Fe-based Amorphous Metallic Coatings Research Articles

Effect of inorganic silicate sealing treatment on corrosion behaviour for HVAF sprayed Fe-based amorphous coatings

ABSTRACT Sealing treatment is an effective, environmentally friendly, and economical coating surface modification technology. To clarify the inorganic silicate sealing mechanism and concentration dependence on corrosion behaviour, HVAF-sprayed Fe-based amorphous metallic coatings (AMCs) were sealed by Na2SiO3 solution with various concentrations, and their microstructure, electrochemical performance, and surface chemistry were characterised in detail. The results showed that Na2SiO3, in the form of a silicon-oxygen bond (Si-O), could effectively bond with the coating surface inside the pore defects. In addition, the optimal concentration was proved to be 1 mol/L, and the corresponding passivation current density could be reduced to (2.38 ± 0.33) × 10−6 A/cm2, which was an order of magnitude lower than that of as-sprayed coating. This was due to the incomplete filling of the sealant at lower concentrations and the release of water vapour at higher concentrations. This work aims to provide guidance for the practical application of silicate sealing treatment.

Effect of spray powder particle size on the bionic hydrophobic structures and corrosion performance of Fe-based amorphous metallic coatings

Fe-based amorphous metallic coatings (AMCs) with different spraying particle size powders were prepared on a 316 L stainless steel by the activated combustion high-velocity air fuel (AC-HVAF) method. The bionic hydrophobic structural characteristics and corrosion behavior of the AMCs after chemical etching and surface modification were studied by micro morphology observation and electrochemical testing. Results show that all AMCs have completely dense structure and a good combination with the 316 L stainless steel. In addition, the particle size shows a significant effect on the surface hydrophobic structure. The number of unmelted particles on the surface decreased with the decreasing particle size. As a result, micro/nanoscale hierarchical hydrophobic structures are constructed according to the Cassie–Baxter model, and an enhanced corrosion resistance is obtained given the particle size of 400 mesh. The TEM results showed that many nanoscale Cr-rich particles are randomly distributed in the AMCs. These Cr-rich particles built a nanoscale hydrophobic structure on the surface and improved the corrosion resistance of the AMCs. With the increase in the temperature and the concentration of the NaCl solution, the corrosion resistance of the hydrophobic AMCs decreased, and the water contact angle of the hydrophobic AMC with 400 mesh particle spraying powder reached 152.11°. Changing the particle size of spraying powder is an effective method to prepare the optimum bionic hydrophobic interfaces for AC-HVAF AMCs.

Effect of bionic hydrophobic structures on the corrosion performance of Fe-based amorphous metallic coatings

Fe-based amorphous metallic coatings (AMCs) were prepared by the activated combustion−high velocity air fuel (AC − HVAF) coating method. A micro/nanoscale hydrophobic surface based on the lotus effect was constructed on the AMCs by chemical etching and surface modification, and the controllable preparation of bionic hydrophobic structures was realized by optimizing the chemical etching time and etching concentration. The influence of surface treatment on the corrosion resistance of the coatings was analyzed by electrochemical testing, and the correlation between the hydrophobicity of the coating structure and its corrosion resistance was determined. Results showed that the surface of the hydrophobic AMCs is composed of micro/nanoscale hierarchical structures of synapses formed by unmelted particles and corrosion products. Several pores and pits were observed after chemical etching. As the chemical etching concentration and etching time increased, the solid−liquid contact fraction and surface energy of the hydrophobic AMCs first decreased and then increased. The water contact angle peaked at 142.52° when the coatings were chemically etched in 3 mol/L HCl solution for 60 min, thereby conforming to the Cassie−Baxter model. The passivation current density and pitting tendency of the hydrophobic AMCs in 3.5% NaCl solution were closely related to the chemical etching process. The prepared hydrophobic interface could form an effective air wall that isolates the coating surface from the corrosive medium and enhances its uniform corrosion resistance. However, chemical etching aggravated the pores and intensified the dissolution of inclusions, thereby increasing the pitting sensitivity of these concavities. The construction of robust and compact bionic hydrophobic interfaces is an effective approach to enhance the uniform corrosion resistance of AMCs.

