Formation Of Passive Film Research Articles

Corrosion behavior of functionally graded cemented carbides with CoNiCrFe binder

The corrosion behavior of functionally graded cemented carbide (FGCC), with CoNiCrFe binder (WC-CoNiCrFe), has been investigated by performing electrochemical measurements and chemical characterization. The results showed that the CoNiCrFe binder was selectively dissolved in alkaline and acidic solutions. In the NaOH solution, the high contents of Ni and Co generated a passive film consisting of Co(OH)2, Ni(OH)2, Co3O4, and NiO that effectively isolated the corrosive medium from the materials. However, the contents of Fe and Cr were too low to provide satisfactory protection. In the HCl solution, the Cr element generated Cr0, Cr2O3, and Cr(OH)3. However, the Cr content was too low to form a passive film. In addition, Co, Ni and Fe in the binder were selectively dissolved in the acidic solution. Moreover, compared with the FGCC, the non-graded cemented carbide (CC) had a higher binder content and a more dispersed WC distribution. Thus, it formed a denser and thicker passive film, which was beneficial for the corrosion resistance in the NaOH solution. However, the CoNiCrFe binder was severely corroded in the HCl solution, which was due to the absence of a passive film. In comparison, the FGCC with a lower binder content and a denser WC distribution, showed a higher corrosion resistance. On the other hand, the WC grain size showed the opposite effect in alkaline and acidic solutions. In the NaOH solution, a small WC grain size increased the number of WC/CoNiCrFe interfaces, which favored the formation of a dense passive film. It, thereby reduced the probability for pitting corrosion. In the HCl solution, the corrosion generally started from grain boundaries with high interface energy as no passive film existed. Thus, a large WC grain size facilitated the improvement of the corrosion resistance. The results from the present study do not only provide a theoretical guidance for an understanding of the anticorrosion mechanism of WC-CoNiCrFe, but also a new approach for the preparation of cemented carbides with high corrosion resistance.

Study on mechanical and corrosion resistance of Ni-W-P composite coating reinforced by cationic cubic boron nitride

In this paper, the oxidative self-polymerization and ionization reactions of dopamine were used to modify the surface of nanoparticle cBN to obtain a cationic nanoparticle DRcBN+, which was co-deposited with Ni-W-P alloy by pulse electrodeposition technology to create a novel Ni-W-P based composite coating. A stronger adsorption force between the cathode and DRcBN+ under the influence of the electric field results from the presence of -NH3+ on the surface of DRcBN+, which prevents structural flaws in the coating brought on by cBN's lack of adsorption force during the process from cathode adsorption to deposition. The research results show that DRcBN+ is embedded in the Ni-W-P alloy coating, which reduces the coating's crystallite size, improves its compactness and microhardness, and enhances its wear resistance and corrosion resistance. The dispersion strengthening caused by nanoparticles may be the cause of the increased microhardness and wear resistance, which is consistent with Hall-Petch theory to a certain extent. In addition, in the immersion test of 3.5 wt% NaCl solution, it was found that DRcBN+ nanoparticles could enhance the long-term corrosion resistance of Ni-W-P alloy coating. This might be because the -NH3+ on the surface of DRcBN+ can electrostatically adsorb electrons and promote the formation of passive film. In addition, the composite coating can achieve corrosion inhibition because of its adsorption of corrosive negative ions (Cl−).

Effect of minor gallium addition on corrosion, passivity, and antibacterial behaviour of novel β-type Ti–Nb alloys

Metastable Ti–Nb alloys are promising bone-implant materials due to improved mechanical biofunctionality and biocompatibility. To overcome increasing bacterial infection risk, alloying with antibacterial elements is a promising strategy. This study investigates the effect of minor gallium (Ga) additions (4, 8 wt% Ga) to as-cast and solution-treated β-type Ti–45Nb-based alloy (96(Ti–45Nb)-4Ga, 92(Ti–45Nb)-8Ga (wt.%)) on corrosion and passive film properties, as well as cytocompatibility and antibacterial activity. The electrochemical properties were evaluated by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and Mott-Schottky analyses in phosphate-buffered saline (PBS). X-ray photoelectron spectroscopy (XPS) was performed to analyze the chemical composition of passive films. Early adhesion and viability of macrophages and Staphylococcus aureus were assessed by nucleocounting and colony-forming unit counting, respectively. The results showed that high corrosion resistance and passive film properties of Ti–45Nb are retained and even slightly improved with Ga. EIS results revealed that Ga addition improves the passive film resistance. XPS measurements of 92(Ti–45Nb)-8Ga show that the passive film contains Ti-, Nb- and Ga-based oxides, implying the formation of mixed (Ti–Nb-Ga) oxides. In addition, marginal Ga ion release rate was detected under free corrosion conditions. Therefore, it can be assumed that Ga species may contribute to passive film formation on Ga-containing alloys. The 92(Ti–45Nb)-8Ga elicited an antibacterial effect against S. aureus compared to cp-Ti at 4 h. Moreover, Ga-containing alloys showed good cytocompatibility with THP-1 macrophages at 24 h. In conclusion, it was demonstrated that Ga additions to Ti–45Nb are beneficial to corrosion resistance and showed promising initial host and bacterial interactions.

