Secondary Passivation Research Articles

Research on the Corrosion Resistance and Cytotoxicity of Medical Forged Co-28Cr-6Mo Alloy

Co-Cr-Mo alloy as a human body implant material has a long history, because of its excellent corrosion resistance and biocompatibility, and is widely used in human hip joint materials. Co-Cr-Mo alloy in the human body is often in a passivation state; the formation of dense oxide film on the alloy surface prevents further corrosion of the alloy. The main component of the passivation film is the oxide of Cr, so a layer of oxide film formed by Cr on the surface of Co-Cr-Mo alloy is the reason for its good corrosion resistance. In biocompatibility, cytotoxicity is the first choice and necessary option for biological evaluation, and cytotoxicity can quickly detect the effect of materials on cells in a relatively short time. Therefore, this research conducted a comparative evaluation on the corrosion resistance and biocompatibility of forged Co-Cr-Mo alloys produced in domestic and foreign alloys in line with medical standards. Three simulated human body fluids and Princeton electrochemical station were selected for corrosion resistance experiments, and it was found that the corrosion resistance of four alloys in sodium citrate solution inside and outside China would be reduced. All the alloys exhibit secondary passivation behavior in Hanks solution, which improves the corrosion resistance of the alloys. According to the self-corrosion potential Ecorr analysis, the corrosion resistance of domestic B alloy is the best, while that of foreign R31537 alloy is poor. In the biocompatibility experiment, the biocompatibility of Co-Cr-Mo alloy was evaluated through the measurement of contact Angle and cytotoxicity reaction. The experimental results show that Co-Cr-Mo alloy is a hydrophilic material, and the contact Angle of foreign R31537 alloy is smaller, indicating that the surface of R31537 alloy is more suitable for cell adhesion and spreading. According to the qualitative and quantitative analysis of the cytotoxicity experiment, the toxic reaction grade of domestic A, B and R31537 alloy is grade 1, the toxic reaction grade of C alloy is grade 2, and C alloy has a slight toxic reaction.

Characterizing the combined impact of nucleation-driven precipitation and secondary passivation on carbon mineralization

The evolution of mineral reactive surface area is one of the primary phenomena controlling the progression and extent of mineral carbonation. The CO2 mineralization begins with nucleation of crystals that provide initial surface area for subsequent growth of the mineral. However, many reactive transport models (RTMs) for CO2 mineralization do not include the nucleation process. The few RTMs that do include it are yet to be validated against experimental data. Similarly, many RTMs ignore passivating effects of the secondary mineral, which coats the surface of the dissolving mineral, slow down the reaction process, and reduce the total extent of carbonation. Furthermore, the combined impact of nucleation and passivation on carbon mineralization is yet to be properly characterized. In this study, we consider the coupled effects of passivation and nucleation on the mineralization extent. The nucleation-driven precipitation model relies on the formation of nuclei to provide a surface area for crystal growth, while a new model is proposed to account for passivation effects. Our analysis shows that (i) omission of nucleation leads to overestimation of extent of mineralization, and (ii) omission of passivation leads to overestimation of host rock reactivity. The model was evaluated via comparison with CO2 mineralization data from the literature and models that ignore these processes. We observed that including nucleation and passivation lead to closer predictions of the CO2 mineralization extent. Therefore, this study highlights the importance of including the coupled nucleation-driven precipitation and secondary passivation in RTMs. The findings from the study can be applied in various scientific and engineering applications such as petroleum production, cement carbonation, CO2 sequestration, chemical weathering, and concrete degradation.

