High-temperature Corrosive Environments Research Articles

The influence of multi-step modification method on the adsorption effect of alkali metal adsorbent in high temperature corrosive environment

In order to mitigate the serious high-temperature corrosion problem caused by gaseous alkali metal salts on the heating surface of boilers, the adsorption effect of kaolinite for alkali metal vapor stood out among many silicon-aluminum-based adsorbents. To improve the adsorption capacity of kaolinite for alkali metal vapor, a novel multi-step modification method was proposed in this reported work. The multi-step modification method consisted of the following four steps, which were pre-intercalation, replacement intercalation, multiple intercalation, and exfoliation, and it was found that the multi-step modification method resulted in the increase of the particle size, the expansion of the interlayer spacing, and the expansion of the porosity structure of kaolinite. The physicochemical analyses showed that the multi-step modification method made it easier to dehydrate kaolinite to produce metakaolinite. In addition, it was found that the multi-step modification method led to the generation of a new crystalline phase of kaolinite, which was formed by the dehydration and condensation of methanol and AlOH groups. It also led to the pre-removal of some of the hydroxyl groups in kaolinite. The insoluble sodium loading capacity of the multi-step modified kaolinite was enhanced by 64.47% and the insoluble potassium loading capacity was enhanced by 29.68% under the adsorption condition of 850 °C for 45 min. Therefore, the multi-step modification method enhanced the alkali metal vapor adsorption capacity of raw kaolinite under high-temperature environment, which laid a certain foundation for subsequent research in this field.

Alumina-based ceramic coatings obtained by the SHS process for high temperature corrosion and wear protection of steel tubulars

Ceramic coatings have much potential for steel tubulars' protection against high temperature corrosion of gases and liquids, often containing abrasive solid particles, and to enhance their integrity in severe power generation, mineral and oil & gas production environments. Combining self-propagating high-temperature synthesis (SHS) with high-speed centrifugal process, alumina-based coatings were produced onto the inner surface of tubulars for these applications. According to the developed SHS batch formulation and the designed centrifugal device and process, well-consolidated composite ceramic layers with a thickness of 1.5–3 mm have been obtained. The developed SHS process does not create hazardous gases, and it takes ∼30 s for the ceramic phase formation. A high-level compaction due to centrifugal forces and a liquid phase sintering approach provided a dense gradient ceramic structure formed by oxide grains cemented by a glassy phase and bonded with a steel substrate through a thin iron layer. The coatings were successfully produced onto the inner surface of carbon and stainless steels’ tubes of up to 12 ft-lengths and standard dimensions from 2 to 3/8″ to 7.5” OD. The coatings were tested, for the first time, in wear and high-temperature corrosive environments, e.g., in steam–H2S–CO2 gases and molten salts, with encouraging results due to high contents of Al2O3 and some other oxides and consolidated ceramic structure. They can be recommended for applications in high-temperature (900 °C and greater) corrosive environments and erosive flows.

Thermal spray coatings for molten salt facing structural parts and enabling opportunities for thermochemical cycle electrolysis

AbstractThermochemical water splitting stands out as the most efficient techniques to produce hydrogen through electrolysis at a high temperature, relying on a series of chemical reactions within a loop. However, achieving a durable thermochemical cycle system poses a significant challenge, particularly in manufacturing suitable coating materials for reaction vessels and pipes capable of enduring highly corrosive conditions created by high‐temperature molten salts. The review summarizes thermally sprayed coatings (deposited on structural materials) that can withstand thermochemical cycle corrosive environments, geared towards nuclear thermochemical copper–chlorine (CuCl) cycles. An assessment was conducted to explore material composition and selection (structure–property relations), single and multi‐layer coating manufacturing, as well as corrosion environment and testing methods. The aim was to identify the critical areas for research and development in utilizing the feedstock materials and thermal spray coating techniques for applications in molten salt thermochemical applications, as well as use lessons learnt from other application areas (e.g., nuclear reaction vessels, boilers, waste incinerators, and aero engine gas‐turbine) where other types of molten salt and temperature are expected. Assessment indicated that very limited sets of coating‐substrate system with metallic interlayer is likely to survive high temperature corrosive environment for extended period of testing. However, within the known means and methods, as well as application of advanced thermal spray manufacturing processes could be a way forward to have sustainable coating‐substrate assembly with extended lifetime. Spraying multi‐layered coating (nano‐structured or micro‐structured powder materials) along with the application of modern suspension or solution based thermal spray techniques are considered to result in dense microstructures with improved resistance to high temperature thermochemical environment.