Corrosion behavior of laser powder bed fusion manufactured Cu-bearing titanium alloy in artificial saliva

This study examined the corrosion behavior of Cu-bearing titanium alloy, Ti-xCu (x = 0,2,5,8 wt%) manufactured through laser powder bed fusion (LPBF) in artificial saliva. The results demonstrated that the Ti-8Cu alloy exhibited exceptional corrosion resistance, attributed to the formation of a protective passivation film facilitated by the copper (Cu) addition. The incorporation of Cu effectively anchored the grain boundaries and resulted in grain refinement, contributing to the enhanced corrosion resistance of the alloy. The Ti-xCu alloy with higher Cu content exhibited higher concentration of finer grains and greater density of dislocations, thereby promoting the formation of more compact passive film and leading to an enhanced corrosion resistance.

Enhancement of the electrochemical corrosion resistance of Ti6Al4V alloy reinforced by nano- and micro-TiC particles through directed energy deposition

Nano- and micro-size TiC ceramic particles were synergistically utilized for the reinforcement of Ti6Al4V alloy, manufactured through directed energy deposition. The corrosion current density of nano/micro-TiC/Ti6Al4V composite decreased by 23.7% and 74.6% compared to Ti6Al4V composites containing only TiC microparticles or only TiC nanoparticles, and by 77.8% compared to pure Ti6Al4V. A stable passive film of TiO2 with higher content of bound water is formed in nano/micro-TiC/Ti6Al4V composite specimen. The excellent electrochemical corrosion resistance of the nano/micro-TiC/Ti6Al4V composite is attributed to the α martensite transformation and the accumulation of [TiCl6]2- complexes, which promotes the formation of TiO2 passive film.

Recrystallization effect on surface passivation of Hastelloy X alloy fabricated by laser powder bed fusion

Post-heat treatment is generally adopted in the additive manufacturing field due to its alleviation of high residual stress and modification of rapid-solidified multilevel heterogeneous microstructure, and the related performance of the heat-treated counterparts calls for a systemic investigation to build a criterion of the heat treatment procedure. In this work, we focus on the heat treatment effects on the recrystallization of the Hastelloy X alloy produced by the laser powder bed fusion (LPBF) method, and the related surface passivation of the heat-treated counterparts is meticulously assessed as well. Results show that the multilevel heterostructure for LPBF Hastelloy X alloy consists of sub-micro dislocation cell substructures with Cr/Mo elemental segregation, fine columnar grains, and periodically-distributed molten pools. After heat treatment, partially and fully recrystallized structures for LPBF Hastelloy X alloys were achieved at 1100 and 1200 °C for 1 h, respectively. Furthermore, the as-built LPBF Hastelloy X alloy shows superior corrosion resistance while the heat-treated one (1100 °C) exhibits the worst in the borate buffer solution. The growth of passive film exhibited a highly linear correlation with the nucleation process controlled by diffusion, and high dislocation density and low angle grain boundary decreased the diffusion coefficient of cation vacancies, augmenting the nucleation sites of the passive film and enhancing its growth rate. Moreover, the micro-galvanic effect resulting from the partially recrystallized microstructure actively facilitated the formation of inhomogeneous porous passive films, leading to the worst corrosion resistance.