Carboxymethyl cellulose enhanced Cr(VI) removal of zero-valent iron via the coating anti-passivation mechanism

Competition for electrons from dissolved oxygen and secondary passivation resulting from the deposition of FeCr hydroxide are two challenges that impede the removal of hexavalent chromium (Cr(VI)) with zero-valent iron (ZVI). In this study, we fabricated micron-sized zero-valent iron modified with carboxymethyl cellulose (CMC-ZVI) via a mechanical ball milling process and demonstrated its improved performance up to eight times for Cr(VI) removal. Owing to the CMC protective film, the inhibition of dissolved oxygen for Cr(VI) removal, aging of ZVI in air, and secondary passivation of ZVI by the deposition of FeCr hydroxides were all inhibited. Even after a week of continuous aging, the CMC-ZVI maintained a stable Cr(VI) removal efficiency of over 50%. The excellent hydrophilicity of CMC enhanced the interaction of CMC-ZVI with water and Cr(VI), thus improving its capacity for Cr(VI) removal. Furthermore, CMC-ZVI not only proficiently reduced and completely removed the high-mobility Cr(VI) from actual electroplating effluent, but also effectively alleviated the inhibition of Cr(VI) removal via CMC-ZVI by aeration and air aging. The efficiency of CMC-ZVI in removing Cr(VI) from actual electroplating effluent was improved by over 41 times compared to pristine ZVI. This advantage has the potential to make the storage and transportation of ZVI more affordable and convenient and increases its potential for practical applications. This study provides an efficient Cr(VI) removal strategy using ZVI for environmental remediation and a strategy for the production of highly reactive, aging-resistant, easily stored, and transported zero-valent iron with the potential for practical applications.

Corrosion behavior and passivation property of CoCrFeNi HEA in a simulated seawater environment containing CO32−/HCO3−

AbstractThis work systematically studied the corrosion behavior and passivation property of CoCrFeNi high‐entropy alloys (HEAs) in a simulated seawater environment. The results reveal that the addition of CO32−/HCO3− results in the secondary passivation of the HEA in NaCl solution, and secondary passivation film possesses higher dissolution rate in comparison with primary passive film. H+ ions ionized by HCO3− facilitate the charge transfer process, thin the thickness of passive film, and increase the disordered degree of the film. Moreover, the presence of HCO3− promotes the nucleation and growth of metastable pits, and enhances the pitting sensitivity. Furthermore, an increase in CO32− concentration accelerates the dissolution of passive film, weakens the compactness and protective performance of the film, and increases the probability that metastable pitting evolves into stable pitting, as well aggravates the corrosion of the HEA. In addition, the corrosion resistance of the HEA is diminished due to the formation of microgalvanic corrosion cells between matrix and inclusion. The selective dissolution of elements occurred in the HEA.

(Corrosion Division Morris Cohen Graduate Student Award) Evolution of Passivity and Passivity-Breakdown for Cr and Cr-Containing Alloys

Among several corrosion prevention strategies for metallic alloys, alloying with so-called ‘passivating’ elements remains the most effective approach. For example, chromium (Cr) remains the key functional alloying element for Fe-based stainless steels, Ni-based Inconel® alloys and Co-based Stellite® alloys, which dominate the family of corrosion resistant alloys (CRAs) used in a wide range of applications. In addition to conventional alloys, recent exploration in the materials community has led to development of an emerging class of alloys, so-called high entropy alloys (known more broadly as multi-principal element alloys (MPEAs)), which contain multiple alloying elements in ‘principal’ fractions – and of which the corrosion performance has not been widely studied to date. It is however challenging to ascertain the key controlling factors responsible for the passivity in MPEAs using conventional electrochemical techniques due to the combination of low sensitivity and the lack of element resolved information. Moreover, the physical and chemical properties of the nano-metric thin passive film upon CRAs are challenging to evaluate using conventional microscopy and spectroscopy. Consequently, several fundamental aspects of passivity of Cr-containing CRAs remain open questions to be elucidated.The work herein sought to investigate the passivity and dissolution of Cr and Cr-containing alloys, including stainless steel and several MPEAs; primarily using an in-situ electrochemical method called atomic-spectroelectrochemistry (ASEC). ASEC uses a modified flow electrochemical cell coupled with an atomic spectrometer, revealing the real-time and element-resolved dissolution rate during electrochemical testing. In addition to ASEC analysis, the project herein employed a range of electrochemical testing, electron microscopy, X-ray photoelectron spectroscopy, and thermodynamic calculations to explore Cr-based passivity.Significance between composition and growth kinetics on the passivity of Cr and Cr-containing alloys was established. It was found that the passive films upon Cr and Cr-containing alloys (including MPEAs) obey a direct-logarithmic growth law, consistent with the assumptions of the point defect model. In addition, the results unveiled a complex interdependence of oxidation rate of material and dissolution rate of the passive film during anodic polarization, found to be an important parameter to evaluate barrier properties of passive films. The influence of microstructural features including grain boundaries and chemical heterogeneities on Cr-based passivity was explored for stainless steel 316L; whereas the role of alloying elements and electrolytic on the evolution of compositional morphology and thickness of the Cr2O3-based passive film was studied for MPEAs exhibiting single-phase microstructures. The origin of passive film breakdown and role of potential and chloride ions on the breakdown mechanism were elucidated for stainless steel 316L; whereas transpassive dissolution and origin of secondary passivation were studied for equiatomic CoCrFeNi and AlTiVCr MPEAs.Such insights advance the knowledge of underlying mechanisms and associated rate controlling factors of passivity in Cr and Cr-containing alloys, aiding towards an ‘oxide-by-design’ paradigm, to guide design of new CRAs.

