results show that, when immersed in Na2SO4 solution, WE43 alloy shows a unique micro-galvanic corrosion behavior. On the other hand, the corrosion rate of WE43 in Na2SO4 solution is much faster than that in NaCl solution, which is the direct opposite of AZ80 and most magnesium alloys. The protection of the surface film maybe the key factor to the unexpected phenomena.

In this study, the influence mechanism of rare earth elements Ce and Nd on the corrosion behavior of Mg alloy was studied by combining experimental characterization with the first-principles calculation. The changes of work function and surface energy of the rare Earth phase and matrix phase in binary rare Earth Mg alloys Mg-0.3%Ce and Mg-0.3%Nd. In addition, the electronic properties of the second rare Earth phase and matrix phase, and the correlation between the interface energy of the second rare Earth phase and matrix and the corrosion behavior of Mg alloys were investigated. The experimental results show that Mg _12 Ce and Mg _12 Nd as the second phases of Mg-0.3%Ce and Mg-0.3%Nd, respectively, have not obvious roles in the galvanic corrosion of the alloys. The corrosion morphology of the two alloys is similar, the matrix undergoes filamentary corrosion through the grain to a large area. The results of the first-principles calculation show that the binding energy of the rare Earth phase is higher than that of the matrix phase, this implied the addition of light rare Earth elements Ce and Nd can improve the stability of the second phase. The work function of different surfaces of the second phase is similar to that of the matrix phase, so limited effect of the rare Earth phase on corrosion behavior has been found. In addition, the addition of rare Earth Ce and Nd will decrease the surface stability of Mg matrix, this result reaffirms the higher corrosion rates of Mg-0.3%Ce and Mg-0.3%Nd over that of the pure Mg.

Mg-Sn-Y alloys with different Sn contents (wt%) were assessed as anode candidates for Mg-air batteries. The relationship between microstructure (including the second phase, grain size, and texture) and discharge properties of the Mg-Sn-Y alloys was examined using microstructure observation, electrochemical measurements, and galvanostatic discharge tests. The Mg-0.7Sn-1.4Y alloy had a high steady discharge voltage of 1.5225 V and a high anodic efficiency of 46.6% at 2.5 mA·cm−2. These good properties were related to its microstructure: small grain size of 3.8 µm, uniform distribution of small second phase particles of 0.6 µm, and a high content (vol%) of (11 2 ¯ 0 )/(10 1 ¯ 0 ) orientated grains. The scanning Kelvin probe force microscopy (SKPFM) indicated that the Sn3Y5 and MgSnY phases were effective cathodes causing micro-galvanic corrosion which promoted the dissolution of Mg matrix during the discharge process. [ABSTRACT FROM AUTHOR]

The poor corrosion resistance of magnesium alloys is one of the major obstacles to their widespread applications in the engineering field and the medical field. A hot deformation process is the main way to improve the corrosion resistance of magnesium alloys. In the present study, X-ray photoelectron spectroscopy (XPS), a scanning electron microscope (SEM), an electron probe micro-analyzer (EPMA) and X-ray diffraction (XRD) were used to investigate the micro-galvanic corrosion behavior and film protection mechanism of hot-rolled Mg-2Zn-2Er-0.3Zr-0.3Mn under 25%, 50% and 75% thickness reductions in Hank’s solution. The results revealed that the best corrosion resistance was obtained in the alloy under a 75% thickness reduction, with a corrosion rate of 0.85 mm/y. The improvement in anti-corrosion was due to the coupling effect of a refined microstructure and dense degradation film on the large deformation hot-rolled alloy. Furthermore, the elements Mn and Er participated in the film formation and stabilized the film structure.

In the absence of any abnormality (standard conditions), the gradient of any mechanical/thermodynamic stress would be intensified at the dissimilar joint due to an abrupt change in the chemical composition. This paper aims to investigate the effect of delocalizing this stress by imposing an optimum chemical gradient within the dissimilar joint. In this work, we computationally demonstrated that a homogenous distribution of magnesium atoms in the aluminum (100) structure with a specific chemical gradient could potentially reduce the susceptibility of the Mg/Al dissimilar joint towards micro-galvanic corrosion. This is achieved through the minimization of the work function gradient within the dissimilar joint. [ABSTRACT FROM AUTHOR]

The 6061-T6 aluminum alloy welding joints were fabricated using gas metal arc welding (GMAW) of various laser powers, and the effect of laser power on the microstructure evolution of the welding joints was investigated. The corrosion behaviors of 6061-T6 aluminum alloy welding joints were investigated in 3.5 wt% NaCl solution using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The results showed that the micro-galvanic corrosion initiation from Mg2Si or around the intermetallic particles (Al-Fe-Si) is observed after the immersion test due to the inhomogeneous nature of the microstructure. The preferential dissolution of the Mg2Si and Al-Fe-Si is believed to be the possible cause of pitting corrosion. When the laser power reached 5 kW, the microstructure of the welded joint mainly consisted of Al-Fe-Si rather than the Mg2Si at 2 kW. The relatively higher content of Al-Fe-Si with increasing in laser power would increase the volume of corrosion pits.

