NiCr Coatings Research Articles

Comparative hot corrosion performance of APS and Detonation sprayed CoCrAlY, NiCoCrAlY and NiCr coatings on T91 boiler steel

Hot corrosion performance of Atmospheric Plasma Spray (APS) and Detonation spray (DSC) CoCrAlY, NiCoCrAlY, and NiCr coatings on T91 steel were investigated at 650 °C for 100 cycles under Na2SO4-K2SO4-Fe2O3 mixed salt deposit in ambient air. The hot corrosion resistance of DSC coatings was found to be superior to their APS counterparts. Chromia and spinel oxides provided excellent corrosion resistance while no α-Al2O3 was observed. DSC NiCr and APS NiCoCrAlY offered the most and the least corrosion resistance, respectively. DFT calculation was performed to validate the thermodynamic stability of each oxide and identify the precise oxide formation.

Novel suspension route to incorporate graphene nano-platelets in HVAF-sprayed Cr3C2–NiCr coatings for superior wear performance

Graphene nano platelets (GNP) have several attractive properties, including excellent lubricity that can be used to develop wear-resistant coatings. Thermally sprayed chromium carbideenickel chromium (Cr3C2eNiCr) coatings are widely employed to impart wear resistance to engineering components. This work attempts to improve the wear resistance of high velocity air fuel (HVAF) sprayed Cr3C2eNiCr coatings by incorporating GNP using a hybrid approach in which Cr3C2eNiCr (powder) and GNP (suspension) are co-axially injected. Two different powder-to-suspension delivery ratios were employed in this study that utilizes a liquid feedstock in tandem with a HVAF system. Furthermore, for comparison, a pure (without graphene) Cr3C2eNiCr reference coating was deposited by the HVAF process using identical spray parameters. The as-sprayed coatings were characterized for their microstructure and phase constitution by SEM/EDS and X-Ray Diffraction. Mechanical properties such as hardness and fracture toughness were evaluated using micro-indentation technique. The hybrid coatings were subjected to dry sliding wear tests and wear performance was compared with reference Cr3C2eNiCr. The GNP incorporated hybrid coatings exhibited lower CoF and lower wear rates than the reference Cr3C2eNiCr coating. Post wear SEM/EDS analysis revealed different wear mechanisms predominant in the investigated coatings. Utilizing the above as a case study, this work provides key insights into a new approach to produce GNP incorporated coatings for mitigating wear.

Improving the corrosion resistance of ferritic-martensitic steels at 600 °C in molten solar salt via diffusion coatings

Molten salt corrosion in the receiver's piping system and storage tanks is still one of the major drawbacks of concentrated solar power (CSP) plants and is currently covered by using expensive high-alloyed steels or Ni-base alloys. The employment of cheaper structural materials combined with protective coatings is an attractive alternative to increase cost-efficiency. The present study investigates the corrosion resistance of three different coatings (a pure Ni, a Cr and a combined Ni + Cr coating) deposited on ferritic-martensitic X20CrMoV12-1 steel during isothermal immersion in molten solar salt in comparison to uncoated Ni-base alloy Haynes 230. Exposure tests were conducted at 600 °C for up to 1000 h. To identify the individual role of the main elements, exposure tests of pure Fe, Ni and Cr were undertaken as well. In addition to the cross-sectional investigations via light-optical microscopy and EPMA, X-ray diffraction measurements were conducted to identify the corrosion products. These were complemented by weight change measurements and chemical analysis of the salts after the exposure tests. The study reveals differences between the corrosion behaviour of the different coating approaches, which is discussed as a function of coating composition and degradation mechanisms. Whereas pure Cr or pure Ni coatings offer hardly any improved protection of the uncoated X20CrMoV12-1 substrate, the combined Ni + Cr coating performs very well and improves the scaling behaviour of X20CrMoV12-1 significantly. The outcome highlights the correlation between the solubility of the pure metals Fe, Ni and Cr in molten solar salt and the corrosion resistance of the investigated coatings.