Effect of pH on corrosion behaviour of CoCrNi MEA imposed by alternating current in Na2CO3/NaHCO3 solution

ABSTRACT The corrosion behaviour and mechanism of CoCrNi medium-entropy alloy (MEA) imposed by alternating current (AC) in HCO3 −/CO3 2− solution with various pH were systematically investigated via a series of corrosion tests. The results showed that with the augment of pH value, the polarisation resistance of the MEA decreased significantly, Ep shifted negatively, and ipI value increased, as well as corrosion rate was enhanced, revealing that the corrosion resistance was weakened. As solution pH increased, the formation of secondary passivation zone became easier, which may be attributed to the reaction conversion process of Cr(III) to Cr(VI) species. Moreover, the increase in pH led to an increase in defect density and a thinning of passivation film, facilitating the corrosion of the MEA. AC interference accelerated the ion exchange process and promoted the formation possibility of stable pitting. Higher iAC and pH inhibited the repassivation of metastable pitting, and increased the pitting susceptibility. Highlights The effect of pH on the corrosion behaviour of CoCrNi MEA with imposed AC was clarified. High pH and iAC accelerated the nucleation and growth of metastable pits. With increased pH and iAC, the stability and protective ability of passive film were weakened. The secondary passivation of the MEA was mainly due to the conversion of Cr(III) to Cr(VI) species.

Effect of Heat Treatment on the Corrosion Resistance of AlFeCoNiMo0.2 High-Entropy Alloy in NaCl and H2SO4 Solutions

The effects of casting and different heat treatment processes on the corrosion resistance of AlFeCoNiMo0.2 high-entropy alloy in 3.5% NaCl (mass fraction) and 0.5 mol/L H2SO4 solutions were investigated using dynamic potential polarization curves, SEM, XRD, XPS, and other test methods. The results show that in the Cl− environment, the cast alloy has the lowest corrosion current density and higher corrosion resistance compared to the annealed alloy. The elements Al and Mo are severely segregated in the crystal and in the grain boundaries, where galvanic corrosion occurs, and the Al-rich phase produces pitting corrosion in the crystal. The main components of its passive film are oxides of Al, Fe, Co, and Mo, and oxides and hydroxides of Ni. In the SO42− environment, the best corrosion resistance is achieved in the 900 °C annealed state of the alloy. Electrochemical test results show that the alloys all undergo secondary passivation, producing two successive product films to protect the metal matrix. Preferential corrosion areas are concentrated in the molybdenum-rich grain boundaries and nearby dendritic regions, reducing the corrosion resistance of the alloy. The main components of the passive film are oxides of Al and Mo; oxides of Fe, Co, Ni; and hydroxides. The Mo element in the passive film prevents the activated dissolution of Fe and produces the protective component MoO3, which inhibits the dissolution of the alloy and improves the stability of the passive film. The presence of Mo elements increases the selective dissolution of Fe, and the aggregation of Mo elements at grain boundaries after annealing weakens the corrosion resistance of the alloy and leads to the dissolution of the passive film. The main components of the passive film are oxides of Al and Mo; oxides of Fe, Co, Ni; and hydroxides.