Corrosion behavior of Ti-0.3Mo-0.8Ni (TA10) alloy in proton exchange membrane fuel cell environment: Experimental and theoretical studies

The Ti-0.3Mo-0.8Ni (TA10) alloy is a titanium-based material known for its favorable corrosion resistance, making it a promising candidate for PEMFC bipolar plates. In order to investigate the electrochemical behavior of the TA10 alloy in a simulated PEMFC environment, the microstructure of the alloy was characterized using scanning electron microscopy and X-ray diffraction. The corrosion resistance was evaluated through potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests in the simulated PEMFC cathode environment. Based on the experimental characterization, the mechanism underlying the improved corrosion resistance of the TA10 alloy in the PEMFC environment was revealed using first-principles calculations. The research findings demonstrate that the cast structure of the TA10 alloy comprises a predominant white α-phase matrix and a minor proportion of gray-black layered β-phase. The α-phase is composed solely of 100 wt% Ti, while the β-phase contains 90.4 wt% Ti and 9.6 wt% Ni. Under the simulated PEMFC environment, the corrosion potentials of TA1 and TA10 were observed to be − 0.424 V and − 0.228 V, respectively, in the anode environment, and − 0.382 V and − 0.234 V, respectively, in the cathode environment, representing an increase of approximately 38.7–46.2 % compared to the TA1 alloy. Importantly, the corrosion current density of the TA10 alloy was reduced by approximately 20.5–36.9 % compared to TA1, indicating enhanced corrosion resistance and chemical stability due to the higher corrosion potential and lower corrosion current density exhibited by the TA10 alloy. Furthermore, electrochemical impedance spectroscopy (EIS) measurements demonstrated a significant increase in the capacitive arc radius of TA10 compared to TA1, with an Rfb value of 7.166 kΩ cm2 for TA10, approximately 13 times higher than the corresponding value of 0.529 kΩ/cm2 for TA1. This substantial increase in the Rfb value signifies the improved corrosion resistance of TA10 compared to TA1. Theoretical calculations revealed that the presence of Mo and Ni elements in the TA10 alloy effectively reduces the overpotential on the Ti surface. Specifically, the overpotential was found to be 0.734 eV for Ti-Mo, 0.569 eV for Ti-Ni, and when Mo and Ni were co-doped, the overpotential further decreased to 0.512 eV. This reduction in overpotential enhances the rate of the cathodic hydrogen evolution reaction and facilitates the formation of an anodic passivation film, thereby significantly improving the corrosion resistance of the TA10 alloy. In conclusion, our study provides comprehensive insights into the electrochemical performance of the TA10 alloy in a PEMFC environment, highlighting its potential as a corrosion-resistant material for bipolar plates in PEMFC applications.

Stepwise induced passivation film formation on the surface of spherical hard carbons enabling ultra-stable potassium ions storage

Potassium-ion storage devices are seen as a form of prospective substitute for large-scale electrochemical energy storage because of the low cost and the fast charge transport in electrolytes. However, the rosy scenario is compromised by developing ultra-stable electrode materials to ensure long-term cycling performance. Herein, with sucrose as the carbon source, we construct two carbon materials, namely hard carbon spherules (HCS) and active hard carbons (AHCS), as anode and cathode materials, respectively, to fabricate high-performance potassium-ion hybrid capacitors (PIHC). Benefiting from the advantageous ionic conductivity and the comparatively stable passivation film, HCS bonding with sodium alginate delivers superior rate performance and high pseudocapacitive proportion. In situ electrochemical impedance spectroscopy and distribution of relaxation time analysis combined with ex situ scanning electron microscope images reveal that the formation of solid electrolyte interface on the surface of the HCS during the initial discharge can be explained as a gradually forming process. As expected, the HCS//AHCS PIHC device delivers a high energy density of 116 Wh kg−1 at a power density of 218 W kg−1 and exhibits an ultra-stable lifespan over 20000 cycles. This study provides a worthy reference for the value-added utilization of biomass in innovative potassium-based energy storage technology.

Biochar Promotes Arsenopyrite Weathering in Simulated Alkaline Soils: Electrochemical Mechanism and Environmental Implications.

Oxidation dissolution of arsenopyrite (FeAsS) is one of the important sources of arsenic contamination in soil and groundwater. Biochar, a commonly used soil amendment and environmental remediation agent, is widespread in ecosystems, where it participates in and influences the redox-active geochemical processes of sulfide minerals associated with arsenic and iron. This study investigated the critical role of biochar on the oxidation process of arsenopyrite in simulated alkaline soil solutions by a combination of electrochemical techniques, immersion tests, and solid characterizations. Polarization curves indicated that the elevated temperature (5-45 °C) and biochar concentration (0-1.2 g·L-1) accelerated arsenopyrite oxidation. This is further confirmed by electrochemical impedance spectroscopy, which showed that biochar substantially reduced the charge transfer resistance in the double layer, resulting in smaller activation energy (Ea = 37.38-29.56 kJ·mol-1) and activation enthalpy (ΔH* = 34.91-27.09 kJ·mol-1). These observations are likely attributed to the abundance of aromatic and quinoid groups in biochar, which could reduce Fe(III) and As(V) as well as adsorb or complex with Fe(III). This hinders the formation of passivation films consisting of iron arsenate and iron (oxyhydr)oxide. Further observation found that the presence of biochar exacerbates acidic drainage and arsenic contamination in areas containing arsenopyrite. This study highlighted the possible negative impact of biochar on soil and water, suggesting that the different physicochemical properties of biochar produced from different feedstock and under different pyrolysis conditions should be taken into account before large-scale applications to prevent potential risks to ecology and agriculture.