Developing partially oxidized NiCr coatings using the combined flame spray and plasma spray process for improved wear behaviour at high temperature

The powders of NiCrBSiFe and NiCr are partially oxidized using a flame spray process and are deposited on MDN321 steel substrate using a plasma spray process. The effect of partial oxidization on microstructure, microhardness, density, bond strength, and porosity of the coatings is analyzed. The friction and wear behaviour of the coatings was assessed using a pin-on-disc tribometer by varying loads (10, 20 and 30 N), sliding velocities (1, 2 m/s) and temperatures (RT, 200, 400 and 600 °C). Worn surfaces of NiCrBSiFe and NiCr coatings consist of oxide phases of SiO2, NiO, Cr2O3 and NiCr2O4 at elevated temperatures. These phases contributed to reducing the wear rate by five folds in coated steels compared to uncoated steels at 600 °C. The wear rate in coating decreases with an increase in temperature. The coefficient of friction was reduced gradually with the temperature in coatings and substrate. The wear rate coefficient of NiCr coating was 1.7 times higher than the NiCrBSiFe coating.

Structure and Tribological Properties of Ni–Cr–Al-Based Gradient Coating Prepared by Detonation Spraying

In this paper, Ni–Cr–Al coatings were deposited using the detonation spraying method. The aim was to investigate how technological parameters influence coating structure formation, phase composition and tribological performances. We observed that the degree to which the barrel is filled with an O2/C2H2 gas mixture strongly influences the chemical composition of manufactured coatings. High degrees of barrel filling led to a decrease in aluminum content in the coating. Filling degrees of 40% and 50% produced sprayed coatings in which only Ni–Cr phases could be found. When the filling degree was reduced up to 25%, Ni–Al phases began to form in the sprayed coatings. Gradient Ni–Cr–Al coatings were produced by gradually reducing the filling degree from 50% to 25%. These coatings are characterized by Ni–Cr near the substrate level with Ni–Cr and Ni–Al phases at higher levels. The results obtained confirm that gradient Ni–Cr–Al coatings exhibit high hardness as well as good wear resistance.

CrO2(OH)2 volatilization rate and oxidation behavior prediction of the NiCr coating in air-H2O environment at 650 ℃

The volatilization rate and lifetime prediction of the NiCr coatings produced by high-velocity oxygen-fuel were investigated at 650 °C in air containing 20 % water vapor. Water vapor could significantly accelerate the consumption of Cr which reduced the service time of the coatings. Oxidation kinetic rate of the NiCr coating and volatilization rate of the CrO2(OH)2 were calculated using classical gas transport theory in the viscous-flow regime. According to the practical oxidation kinetic rate, we built a relationship to predict the lifetime of the NiCr coating.

Microstructure and Properties of Cold Sprayed NiCrAl Coating on AZ91D Magnesium Alloy

Herein, a NiCrAl coating was prepared on the AZ91D magnesium alloy by cold spraying technology. The microstructure, wear resistance, and corrosion resistance of the cold sprayed NiCrAl coating were studied and compared with two NiCrAl coatings prepared by plasma spraying. The results showed that the porosity of the two-plasma sprayed NiCrAl coatings was 3.21% and 2.66%, respectively, while that of the cold sprayed NiCrAl coating was only 0.68%. The hardness of the cold sprayed NiCrAl coating (650 HV0.1) was higher than those of the two-plasma sprayed NiCrAl coatings (300 HV0.1, 400 HV0.1). In the abrasion resistance test, the cold sprayed NiCrAl coating showed a lower friction coefficient (0.346), less wear volume (3.026 mm3), and superior wear resistance accordingly compared with the two-plasma sprayed NiCrAl coatings. Moreover, the scanning electron microscopy (SEM) morphology at the bottom of the wear trace of the cold sprayed NiCrAl coating showed a compact mechanically mixed layers (MML) structure, and its wear mechanism was mainly abrasive wear, with some fatigue wear. In the electrochemical test, the corrosion current density of the cold sprayed NiCrAl coating (4.404 × 10−2 A·cm−2) was much lower than those of two plasma sprayed coatings (25.96 A·cm−2, 26.98 A·cm−2), indicating that the cold sprayed NiCrAl coating had superior corrosion resistance. Therefore, preparing a cold sprayed NiCrAl coating is a feasible method to comprehensively improve the wear resistance and corrosion resistance of the AZ91D magnesium alloy.