Electrochemical Corrosion Behavior of Passivated Precipitation Hardening Stainless Steels for Aerospace Applications

Precipitation-hardening (PH) stainless steels (SS) are widely used in various aerospace applications. These steels exhibit good mechanical and corrosion resistance. The electrochemical behavior of 15-5PH, 17-4PH, Custom450 and AM 350 stainless steels passivated with citric and nitric acid baths for 60 and 90 min at 25 and 49 °C were evaluated in 5 wt.% sodium chloride (NaCl) and 1 wt.% sulfuric acid (H2SO4) solutions. The electrochemical behavior was studied with potentiodynamic polarization curves (PPC) according to the ASTM G5-13 standard. The results indicated that there are two characteristic mechanisms that are present in the potentiodynamic polarization curves. When the PHSS is immersed in an H2SO4 solution, there is a secondary passivation, and in the NaCl solution, there is a pseudo-passivation (not stable passivation film). The current densities in the NaCl solution were between 10−4 and 10−5 mA/cm2, while those of H2SO4 were recorded around 10−2 and 10−3 mA/cm2. Citric acid does work as a passivating solution, and in some cases, the corrosion resistance of the stainless steel was comparable to that of nitric acid.

Synthesis and corrosion behavior of Mo15Nb20Ta10Ti35V20 refractory high entropy alloy

The Mo15Nb20Ta10Ti35V20 refractory high entropy alloy with BCC structure was prepared by vacuum arc melting. The synthesized Mo15Nb20Ta10Ti35V20 demonstrated better corrosion resistance compared to 316 stainless steel in chloride corrosion environment, whose better corrosion resistance is attributed to the complex passive film structure and shows the characteristics of passivation-transpassivation-secondary passivation on the polarization curve. Mott-Schottky curves analysis and XPS results show that the primary and secondary passive films have different P-N junctions which were composed of different oxides showing bilayer structure characteristics. Through the thermodynamic calculation, the relationship between the competitive growth of multiple principal elements in RHEA were analyzed. The dynamic changes of TiO2 and Ti2O3 contents were responsible for the secondary passivation phenomenon.

Broadening the pH range of molecular oxygen activation for zero-valent aluminum by ball-milling surface modification

In the zero-valent aluminum (Al0)/O2 system, due to the oxide film covering the Al0 surface and secondary passivation occurring during the reaction, the drawbacks of long induction period (∼2 h) and requiring strong acidic conditions (pH < 4) limit its wide application. It is possible to achieve efficient activation of molecular oxygen over a wide pH range for Al0 by ball milling (BM) surface modification. In this study, a novel wide-pH-applicable and zero-induction-period Al0-based composite was synthesized with one-pot BM method by directly mixing solid-phase microscale Al0, ethylenediaminetetraacetic acid ferric sodium salt (FeEDTA) and carbon nanotubes (CNTs). The excellent performances under different reaction conditions and the characterizations of Al0-based composite were analyzed. At initial pH of 2–10, the removal efficiency of phenol by Al0-based composite was more than 95% under atmospheric conditions without any induction period. Through BM process, the oxide layer of Al0 was destroyed by FeEDTA and further activating Al0 surface and increasing its reactivity. Meanwhile, CNTs were attached on the fresh Al0 surface to prevent secondary passivation and catalyze the reduction reaction between inert Al0 and O2. This study provides a green and convenient strategy for realizing the extensive application of Al0/O2 system.