Pitting corrosion mechanism of Cl−- and S2−-induced by oxide inclusions in Fe-based amorphous metallic coatings

Fe-based amorphous metallic coatings (AMCs) were prepared by activated combustion-high velocity air fuel (AC-HVAF) method. The corrosion mechanism of AMCs under different corrosion conditions was investigated using corrosion electrochemical tests. The process of pitting corrosion induced by oxide inclusions was uncovered by molecular dynamics (MD) simulation. Results show that the passivation current density of the AMCs increases with increasing concentration of S2−. After adding NaCl, variation in the concentration of S2− shows minimal effect on the corrosion. Cl− is the main factor that affects the pitting corrosion of the AMCs when Cl− and S2− coexist. The pitting corrosion of AMCs is related to oxides of Fe, Cr and Mo. Cr2O3 inclusion is one of the sensitive phases to induce the pitting initiation for AMCs. Cl− and S2− exhibit the strongest diffusion force and the greatest negative adsorption energy on the (001) surface of Cr2O3 inclusion during the MD calculation. Pitting corrosion induced by Cl− occurs through the penetration mechanism and adsorption at the interface with metal oxides. The addition of S2− does not affect the molecular force and pitting tendency between Cr2O3 inclusion and NaCl. Hence, Cl− primarily affects the pitting corrosion of AMCs in mixed solutions. Results of simulation and experiment provide a suitable method for solving the sensitive location of pitting initiation induced by oxide inclusions in Fe-based AMCs.

Particle in-flight status and its influence on the properties of supersonic plasma-sprayed Fe-based amorphous metallic coatings

In order to optimize the supersonic plasma spraying process more scientifically, it is important to clearly understand the spreading behavior of molten droplet. In this study, the effects of spray parameters on the in-flight behavior and spreading behavior of Fe-based amorphous droplets were investigated. Additionally, since the morphology of the splats is a very important factor which determines the final properties of the coatings, the splats in this study were characterized in terms of their circularity, eccentricity, solidity, and spread factor. The porosity and micro-hardness of the Fe-based amorphous coatings were also evaluated. Results show that the spraying power is the most important parameters that influence the surface temperature of the Fe-based amorphous droplets, while the main factor affecting the in-flight velocity is the Ar flow rate. The melting index of in-flight particles reaches the maximum value at the spraying power of 60 kW and Ar flow rate of 110 L/min. The droplets present disk-shaped splats with the lowest solidity and eccentricity, and highest circularity and spread factor with the average velocity of 445 m/s and surface temperature of 2780 K. This is very crucial to reduce the defects of as-sprayed coatings and form fine-lamellar-structured supersonic plasma-sprayed coatings with outstanding mechanical properties.

Microstructures and Tribological Properties of Fe-Based Amorphous Metallic Coatings Deposited via Supersonic Plasma Spraying

The effects of the Ar flow rate and spraying power of a supersonic plasma spraying process on the microstructures and amorphous phase contents of Fe48Cr15Mo14C15B6Y2 amorphous coatings were systematically investigated. The tribological properties of the coatings were evaluated in pin-on-disk mode using a sliding tribometer. The results show that the amorphous phase content and microhardness initially increase with the Ar flow rate and then gradually decrease. However, the amorphous phase content and microhardness increase with the power. In particular, the amorphous phase content of the coating reaches 96.78% with a spraying power of 62 kW and a 110 L min−1 Ar flow rate. Tribological testing demonstrates that the coatings exhibit similar steady-state coefficients of friction (0.75–0.82) with a total test time of 20 min and an applied load of 20 N. However, the wear rates vary with the spraying parameters. In particular, the relative wear rate of the coating can be enhanced up to sixfold under optimal spraying conditions, resulting in excellent wear resistance. Detailed analysis of the coating wear surfaces indicates that the dominant wear mechanisms are abrasive and oxidative wear. Moreover, delamination may occur during the wear process.