Effect of Laser Surface Melting on Microstructure of Cold Sprayed Ni20Cr Coatings

Effect of Laser Surface Melting on Microstructure of Cold Sprayed Ni20Cr Coatings

Erosion-Corrosion Protection Due to Cr3C2-NiCr Cermet Coating on Stainless Steel

Cr3C2–NiCr coatings have been used extensively to combat the erosion corrosion of hydro power turbine blades made of stainless steel. Cr3C2–NiCr coatings are also used in aqueous corrosive environments due to the high corrosion resistance rendered by the NiCr binder. In this investigation, both erosion and corrosion environments are introduced to cermet coating to study corrosion behavior using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. The cermet coatings are useful for reducing the risk of deterioration of mechanical properties of hydro power turbines due to the continuous exposure to the erosive and corrosive action of the corrosive environment containing silt. It was observed that Cr3C2–NiCr coating offered a reasonable improvement in corrosion resistance when compared to bare substrate. The corrosion behavior of the coating was studied in a 150 mL solution of 0.1 M NaCl with 2 gms of quartz particles (0.2–0.8 mm) at various rotation speeds (3000, 4500, 6000 rpm) of the solution over a 1 h immersion using potentiodynamic polarization and EIS studies in a specifically designed experimental set-up for erosion corrosion. When compared to the bare stainless steel samples at 3000 rpm and 6000 rpm, the coating showed the highest improvement at 6.57 times and the least improvement at 3.79 times, respectively.

Effect of Ni60-WC on Properties of NiCrAl Coating

Ni60-WC reinforced NiCrAl coatings were prepared on the TC4 alloy by laser cladding technique with different mixed powders: 95%NiCrAl-5%Ni60-WC, 85%NiCrAl-15%Ni60-WC, 75%NiCrAl-25%Ni60-WC (wt.%). The microstructure, phases compositions, microhardness, wear properties were studied. The results showed that the mainly consists of dendrites and bulk like compounds crystal and some of flower-like microstructure formed. The XRD results indicated TiC, VC metalloceramics, Cr0.2Ti0.8C, NiTi2, Ni2W4C and AlFe3C intermetallic compounds generated. The microhardness of the coatings are 881HV0.3, 778HV0.3 and 788HV0.3, respectively. Compared with the substrate hardness about 200HV0.3, the hardness of coatings have been improved significant. The friction coefficient of coatings are 0.35, 0.42, and 0.42, respectively, and the wear rates of the coatings are 3.86×10-7mm3N-1m-1, 4.51×10-7mm3N-1m-1 and 4.34×10-7mm3N-1m-1, respectively.

Influence of the detonation-spraying mode on the phase composition and properties of Ni-Cr coatings

Influence of the detonation-spraying mode on the phase composition and properties of Ni-Cr coatings

Microstructure and Property Modification of Cold Sprayed Coatings Using Different Grain Sizes of Cr3C2-25(Ni20Cr) Composite Powder

This article describes the microstructure and mechanical properties of Cr3C2-25(Ni20Cr) cermet coatings cold sprayed on Al 7075 alloy substrates. The aim of the study was to determine the effect of three different powder grain sizes and three different standoff distances on the microstructure (phase composition, surface topography, volume fraction and distribution of the Cr3C2 phase in the Ni20Cr matrix and porosity) and the mechanical properties (hardness and abrasive wear resistance) of the Cr3C2-25(Ni20Cr) coatings. The examinations revealed that the thickness of the coatings obtained with same duration and feed rate decreased with increasing powder grain size and standoff distance. The coatings produced from powders with smaller particles had higher hardness and more compact structures than the coatings obtained from powders where the particles were larger (more than 40 μm). The coatings produced from finer powders were also more resistant to abrasive wear.