Evaluation of welded joints of austenitic stainless steels at high temperatures in the presence of SO2, SO3, and O2

This study undertakes a comprehensive examination of crack formation in welded joints of austenitic stainless steel 304, particularly in high-temperature corrosive environments prevalent in sulfuric acid production units. Initially, an investigation was conducted on a steam superheater that had been operational for about 19 years, where a crack formation was discovered. This unit, subjected to a working temperature of approximately 620 °C and exposure to gases composed of SO2, SO3, and O2, allowed for the analysis of corrosion products formed during the crack propagation. Building on the initial findings, a more controlled study was undertaken, focusing on the cracks formed in welded joints using the coated electrode E308L for joining 304H stainless steel plates under similar corrosive conditions. The specimens, constructed using materials typically utilized in sulfuric acid production units, were designed based on the double-beam principle, enabling exposure to both tensile stresses and a corrosive environment at high temperatures. Over a span of 122 days, or roughly 3000 h, microstructural analyses were conducted, revealing that the formation and propagation of cracks preferentially occurred through fragile microconstituents, notably the sigma phase. These findings mirrored the sigma phase formation observed in the initial superheater study and point to a stress corrosion process acting synergistically with sigma phase development, and consequently seen as a probable cause of failure. This comprehensive evaluation, therefore, indicates a significant role of the sigma phase and corrosive environments in influencing crack formation and propagation in welded joints of austenitic stainless steels, which require further studies for preventative strategies in industrial setups.

Recent progress on corrosion mechanisms of graphene-reinforced metal matrix composites

Abstract For components serving in high temperature, humidity, and salinity marine corrosive environment, it is vital to analyze the causes of corrosion behavior and corrosion mechanisms. Metal matrix composites (MMCs) are commonly used materials for offshore equipment. In this work, the corrosion factors of MMCs in marine environments are analyzed from the characteristics of high temperature, humidity, and salinity service environment, and the corrosion mechanisms are summarized. Graphene (Gr) has excellent comprehensive properties and great potential for applications in metal protection materials. In recent years, research into Gr anti-corrosive applications encompasses two aspects: pure Gr coatings and Gr composite coatings. Gr applied in MMCs is yet to be extensively studied. Therefore, this study analyzes the corrosion resistance of Gr–metal composites and discusses the corrosion resistance mechanisms of Gr-reinforced MMCs, which provides a reference for the design of Gr-reinforced metal composites and the optimization of corrosion resistance performance. Finally, future development directions for Gr–metal composites are proposed, and the critical factors such as defects, dispersion, content, size, arrangement, interface, and conductivity of Gr in the composites affecting their anti-corrosion properties are discussed.