Enhancement of Corrosion Resistance in Carbon Steels Using Fe-Based Amorphous Metallic Coatings Under High-Temperature Flowing Water

Carbon steels are the backbone material of modern industry applications owing to excellent forming, machining, and welding properties. However high-temperature flowing water condition induces complex corrosion phenomena involving destructive electrochemical reactions and mechanical wear for carbon steels thus a cost-effective countermeasure for the corrosion of carbon steels is highly required. In this study, to enhance the corrosion resistance in carbon steels under high-temperature flowing water, Fe-based amorphous metallic coatings (AMC) are deposited on carbon steels using high-velocity oxygen fuel (HVOF) technique. The Fe-based AMC gathers tremendous interests as they exhibit excellent corrosion and wear resistance thus it is expected that it provides effective corrosion protection ability for carbon steels even in severe environments including nuclear power plants. To examine the corrosion resistance of the Fe-based AMC, rotating cage corrosion (RCC) test was designed with a magnedrive-installed autoclave and recirculation loop system. The temperature for each test was 125, 150, 175, and 200 oC, respectively. To simulate power plant water chemistry, pH 9.3 ETA solution was continuously supplied. The weight loss of each specimen was measured after 2 weeks of elapsed time and the result shows that the Fe-based AMC exhibits excellent RCC resistance at all temperature and most effective at 150 oC as shown in Fig. 1. Further, while microstructure evolution of carbon steels displays the formation of porous Fe3O4 at the surface, the Fe-based AMC displays the formation of thin Cr2O3 layers meaning passivity as shown in Fig. 2.. At 200 oC, the formation of FeCr2O4 was also observable (Fig. 3) The early conjecture is that the Fe-based AMC is able to protect carbon steels by forming passive oxide layer at its surface. Previous research argued that Cr and its passive layer is the most effective RCC barrier in high-temperature. Thus, further investigation using transmission electron microscopy and X-ray photoelectron spectroscopy will be carried out to figure out their stoichiometry and detailed microstructure evolution. Figure 1

Electrochemical Properties of Fe-Based Amorphous Metallic Coatings in Aqueous Media

Coating as an effective corrosion barrier for various industrial application is one of the major research trend in electrochemistry. Many types of coating materials and techniques have been suggested and a high-velocity oxy-fuel (HVOF) technique has adequate potential. Fe-based amorphous metallic coating (AMC) is a non-crystalline alloy coating with spatially dispersed nanocrystalline structure. As they are known as an effective anti-corrosion and anti-wear resistance, some research groups have figured out their corrosion behavior in corrosive environments. In this study, we experimentally demonstrated the corrosion behavior of Fe-based AMC in seawater and alkaline medium environments using various electrochemical tests including linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS). Fig. 1. Displays the powder morphology of Fe-based AMC feedstock material. Spherical shape with ca. 30 um size particles can be observed. The chemical composition is 54.05Fe-18.31Cr-11.34W-12.5Mo-2.8Mn. After the spray process, surface and cross section morphologies are analyzed by SEM (Fig. 2). Fig. 3. Displays the corrosion behavior of the substrates and the coatings in seawater and alkaline media, respectively. The results indicates that corrosion resistance of Fe-based AMC is not remarkable in both media. Fig. 4. Also displays the corrosion resistance is lower and double layer capacitance is higher compared to those of the substrates. Thus, likewise to other research results, the as-sprayed Fe-based AMC exhibits less corrosion resistance compared to carbon steels. However, their promising potential in high-temperature alkaline media and flowing conditions will be figured out as our future works. Figure 1

Fabrication and characterization of supersonic plasma sprayed Fe-based amorphous metallic coatings