Erosion corrosion behaviour and mechanical properties of wire arc sprayed Ni-Cr and Ni-Al coating on boiler steels in a real boiler environment

ABSTRACT The aim of the current investigation is to evaluate the erosion-corrosion (E-C) performance of wire arc sprayed Ni-20Cr and Ni-5Al coatings on T22 and SA516 boiler steels in a real boiler environment. The microstructure, mechanical properties and high temperature E-C performance of the deposited coatings were investigated. The E-C behaviour of the test samples was determined in the superheater zone of the coal fired boiler at 750°C for fifteen cycles, where each cycle involved 100 hours of heating followed by 1hour of cooling. Thickness loss data was used to determine the E-C behaviour of the coating. XRD, SEM/EDS were used to analyse the test samples. Ni-20Cr coated SA516 steel decreased maximum E-C rates by 98.05%, while Ni-5Al coated T22 steel reduced E-C by 68.63% than the base steel. This higher E-C resistance is due to the formation of protective phases (Cr2O3, NiO and Al2O3) and better mechanical properties of the coatings.

Microstructure of HVOF-sprayed Ag–BaF2⋅CaF2–Cr3C2–NiCr coating and its tribological behavior in a wide temperature range (25 °C to 800 °C)

Ag–BaF2⋅CaF2–Cr3C2–NiCr composite powders were prepared by physically blending commercial BaF2⋅CaF2–Cr3C2–NiCr and Ag powders. Ag–BaF2⋅CaF2–Cr3C2–NiCr composite coatings were deposited on Inconel 718 alloy substrate by high velocity oxy-fuel (HVOF) spraying. The friction and wear behavior of the coatings under dry sliding against Si3N4 balls from 25 °C to 800 °C was evaluated with a ball-on-disk high temperature tribometer. The microstructure and composition of the samples were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectrometer. Results showed that the composite coatings were mainly composed of hard phase of Cr3C2, binder phase of NiCr, high-temperature lubrication phase of fluorides and low-temperature lubrication phase of Ag. The fluorides existed in the forms of both crystal particles and amorphous state, while the silver featured as typical thermally sprayed splats. Due to the high flame temperature, some fluorides have been oxidized to chromates and around 30 wt% of Ag was lost during spraying. In addition, it was found that Ag content had an important influence on the composite coating, and an appropriate dosage of metallic silver could effectively improve the tribological performance of the coating. The generation of AgCrO2 at moderate (500 °C and 650 °C) temperature and BaCrO4 at high temperature (800 °C) could contribute to the decline in friction coefficients and wear rates of Ag–BaF2⋅CaF2–Cr3C2–NiCr coatings.

Microstructure and cavitation erosion behavior of HVOF sprayed ceramic-metal composite coatings for application in hydro-turbines

Cavitation erosion commonly occurs in hydro-turbine blades and has been a crucial issue for operation of hydro-turbines. In this paper, high-velocity oxygen-fuel (HVOF) spraying was applied to produce Cr3C2–NiCr and WC-CoCr ceramic-metal coatings. The microstructure and the relationship between the flow velocity and cavitation erosion behavior of the coatings were evaluated, and their cavitation erosion mechanisms at different flow velocities were discussed. Experimental results showed that the WC-CoCr coating exhibited slightly lower porosity and significantly higher microhardness as compared to the Cr3C2–NiCr coating. Both ceramic-metal coatings were combined well with the steel substrate and showed an increase in the volume loss rates with increasing flow velocity. The volume loss rates for the WC-CoCr coatings were lower than that of the Cr3C2–NiCr coatings at each flow velocity. The critical flow velocity of the Cr3C2–NiCr coating was in the region of 33.5–41.9 m s−1. The cavitation erosion process of the Cr3C2–NiCr coatings mainly included removal of chromium carbide particles, coalescence of the craters and brittle detachment of the coating, while the failure mechanism of the WC-CoCr coatings was erosion of the binder phase at 23.4 m s−1, cavitation pits interlinking at 33.5 m s−1, and detachment of the WC particles at 41.9 m s−1.