Phase evolution and hot corrosion behavior of Yb2O3 and CeO2 co-doping YSZ ceramics under high temperature

The phase instability of 6–8 wt% Y2O3 partially stabilized ZrO2 (YSZ) in high temperature and molten salt corrosion environment seriously limits its application above 1200 °C. In this study, the chemical co-precipitation method prepared Yb2O3 and CeO2 co-doping YSZ (YbCeYSZ) ceramic materials to improve the above issues. It was proposed to improve the defect of high-temperature phase instability of CeO2-doped YSZ materials by adding Yb2O3. The phase stability, thermal insulation properties, and Na2SO4 + V2O5 molten salt corrosion behavior of YbCeYSZ ceramic materials were studied. The results showed that 2.5 mol% Yb2O3-4 mol% CeO2-YSZ material (2.5Yb4CeYSZ) had no monoclinic phase (m-ZrO2) after holding at 1500 °C for 120 h due to forming more defect clusters in the system. Meanwhile, the 2.5Yb4CeYSZ had excellent thermal insulation performance and molten salt corrosion resistance, which was attributed to the strong phonon scattering in the 2.5Yb4CeYSZ material system and the low reactivity between the stabilizer and the corrosive salt.

Investigation on deterioration mechanism of geopolymer cemented coal Gangue-Fly ash backfill under combined action of high temperature and salt corrosion environment

The geopolymer cemented coal gangue-fly ash backfill (GCGFB) is pumped to the goaf to maintain the stability of surface subsidence, which is inevitably affected by the high temperature caused by geothermal heat and ion erosion of mine water. To study the deterioration mechanism of GCGFB caused by salt corrosion and high temperature of mine water, in this study, a customized temperature-controlled corrosion chamber was used to carry out the corrosion experiment of GCGFB. A three-factor and three-level orthogonal experiment was designed. The uniaxial compressive strength (UCS), relative dynamic elastic modulus (RDEM) and weight deterioration of GCGFB under different combinations of temperatures, solution concentrations, and solution types were analysed. The deterioration mechanism of GCGFB was analysed through the testing of scanning electron microscopy (SEM)-energy dispersive spectrometer (EDS) and X-ray diffraction (XRD). From the results, the most significant factors on the deterioration of GCGFB is solution type. Cl− and SO42− caused chemical-physical erosion on GCGFB, and the UCS, RDEM and weight of GCGFB increased first and then decreased with ages. The reason for the increase was due to the compacting effect of Friedel 's sat, ettringite, and gypsum filled in GCGFB. The reason for the decrease was due to the expansion pressure on the inner wall of the pores of the GCGFB caused it to crack or even peel off. The GCGFB was corroded in 20 % Na2SO4 solution at 40 °C for 90 days, and the UCS increased by 164.90 % compared to that of the uncorroded specimen. Mg2+ brought chemical erosion on GCGFB, and a decreasing law of the UCS, RDEM and weight with ages was shown. During the Mg2+ erosion, there are more and more M−S−H in matrix, which reduced the density of specimen apparently. The GCGFB was corroded in 5 % MgCl2 solution at 40 °C for 180 days, and the macropore area distribution of GCGFB is increased to 23.499% than that of uncorroded group. The expansion stress of corrosion products would squeeze the inner pore wall, resulting in microcracks inside GCGFB and high temperatures would accelerate the migration of ions and the formation of corrosion products. Overall, this study can provide a new insight or valuable data for the designing of ultra-high performance, low-caron, eco-friendly, and durable GCGFB structures in the harsh mining environments.

Hollow fiber polytetrafluoroethylene membrane heat exchanger with anti-corrosion properties