Irradiation enhanced corrosion resistance of CrN/TiSiN multilayers synthesized by cathodic arc ion plating

The effect induced by irradiation is often considered as ‘negative impact’. Irradiation damage normally leads to defects in materials, including vacancies, clusters and interstitials, therefore resulting in the attenuation of stability such as corrosion resistance. Here, multilayered CrN/TiSiN coatings have been fabricated by cathodic arc ion plating and the corrosion properties of prepared coatings have been investigated before and after irradiation. Microstructure of irradiated samples revealed amorphization on the surface layer induced by irradiation. The corrosion behavior was studied by electrochemical tests, and multilayered CrN/TiSiN coatings revealed enhancement in corrosion resistance as the corrosion potential increased and corrosion current decreased in all irradiated samples. It turned out that the multilayered CrN/TiSiN coatings deposited with a bias voltage of 150 V and Ti80Si20 targets revealed the best corrosion resistance after irradiation. Given that, surface morphology analysis of corroded surface was conducted to provide further evidence. This irradiation enhanced corrosion resistance of multilayered CrN/TiSiN coatings could be explained by the rapid formation of passive films upon the irradiation induced amorphous layer during corrosive process, which function as barriers between the substrate and the corrosive solution. Our results reveal profound insights into corresponding corrosion mechanism of nitride coatings based on the microstructure investigation of corroded coatings and contribute to the understanding of corrosion behaviors of irradiated nitride coatings.

Corrosion behaviors of CrFeNiNbx medium entropy alloys with hypereutectic and hypoeutectic structures

In this paper, the corrosion resistance and corresponding corrosion mechanisms of CrFeNiNbx (x = 0.2, 0.4) hypoeutectic and hypereutectic medium entropy alloys (MEAs) in 3.5 wt% NaCl solution were systematically investigated. It is found that, both CrFeNiNb0.2 and CrFeNiNb0.4 MEAs are composed of FCC and Laves phases, and they show excellent and similar corrosion resistance in 3.5 wt% NaCl solution due to ultrahigh oxidized (Cr, Nb) concentration in their passive films. However, the FCC and Laves phases in these MEAs have significant difference in corrosion behaviors, which is possibly influenced by the diversity in chemical properties and formation of passive films on these phases. This work is beneficial for designing high-performance eutectic-related MEAs with outstanding corrosion resistance.

Influence of UV light on the corrosion behavior of AA6061-T6 in a CMA solution

The effect of ultraviolet (UV) light illumination on the corrosion behavior of AA6061-T6 in an alkaline snowmelt-agent solution (CMA solution) was investigated using electrochemical and spectroscopic techniques. The corrosion products and passive films formed under different exposure conditions were n-type semiconductors. Exposure for 1 and 2 weeks facilitated the corrosion process. Under UV light illumination, the extent of corrosion and size of corrosion pits were larger than those in the dark; this was attributed to the facilitation of the dissolution of AlFeSi intermetallics and formation of fewer oxides such as Al2O3 under UV light illumination. After exposure for 4 weeks, under UV light illumination, the formation of passive films was fewer defects, and more compact structures offered more protection to the aluminum substrates, resulting in slightly suppressed corrosion.

Femtosecond laser induced periodic nanostructures towards enhanced anti-corrosive property of titanium

Femtosecond laser was employed to induce periodic nanostructures on the surface of titanium to improve its corrosion resistance. The effect of the induced structures on the corrosion resistance of the titanium was investigated. The micromorphology, wetting and microstructural features (e.g., phase content, grain size and grain boundary) and corrosion behavior of the titanium surface before and after femtosecond laser treatment were systematically investigated. The results revealed that femtosecond laser induced periodic nanostructures enhanced the corrosion resistance of the titanium in 3.5 % NaCl solution. The potential mechanisms could be attributted to the low proportion of high-angle grain boundaries (HAGBs), the formation of passive film with higher fraction of TiO2 and the air cushion effect on structured surface, which slows down the corrosion rate.