Tribological assessment of NiCr, Al2O3/TiO2, and Cr3C2/NiCr coatings applied on a cylinder liner of a heavy-duty diesel engine

Internal combustion engines consume about 90% of fuel refined from crude oil which supplies 30% of the annual global flow of energy. Heavy-duty diesel engines are the primary source of power used in highways, marine, railroads, and power stations. The right coating can improve the tribological properties of cylinder liners and increase the mechanical efficiency of an engine. Also, it can help to extend the maintenance periods, and enhance the reliability of the vehicles. In this research, tribological and economic evaluations were performed for coated and uncoated substrates from a cylinder liner of a heavy-duty diesel engine, aiming to lower friction, wear rate, and maintenance cost. A reciprocating friction test was conducted under dry condition using Wolfram carbide (tungsten carbide) ball applied a 10 N normal load on a ball on disk geometry. The cylinder liner was made of gray cast iron, and the substrates obtained were coated with three different coating materials (Cr3C2/NiCr, NiCr, and Al2O3/TiO2) through the thermal spray and high-velocity oxy-fuel coating process. Tribological evaluations showed that the substrates coded with Al2O3/TiO2 and Cr3C2/NiCr had the lowest friction coefficient and wear rate. The most economical coating was Al2O3/TiO2, being able to supply about 61% lower coefficient of friction and 94% less wear rate relative to the uncoated sample, for the price of one-third of the Cr3C2/NiCr coating and one half of a new gray cast iron cylinder liner.

Microstructure and Selected Properties of Cr3C2-NiCr Coatings Obtained by HVOF on Magnesium Alloy Substrates.

In present work the Cr3C2–NiCr coating was deposited on magnesium alloy substrate with high velocity oxygen fuel (HVOF) spraying. The microstructure of the samples has been characterized by means of electron microscopy, SEM and phase composition analysis carried out. The porosity of coatings has been also estimated. Finally, tests of selected mechanical properties, such as instrumented indentation, abrasive erosion have been performed. The results of the investigations confirmed that dense, homogeneous and well-adhered Cr3C2–NiCr cermet coating is possible to obtain onto the magnesium AZ31 alloy substrate. Moreover, the coatings exhibit high resistance to erosion.

Investigation of PVD thin films as hydrogen barriers in aluminized press hardened steels (PHS)

The reduction of hydrogen intake of aluminized boron press hardened steel Usibor®1500 during austenitisation at 900 °C under N2 humid atmosphere has been explored by applying thin physical vapour deposition (PVD) coatings. A wide range of metallic coatings have been explored including Au, Cu, Ti, Y, Mo, W, Cr, Ni, Ni0.8Cr0.2, Ni0.4Cr0.6, Inconel 690 and stainless steel 316L as well as some nitrides WN, CrN and oxides Al2O3 and SiO2. W, Y and Ti enhance the increase of hydrogen intake, SiO2 and Al2O3 have no effect while Au, Cu, Mo, CrN, WN, Cr, Ni and NiCr alloys provide a decrease of the hydrogen ingress. The barrier effects of these coatings are discussed as a function of their thicknesses and chemical composition of the NiCr coatings. The most efficient barrier to hydrogen intake is obtained in this study for Ni0.8Cr0.2 coating with a reduction higher than 50% for an optimum coating thickness of 200 nm. The structure and the surface of the latter coating are investigated after austenitisation by a combination of techniques.

Fireside corrosion on T24 steel pipes and HVOF NiCr coatings exposed to different salt mixtures

Fireside corrosion of T24 steel and HVOF NiCr-coated counterparts is investigated. The samples were exposed to different deposits of NaCl, KCl, Na2SO4 and K2SO4 at 650 °C for 360 h in dry air. The deposit with the highest chloride content (NaCl-KCl) was the most aggressive followed by the ones combining chlorides and sulfates. In contrast, the sulfate-based salts did not produce severe damage. The NiCr coating maintained the integrity of the substrate but the chlorides induced a great attack on the coating. The formation of K2CrO4 and the penetration of chlorides are identified as the main factors of this attack.

Surface Engineering Analysis of D-Gun Sprayed Ni-20Cr Coating in Aggressive Environment

Surface Engineering Analysis of D-Gun Sprayed Ni-20Cr Coating in Aggressive Environment