Effect of ceramic types on the microstructure and corrosion behavior of titanium matrix composites produced by selective laser melting

To find out which ceramic particles can efficiently improve the corrosion resistance of commercial pure titanium (CP-Ti), the ex-situ formed Ti-2.5TiN composite and in-situ synthesized Ti-2.5TiB composite were developed by selective laser melting (SLM) and their corrosion behavior in 3.5 wt% NaCl solution was investigated. The open-circuit potentials, polarization curves, and electrochemical impedance spectroscopy were measured and compared. The results demonstrated that the corrosion resistance of SLM-produced Ti-2.5TiB composite possesses is better than that of the SLM-produced Ti-2.5TiN and CP-Ti samples after immersion for 0 day (without immersion). Due to tiny TiB and TiB 2 particles acting as the micro-cathode in the titanium matrix, anodic dissolution of the titanium matrix was prominently facilitated in the early corrosion stage, followed by rapid passivation on the surface of the titanium matrix. However, the corrosion resistance of SLM-produced Ti-2.5TiN composite possesses is better than the SLM-produced Ti-2.5TiB and CP-Ti after immersion for 5 and 10 days. The fine grain size, the δ-TiN phase, and the higher relative density can help the Ti-2.5TiN surface to form a stable passive film. • CP-Ti, Ti-2.5TiN and in-situ Ti-2.5TiB composites are produced by selective laser melting. • Ti-2.5TiN has higher relative density and finer grain size than Ti-2.5TiB composite. • Secondary passivation behavior confirms that the passive film is a double-layer structure. • Refinement grains and δ-TiN phase improve corrosion resistance of Ti-2.5TiN. • The effect of ceramic particle types on the corrosion resistance of CP-Ti was discussed.

Corrosion of metallic bipolar plates accelerated by operating conditions in a simulated PEM fuel cell cathode environment

Metallic bipolar plates (BPs) are extensively applied in fuel cell stacks to achieve an extremely high power density, and are thus suitable for vehicle applications. Corrosion of metallic BPs and contamination on membrane electrode assemblies have become key issues. Automotive operating conditions significantly accelerate BP corrosion and surface destruction. Accelerated stress tests, simulating dynamic load and startup-shutdown, are conducted to reveal the operation-induced corrosion mechanisms of typical SS316L BPs. The dynamic potential in the normal range (0.6–0.95 V) accelerates the local breakdown and damage of the passivation layer, leaving pits rich in Cr-species. The abundant Cr-species in the pits prevents further dissolution of Fe-speices and relieves local corrosion. The startup-shutdown condition extends the cathode potential to the trans-passivation and secondary passivation regions. Startup-shutdown drives BP into complex evolution stages of corrosion and passivation. Frequent alternation of potentials between passivation and trans-passivation regions accelerates alternant dissolution of outer Fe-species and Cr-species, thus causing global destruction represented by flocculent microstructure and interlinked pits. The ultrahigh cathode potential in startup-shutdown may induce an obvious surface film via strong secondary passivation. This paper can further guide the test protocols to evaluate the durability of metallic BP and condition optimization to avoid extreme corrosion damage.

Unveiling the transpassive film failure of 3D printing transition alloys

The failure of transpassive films on transition metal alloys was investigated. The results show that the failure of the transpassive film on 3D printing nickel-based superalloy in flowing NaNO3 solution is dominated by self-induced cracking of the film on niobium-segregated phases and the erosion-induced degradation on the film assisted by cation vacancy condensation at the metal/barrier layer interface. Secondary passivation induces the formation of a transpassive film, the nature of which has been debated for decades. The findings are consistent with a bi-layer structure comprising a point-defective barrier layer that grows into the substrate alloy and a precipitated outer layer.