Effect of surface preparation by high-temperature hydrogen annealing on the passivation of Ni-20 at.% Cr alloy in sulfuric acid

The effect of high-temperature annealing under hydrogen gas (H2) surface preparation on a binary polycrystalline Ni-20 at.% Cr alloy was investigated in comparison with conventional mechanical grinding surface preparation. Duplex surface films with inner Cr(III) oxide and outer Ni(II) and Cr(III) hydroxide layers natively formed before and after H2 annealing were characterized by X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectroscopy. A thinner and less homogeneous inner barrier layer formed after H2 annealing caused an oxidation peak at the active/passive transition in the linear sweep voltammetry measurement in a 0.5 M H2SO4 solution. Despite this high oxidation peak, passivation of the H2 annealed surface resulted in the formation of a more corrosion resistant inner layer than on the mechanically ground surface, as demonstrated by electrochemical impedance spectroscopy. The corrosion resistant passive film formation of the H2 annealed surface is attributed to the increased Cr enrichment of the inner layer caused by the selective Ni dissolution providing corrosion resistance to the initially weakly protected sites of the natively formed surface film.

Cover Picture: Materials and Corrosion. 4/2023

Cover: Second-harmonic generation (SHG) was used to monitor in-situ compositional changes to carbon steel surfaces immersed in aqueous solutions and during electrochemical experiments. For passive film formed on carbon steel samples, the response in SHG intensity showed a peak in presence of Fe3O4, while it shows a decrease in presence of Fe2O3. Upon application of cathodic protection for 30 minutes, SHG response revealed the formation and growth of a Fe3O4 film on the carbon steel surface. In this work, the results obtained with SHG were compared with in-situ active reflectance spectroscopy measurements. More detailed information can be found in: Federico Martinelli-Orlando, Eric Dénervaud, Rachel Grange, Ueli Angst, Second-harmonic generation technique for in-situ study of passive film formation on carbon steel surfaces in aqueous solutions, Materials and Corrosion 2023, 74, 508.

Microstructure and corrosion behavior of the Ti-V-Cr-Nb high-entropy alloys in 3.5 wt% NaCl solution

Novel high-entropy alloys (HEAs) of the Ti(30–x)VCrNb(x) (x = 0, 5, 10, 15, and 20 at%) system were designed, synthesized, and characterized. The microstructural features were investigated by XRD, SEM, and EDS. The corrosion behavior was evaluated in 3.5 wt% sodium chloride (NaCl) solution of pH 5 at 25 °C by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The investigated HEAs exhibited a single-phase BCC solid solution with dendritic microstructures. The polarization curves showed the Ti10V35Cr35Nb20 (Nb20) HEA exhibited the best corrosion resistance with the lowest corrosion current density (2.24 µA/cm2) and corrosion rate (1.15 × 10–2 mm/yr). The interpretation of an equivalent electric circuit (EEC) suggested the formation of a stable passive oxide film on the HEAs, as indicated by good capacitive behavior and high impedance values (>104 Ω cm2) at low frequencies and phase angles close to − 80°. This study demonstrated that an appropriate Nb content can stabilize the BCC phase and significantly enhance the corrosion resistance of the HEAs.

Investigation on the mechanical and pitting corrosion behaviour of solution-treated 22% Cr lean duplex stainless steel with different Mn/N ratio

ABSTRACT Effect of Mn/N ratio on the microstructure evolution, mechanical and pitting corrosion properties of solution-treated 22% Cr lean duplex stainless steel (DSS) was investigated. As the Mn/N ratio decreased, tensile and yied strength, as well as elongation,increased due to the strengthening solid solution effect of more nitrogen addition, and two-phase equilibrium was obtained at 17.80 Mn/N ratio. The drop in the amount of Cr2N, more dislocation cell formation in ferrite and the increase of slip bands in austenite contributed to an increase in toughness with increasing Mn/N ratio from 3.28 to 17.80, whereas more σ precipitates and ferrite fraction resulted in a decrease of it at 54.14 Mn/N ratio. Moreover, higher pitting corrosion resistance was obtained for 17.80 Mn/N ratio DSS than that for 2205 DSS due to the thicker and more compact passive film formation, but higher Mn/N ratio of 54.14 decreased it with more loose MnO formation.