The corrosion behavior of Mg-10Gd–xZn (x = 2, 6 wt.%) alloys in 0.5 wt.% NaCl solution was investigated. Microstructures of both the alloys consisted of (Mg,Zn)3Gd phase and lamellar long period stacking ordered (LPSO) phase. The morphology of the second phase at the grain boundary differed in both alloys: it was a continuous network structure in Mg–10Gd–6Zn, whereas it was relatively discrete in Mg–10Gd–2Zn. The dendrites were finer in size and highly branched in Mg–10Gd–6Zn. The corrosion results indicated that the increase in Zn content increased the corrosion rate in Mg–10Gd–xZn alloys. Micro-galvanic corrosion occurred near the grain boundary in both alloys initially as the grain boundary phase was stable and acted as a cathode, however, filiform corrosion dominated in the later stage, which was facilitated by the LPSO phase in the matrix. Severe micro-galvanic corrosion occurred in Mg–10Gd–6Zn due to the higher volume of second phase. The stability of the second phase at the grain boundary was altered and dissolved after the long immersion times. Probably the NaCl solution chemically reacted with the grain boundary phase and de-stabilized it during the long immersion times, and was removed by the chromic acid used for the corrosion product removal.

In the absence of any abnormality (standard conditions), the gradient of any mechanical/thermodynamic stress would be intensified at the dissimilar joint due to an abrupt change in the chemical composition. This paper aims to investigate the effect of delocalizing this stress by imposing an optimum chemical gradient within the dissimilar joint. In this work, we computationally demonstrated that a homogenous distribution of magnesium atoms in the aluminum (100) structure with a specific chemical gradient could potentially reduce the susceptibility of the Mg/Al dissimilar joint towards micro-galvanic corrosion. This is achieved through the minimization of the work function gradient within the dissimilar joint.

The influence of crystal orientation on the corrosion behaviour of twin roll cast (TRC) AZ31 Mg alloy sheet has been investigated using correlative microscopy and immersion testing in 3.5% NaCl. Corrosion initiates as a consequence of the microgalvanic coupling between cathodic intermetallic phases, including sub-micron sized, rosette-shaped Al 8 Mn 5 particles, β-Mg 17 (Al,Zn) 12 phase and the Mg matrix. Subsequent development of lateral filiform-like corrosion fronts are observed running along the dendrite arms whilst being constrained by the Al- and Zn-rich interdendritic boundaries. Prolonged immersion showed variable corrosion depths across the surface. By correlating with EBSD analysis, it was confirmed that the corrosion depth is considerably higher in α-Mg grains with non-basal planes on the surface. The severity of attack suffered by α-Mg grains with non-basal planes on the surface is on average twice that for grains with basal or near-basal with planes on the surface. At the same time, corroded α-Mg grains with basal planes on the surface display nanoscale, canyon-like corrosion features developing into the alloy perpendicularly. A mechanism has been proposed to explain the sequential corrosion events. [ABSTRACT FROM AUTHOR]

Effect of phase structure and composition on the passive film properties and pitting behavior of L2 1 -strengthened (CoCrFeNi) 86 Al 7 Ti 7 high-entropy alloy (HEA) in 3.5 wt.% NaCl was investigated. Results shown that the bilayer amorphous passive film enriched in Cr-, Fe-oxides/hydroxides on (Fe, Cr)-rich FCC phase and that enriched in Ti-, Ni-oxides on (Ni, Al, Ti)-rich L2 1 phase both contributed to the well pitting resistance. There was a micro-galvanic corrosion effect between FCC phase (cathode) and L2 1 phase (anode), which in synergy with Cl- broken the passive film on L2 1 phase at two-phase interface and resulted in pitting nucleation. • Pitting behavior of L2 1 -strengthened (CoCrFeNi) 86 Al 7 Ti 7 HEA in NaCl was studied. • Well pitting resistance resulted from the bilayer amorphous passive film. • Passive film on FCC phase was better protective than that on L2 1 phase. • There was a MGC effect between FCC phase (cathode) and L2 1 phase (anode). • Synergy of MGC and Cl- caused pitting to nucleate on L2 1 phase at FCC/L2 1 interface. [ABSTRACT FROM AUTHOR]

• Corrosion fatigue failure mechanism of the high-strength steel weld joint was proposed. • The Volta potential variation among various subzones induced the micro-galvanic corrosion. • The micro-galvanic corrosion effect played a predominant role in the corrosion fatigue cracking of the weld joint. • The micro-galvanic corrosion between WM and CGHAZ was confirmed by SKP and SECM. The local electrochemical and corrosion fatigue cracking behavior of the weld joint of Q690qE high-strength bridge steel was investigated in simulated marine environment. It reveals the weld joint was vulnerable to corrosion fatigue failure in polluted marine atmosphere with the corrosion fatigue life considerably lower than that of parent metal. The micro-galvanic corrosion effect between CGHAZ and WM played a dominant role in the corrosion fatigue failure of the weld joint, leading to the final fracture mostly at CGHAZ. The micro-galvanic corrosion was caused by Volta potential variation among various subzones, which was evidenced by local electrochemical techniques. [ABSTRACT FROM AUTHOR]

The underlying mechanism of solute redistribution during heat treatment in affecting corrosion behavior of Mg-1.19Al-0.28Ca-0.44Mn (wt%) alloy was systematically clarified. The supersaturated Al and Ca solutes in matrix effectively alleviate the driving force of micro-galvanic corrosion caused by Al-Mn phases, based on positive shift of matrix potential and film densification. After aging treatment, monoatomic-layer G.P. zones realize higher strength and better corrosion resistance of alloy as compared to nanosized Al 2 Ca, due to higher number density and less solute consumption in matrix. Our work provides guidance to minimize corrosion loss of Mg by regulating nucleation and growth of precipitates. [Display omitted] • Underlying corrosion mechanism is clarified by atomic characterization & simulation. • The supersaturated Al and Ca solutes alleviate the driving force of galvanic effect. • Mn element plays a negative effect in corrosion, due to formation of Al-Mn phases. • G.P. zone is the key to attaining strength-corrosion synergy of dilute Mg alloy. [ABSTRACT FROM AUTHOR]