Fe-based amorphous metallic coatings with a nominal composition of Fe48Cr15Mo14C15B6Y2 were fabricated by supersonic plasma spraying (SPS). The effects of the Ar flow rate and spraying power on the microstructure and amorphous content of the coatings were systematically investigated. The thermal stability, microhardness and porosity of the coatings were also experimentally determined and characterized. The results demonstrate that the coatings present denser layered structure with a thinner intersplat region, and the elements inter-diffusion zone exist between the coatings and the substrate. It is also found that the amorphous content of coatings is as high as 90% and the coatings exhibit high thermal stability with an onset crystallization temperature of 963K. The amorphous content and microhardness initially increase and then decrease gradually with the increase of Ar flow rate whereas the porosity decreases firstly and then increases. However, the amorphous content and microhardness increase with the increase of the power whereas the porosity decreases. In particular, the amorphous content and microhardness can be reached 96.78%, and 1005HV0.1 respectively, whereas the porosity is only 0.85% at the spraying power of 62kW and the Ar flow rate of 110L/min.

Influence of Cold Gas Spray process conditions on the microstructure of Fe-based amorphous coatings

Fe-based amorphous metallic coatings were prepared by Cold Gas Spray process. Through this study, the effects of the process conditions such as spraying distance, gas pressure and temperature on the microstructure of as-sprayed coatings are evaluated. Microstructural studies show that the coatings can present a densely layered structure with porosity below 0.5% and thickness around 800μm depending on the process conditions. Precipitation of nanocrystals in as-sprayed coatings is observed and present results show its dependence on the thermal and kinetic energy implicated in the process. In general, when gas temperature and pressure decreased, in the studied range, coatings displayed a dense and amorphous structure.

Effect of porosity sealing treatments on the corrosion resistance of high-velocity oxy-fuel (HVOF)-sprayed Fe-based amorphous metallic coatings

High-velocity oxy-fuel-sprayed FeCrMoMnWBCSi amorphous metallic coatings were sealed with sodium orthosilicate (Na 3SiO 4), aluminium phosphate (AlPO 4), and cerium salt sealants. The microstructure of the sealed coatings was characterised by scanning electron microscopy, energy dispersive spectrometer, and X-ray diffraction. Corrosion behaviour was examined using electrochemical methods of potentiodynamic polarisation, cyclic polarisation, electrochemical impedance spectroscopy, and Mott–Schottky tests. The results indicated that the uniform corrosion resistance of the three sealed coatings was enhanced greatly, and the passive current densities were decreased by one order of magnitude after the sealing treatments. The AlPO 4 sealant can penetrate the coatings by no less than 50 μm and enhance their hardness, which exhibited a more uniform corrosion resistance, fairly good pitting corrosion resistance, and can be applied in long-term corrosive and/or abrasive environments. The cerium salt-sealed coating showed better pitting corrosion resistance but inferior corrosion resistance in the local regions of micro-cracks, which was practically used for temporary corrosion protection. The Na 3SiO 4-sealed coating showed better uniform corrosion resistance and inferior pitting corrosion resistance, which can be applied in short-term corrosion environments. The stability of the passive film affected the corrosion behaviour of the sealed coatings. The AlPO 4-sealed coating performed better as a protective passive film during the long-term immersion test for a lower defect concentration and a more protective passive film.

Slurry erosion–corrosion behaviour of high-velocity oxy-fuel (HVOF) sprayed Fe-based amorphous metallic coatings for marine pump in sand-containing NaCl solutions

Corrosion and slurry erosion–corrosion (E–C) behaviour of FeCrMoMnWBCSi amorphous metallic coatings (AMCs) compared with 304 stainless steel was investigated by static electrochemical measurements and weight loss tests under rotating conditions in simulated seawater. E–C rates of the AMCs increased with flow velocity, particle size, sand content and NaCl concentration. The AMCs are preferentially attacked at coating defects. The superior E–C resistance of AMCs can be attributed to the high micro-hardness, alloying elements and amorphous microstructure. The AMCs with better slurry E–C resistance may be good candidates to solve the E–C problems of marine pumps in sand-containing seawater.