Azo‐Initiator‐Induced Cascade Defect Passivation for Efficient and Stable Planar Perovskite Solar Cells

Remarkable progress in perovskite solar cells (PSCs) has been made by virtue of surface and bulk modification for defect control of perovskite layers, however, the interior defects in the perovskite layer can be hardly passivated by means of conventional passivation strategies, which makes perovskites prone to decomposition with inferior device performance. Here, the authors propose a cascade defect passivation strategy to doubly reduce perovskite defects by introducing a series of azo radical initiators. The primary passivation process of the azo compounds is realized on the perovskite surfaces through strong coordination interactions between Pb2+ and carbonyl/cyano groups. The secondary passivation occurs after thermal treatment, and the decomposed products diffuse into the interior defects of the initial passivated perovskite films through grain boundaries to accomplish the cascade defect passivation. Excitingly, this passivation strategy inhibits unwanted defect‐assisted recombination effectively, and thus CH3NH3PbI3 (MAPbI3)‐based inverted PSCs exhibit a significant increase in power conversion efficiency (PCE) from 16.92% to 19.69%. Moreover, the dual‐passivated PSCs show only 10% PCE loss after 3000 h in an inert atmosphere. Overall, the first proposed cascade defect passivation strategy is highly efficient in promoting defect healing of perovskites, demonstrating a new way to prepare high‐quality perovskite films for high‐performance PSCs.

High-speed-rate direct energy deposition of Fe-based stainless steel: Process optimization, microstructural features, corrosion and wear resistance

Different types of the Fe-based stainless steel coatings were prepared by a new laser additive manufacturing technology, i.e. high-speed-rate direct energy deposition (HDED), at different deposited speeds. Subsequently, typical microstructural features, corrosion and wear behaviors were investigated. Results indicated that all the coatings were dominantly composed of ultra-fined structures with the grain size less than 2 μm. The HDEDed coating manufactured at the deposited speed of 60 m/min exhibited superior corrosive resistance with the ranking Rt (about 463.185 kΩ·cm2), which is about four time that of coating manufactured at the deposited speed of 30 m/min (about 123.910 kΩ·cm2). In addition, the lowest repassivation current density (0.16 μA/cm2) also indicates the superior secondary passivation capability. The micro-hardness of the coatings manufactured at higher deposited speeds (50 and 60 m/min) was 20% higher than that of the coatings manufactured at lower deposited speed (30 and 40 m/min). However, the wear resistance of the coatings would reach a threshold value when the deposited speed attained a certain level with the other process parameters remaining constant.

Combining in-situ formed PbI2 passivation and secondary passivation for highly efficient and stable planar heterojunction perovskite solar cells

Since the power conversion efficiencies of organic-inorganic hybrid perovskite solar cells have a rapid development due to their high light absorption coefficient, tunable band gap and low fabrication cost, they have received much attention. The thin film formation and properties have significant effects on the final device performance. While how to control the thin film evolution and introduce film passivation is a critical issue to improve the device efficiency. In this study, we studied the controlled in-situ PbI 2 formation during the thin film evolution, and its effect on thin film properties, as well as their passivation effects on the photovoltaic device performance. The final thin film properties are critically related with the precursor used and chlorine inclusion could change the final thin film compositions, and affect the solar cell performance. We further perform a secondary passivation by reacting the excess PbI 2 with large organic cation to form 2D perovskite. Based on in-situ PbI 2 passivation and secondary 2D perovskite passivation, the devices exhibit a higher PCE of 21.1% and excellent V oc of 1.14 V. • Controlled in-situ PbI2 formation during the film evolution is achieved. • 2D perovksites as secondary surface passivation by reacting the excess PbI2. • A higher PCE of 21.1% and excellent V oc of 1.14 V are achieved via controlled in-situ PbI2 and 2D perovksite passivation.

Effect of secondary passivation on corrosion behavior and semiconducting properties of passive film of 2205 duplex stainless steel

The effects of secondary passivation on the electrochemical behavior of passive films on the surface of 2205 duplex stainless steel were studied by potentiodynamic polarization, EIS and Mott–Schottky analysis. The composition and surface morphology of the passive film were analyzed by ICP-MS, ARXPS and SEM. These results manifested that the passive film formed by secondary passivation has good pitting resistance, due to the increase of the content of oxidized Fe3+, Cr3+ and Mo4+ in the passive film, which improves and increases the dissolution and migration rate of metal cations in the passive film, thus improving the compactness and stability of the passive film. The Mott–Schottky plots demonstrated that the five kinds of passive films have different semiconductor properties. The estimated doping density shows that the donor density of the passive film was significantly reduced by secondary passivation. In addition, Cl− and SO42− are permeated in the passivation layer and their adsorption on the surface of the passive film can be inhibited by the secondary passivation.