Microstructural characteristics, mechanical properties and corrosion performance of super austenitic stainless steel 904L produced by wire arc additive manufacturing

This study involves the fabrication of a super austenitic stainless steel 904 L thin wall using wire arc additive manufacturing (WAAM) technology. The microstructure, mechanical properties, and localized corrosion performance of WAAM-processed 904 L were investigated. The average surface roughness varied between 22.43 and 28.51 µm due to the presence of delineations in the wall. Microstructural analysis revealed the presence of equiaxed, columnar, and elongated dendrites along the building direction (BD) of the as-deposited wall. Secondary phases were not noticed in the as-deposited wall due to repeated thermal cycles. Energy-dispersive X-ray spectroscopy (EDS) analysis confirmed the uniform dissolution of elements such as Ni, Cr, and Mo within the austenitic matrix. The hardness mapping in WAAM 904 L indicated the variation at the top, middle, and bottom regions and was 220 ± 11 HV, 235 ± 9 HV, and 260 ± 8 HV, respectively. The tensile test outcomes of WAAM 904 L specimens at 0°, 45°, and 90° orientation showed lower anisotropy and observed maximum tensile properties in the horizontal direction (0 °), i.e., YS= 312 ± 12 MPa, UTS= 658 ± 24 MPa & %E = 36 ± 1.5%. The mode of fracture was ductile in nature, with dimples and microvoids. The Potentiodynamic polarization (PDP) tests in 3.5% NaCl solution highlighted the formation of stable passive film on the surface of the as-produced 904 L specimens, and superior resistance to pitting corrosion was noticed. Electrochemical impedance measurements revealed the stability of WAAM specimens and showed a similar trend to PDP outcomes. The degree of sensitization (DOS) of the WAAM 904 L samples was lower, and the corrosion rate ranged between 0.07 and 0.70 mils per year (mpy).

Unusually high corrosion resistance in Mo CrNiCo medium entropy alloy enhanced by acidity in aqueous solution

• A series of ultra-corrosion-resistant medium entropy alloys were developed. • The corrosion resistance of our alloys unusually increases with acidity in Cl − containing solutions. • The unique anti-corrosion behaviour results from the formation of a protective and compact medium entropy oxide film, which can be attributed to the synergic effect of the chemical complexity on absorption of H 2 O and O 2 . High corrosion resistance of alloys is essential for their structural applications; however, most alloys suffer from degradation of their corrosion resistance with the increasing acidity of their surroundings. Nonetheless, we developed a series of medium-entropy alloys (MEAs) in this work, which exhibit high strength, superior fracture toughness and ultra-high corrosion resistance, outperforming the variety of corrosion resistant alloys hitherto reported. Most interestingly, our MEAs exhibit an unusual anti-corrosion behavior and their corrosion resistance increases with acidity in Cl − containing solutions. Through extensive thermodynamic calculations, density functional theory (DFT) simulations and experiments, we reveal that the unusual anti-corrosion behavior of our MEAs can be attributed to their surface chemical complexity, which facilitates the physio-chemical-absorption of H 2 O and O 2 and thus the rapid formation of metastable medium entropy passive films that contain the lowest amount of defects, as compared to the passive films on conventional alloys reported in the literature.

Passive film formation and corrosion resistance of laser-powder bed fusion fabricated NiTi shape memory alloys

Electrochemical tests and surface analysis measurements were performed to study the corrosion behavior in a 0.9 wt.% NaCl solution at 37 °C of three NiTi shape memory alloys fabricated by laser-powder bed fusion (L-PBF). The passive film characteristics and corrosion resistance of L-PBF NiTi showed different features as a function of their preparation process settings. The passivation rate for L-PBF NiTi surfaces including defects, such as keyhole pores and cracks which showed high electrochemical activity accelerating the passivation reaction process, was higher in the early stages of immersion, but the corrosion resistance provided by such a rapidly formed passive film containing higher defect density is lower than that for an initially defect-free surface. The thickness of the passive film including a higher defect density does not necessarily relate to the corrosion resistance. The L-PBF NiTi prepared at a linear energy density of 0.2 J/m and volumetric energy density of 56 J/mm3 shows the least defects. Also, an outer Ti-rich and inner Ni-rich dense and corrosion protective passive film could be obtained for these L-PBF NiTi samples, which also results in a relatively low Ni ion release rate. A passive film model based on thickness, composition and defect density properties as a function of processing conditions is proposed to explain the difference in corrosion resistance of the various L-PBF NiTi.