Microstructure and Electrochemical Behavior of Fe-Based Amorphous Metallic Coatings Fabricated by Atmospheric Plasma Spraying

A Fe48Cr15Mo14C15B6Y2 alloy with high glass forming ability (GFA) was selected to prepare amorphous metallic coatings by atmospheric plasma spraying (APS). The as-deposited coatings present a dense layered structure and low porosity. Microstructural studies show that some nanocrystals and a fraction of yttrium oxides formed during spraying, which induced the amorphous fraction of the coatings decreasing to 69% compared with amorphous alloy ribbons of the same component. High thermal stability enables the amorphous coatings to work below 910 K without crystallization. The results of electrochemical measurement show that the coatings exhibit extremely wide passive region and relatively low passive current density in 3.5% NaCl and 1 mol/L HCl solutions, which illustrate their superior ability to resist localized corrosion. Moreover, the corrosion behavior of the amorphous coatings in 1 mol/L H2SO4 solution is similar to their performance under conditions containing chloride ions, which manifests their flexible and extensive ability to withstand aggressive environments.

Microstructure and Wear Resistance of Fe-Based Amorphous Metallic Coatings Prepared by HVOF Thermal Spraying

Amorphous metallic coatings with a composition of Fe48Cr15Mo14C15B6Y2 were fabricated by means of high velocity oxygen fuel (HVOF) thermal spraying process. The microstructure and wear performance of the coatings were characterized simultaneously in this article. It is found that the coatings present a dense layered structure with the porosity below 1.5%. The coatings primarily consist of amorphous matrix and some precipitated nanocrystals, though a fraction of Fe-rich phases and oxide stringers also formed during deposited process. High thermal stability enables the amorphous coatings to work below 920 K temperature without crystallization. Depending on the structural advantage, the amorphous coatings exhibit high average microhardness of 997.3 HV0.2, and excellent wear resistance during dry frictional wear process. The dominant wear mechanism of amorphous coating under this condition is fatigue wear, leading to partial or entire flaking off of the lamellae. In addition, the appearance of oxidative wear accelerates the failure of fatigue wear.

Formation and corrosion behavior of Fe-based amorphous metallic coatings prepared by detonation gun spraying

Amorphous metallic coatings with a composition of Fe 48Cr 15Mo 14C 15B 6Y 2 were prepared by detonation gun spraying process. Microstructural studies show that the coatings present a densely layered structure typical of thermally sprayed deposits with the porosity below 2%. Both crystallization and oxidation occurred obviously during spraying process, so that the amorphous fraction of the coatings decreased to 54% compared with fully amorphous alloy ribbons of the same component. Corrosion behavior of the amorphous coatings was investigated by electrochemical measurement. The results show that the coatings exhibit extremely wide passive region and low passive current density in 3.5% NaCl (mass fraction) and 1 mol/L HCl solutions, which illustrates excellent ability to resist localized corrosion.

Formation and corrosion behavior of Fe-based amorphous metallic coatings by HVOF thermal spraying

Amorphous coatings with a composition of Fe 48Cr 15Mo 14C 15B 6Y 2 were prepared by means of high velocity oxygen fuel (HVOF) thermal spraying under different conditions. Microstructural studies show that the coatings present dense layered structure and low porosity with a fraction of nanocrystals precipitated. The porosity and amorphous fraction of the coatings decrease as the kerosene and oxygen flow increase within the parameter range examined. Corrosion behavior of the amorphous coatings was investigated by electrochemical measurement. The results show that the coatings are spontaneously passivated with wide passive region and low passive current density in 3.5% NaCl, 1 N HCl and 1 N H 2SO 4 solutions, and exhibit an excellent ability to resist localized corrosion. However, the corrosion resistance of the coatings decreases in 1 N NaOH solution with lower transpassive potential and passive region. In addition, the optimal spraying parameter improves the corrosion resistance of the amorphous coatings obviously due to the proper proportion of porosity and amorphous fraction.