Insight into electrochemical passivation behavior and surface chemistry of 2205 duplex stainless steel: effect of tensile elastic stress

The effect of tensile elastic stress on passivation behavior and surface chemistry of 2205 duplex stainless steel is investigated. Elastic stress increases the potential drop at the film/solution interface, donor density and defects diffusion coefficient, resulting in the increase of passive current density. High elastic stress (≥60%σs) leads to the generation of soluble CrO3, the increase of oxidized Cr and Fe(Ⅱ) content and the decrease of film thickness, resulting in the quasi-steady-state current density no longer independent of potential. Passive film become denser from spontaneous passivation to primary passivation and Fe(III) compounds dominate the passive film under secondary passivation.

Effect of boronizing process of AISI 321 stainless steel surface on its corrosion resistance in acid environment (pH = 1)

The paper presents results of research under the effect of surface thermo-chemical treatment on the cor-rosion resistance of X6CrNiTi18-10 steel. The corrosion resistance of the surface layers of stainless steel obtained as a result of thermo-chemical treatment (boronizing process) was assessed using the method of progressive thinning, which consists in performing corrosion tests on deeper and deeper areas of the surface layer. This method allowed for the determination of changes in individual characteristic corro-sion parameters read from the potentiokinetic polarization curves and the determination of the depth profiles of these parameters. In the paper, results of tests of X6CrNiTi18-10 steel resistance to general corrosion, performed in acidified sulphate solutions (pH = 1) have been presented. The thickness of the surface layer was assessed on the basis of structural tests and changes in microhardness on the cross-section of the material. It has been found that the extremely high hardness of the boron layer was accompanied by deterioration of the corrosion resistance. The general corrosion rate of the surface layer is 3-4 times higher than the corrosion rate of the material core (substrate). The characteristics of the pas-sive state of steel are particularly worsened, which is manifested by an increase in the value of the critical passivation current, the minimum current in the passive range and by limiting the tendency to secondary passivation.

Microstructure evolution and properties of NiTiCrNbTax refractory high-entropy alloy coatings with variable Ta content

In this work, the novel NiTiCrNbTax (x = 0.1, 0.3, 0.5, 1) refractory high-entropy alloy (RHEA) coatings were successfully synthesized on Ti6Al4V by laser cladding technology. The effect of Ta content on microstructure, thermal stability, micro-hardness and corrosion resistance of RHEA coatings was investigated in detail. The results showed that NiTiCrNbTax RHEA coatings were composed of BCC, NiTi2-Laves and Cr2Nb-Laves phases. With Ta content increasing, the formation of BCC phase was promoted while the precipitate of Cr2Nb-Laves phase was affected. The average micro-hardness values of Ta0.1, Ta0.3, Ta0.5 and Ta1.0 RHEA coatings were calculated as ~922.8 HV0.3, ~985.1 HV0.3, ~851.3 HV0.3 and ~825.4 HV0.3, respectively. The decrease of micro-hardness value might be ascribed to the reduction of hard Laves phase. After annealing at 900 °C for 3 h, the phase compositions of all RHEA coatings remained unchanged but the grains of Cr2Nb-Laves phase were refined obviously. Besides, RHEA coatings exhibited excellent corrosion resistance and the minimum corrosion current density was 1.08 × 10−7 A/cm2. Moreover, the secondary passivation phenomenon emerged in Ta0.5 and Ta1.0 coatings, and the passive film mainly consisted of the oxidation state of Ti, Cr, Nb, Ta and the sub-oxide state of Nb, Ta.