Thin hollow polytetrafluoroethylene (PTFE) fibers (thickness: 200 μm; outer diameter: 1.3 mm; inner diameter: 0.9 mm) were produced by a specially designed die via stretching and sintering, and assembled into a shell-and-tube membrane heat exchanger. The effects of the flow velocities on the heat transfer performance and thermal resistance were studied in a water-water system. The maximum overall heat transfer coefficient (OHTC) was up to 391 W/(m2‧K) (flow velocities: 0.24 m/s for hot water; 0.11 m/s for cold water). The OHTC became larger (increased from 229 to 391 W/(m2‧K)) at a higher cold water flow velocity (increased from 0.02 to 0.11 m/s) on the shell side when maintaining the hot water flows on the tube side. The OHTC increased from 119 to 391 W/(m2‧K) first and then decreased to 298 W/(m2‧K) with the increase of the hot water flow velocities (from 0.08 to 0.40 m/s) on the tube side, while maintaining the cold water flow velocity on the shell side. The thermal resistance results showed that the increased flow velocity (0.11 m/s) on the shell side significantly reduced the thermal resistance to 2.56 × 10−3 K m2/W on the shell side. Furthermore, the flow turbulence on the tube side is favorable for the heat transfer, and the optimum flow velocity on the tube side was 0.24 m/s. The hollow PTFE fibers were extremely stable and durable in corrosive solutions, with less than 1% changes in bursting strength, tensile strength at break, and elongation at break. This work opens new opportunities for the fabrication and application of hollow fiber PTFE membrane heat exchangers to reuse waste heat in high temperature corrosive environments.

Applications and Developments of Thermal Spray Coatings for the Iron and Steel Industry.

The steel making processes involves extreme and harsh operating conditions; hence, the production hardware is exposed to degradation mechanisms under high temperature oxidation, erosion, wear, impact, and corrosive environments. These adverse factors affect the product quality and efficiency of the steel making industry, which contributes to production downtime and maintenance costs. Thermal spray technologies that circumvent surface degradation mechanisms are also attractive for their environmental safety, effectiveness and ease of use. The need of thermal spray coatings and advancement in terms of materials and spray processes are reviewed in this article. Application and development of thermal spray coatings for steel making hardware from the molten metal processing stages such as electric arc and basic oxygen furnaces, through to continuous casting, annealing, and the galvanizing line; to the final shaping process such as cold and hot rolling of the steel strips are highlighted. Specifically, thermal spray feedstock materials and processes that have potential to replace hazardous hard chrome plating are discussed. It is projected that novel coating solutions will be incorporated as awareness and acceptance of thermal spray technology grows in the steel making sectors, which will improve the productivity of the industry.

Development of alumina-based hybrid composite coatings for high temperature erosive and corrosive environments

Low alloy steels are being used for structural applications exposed to extreme environmental conditions such as, high temperature, corrosive and erosive environment in petrochemical industry, turbomachinery, chemical reactors, and nuclear industry. These extreme environment conditions severely affect the performance of critical components. In this context, it is attempted to develop the alumina (Al2O3) based hybrid composite coatings on ASME SA387-22-2 steel to mitigate the effect of high temperature erosion and corrosion. High-velocity oxygen-fuel (HVOF) thermal spray technique is used to develop the coatings. The physical characteristics of the coatings are recorded in terms of density measurement, thermogravimetric analysis, and surface wettability analysis. Hardness and elastic modulus of the coatings are estimated at room temperature using nanoindentation and microhardness at high temperature (400 °C) using Vickers hardness tests. Metallurgical characterization and eroded surface analysis are done using Scanning electron microscopy (SEM), Energy dispersive X-ray spectroscopy (EDS), Raman spectroscopy and X-ray diffraction (XRD). High temperature (400 °C) erosion performance of the coatings is recorded using solid particle Air-jet erosion tester. The Corrosion performance of the coatings are analyzed using the potentiodynamic polarization technique and electrical impedance spectroscopy (EIS). Significant improvement in erosion performance (Erosion reduction: ≈ 96% at 30° and ≈63% at 90°) and corrosion resistance (≈92% reduction in corrosion rate) is recorded for the coating reinforced with 0.8 wt% ceria (CeO2) and 3 wt% hexagonal boron nitride hBN as compared to the bare steel. The improved erosive and corrosive characteristics of the coatings are attributed to the refined microstructural aspects, in-situ generation of hard second phases, and induction of hydrophobicity in the composite structure.

A high-performance lung-inspired ceramic 3D-printed heat exchanger for high-temperature energy-efficient systems

The thermal-to-electric conversion efficiency of next-generation supercritical carbon dioxide and concentrated solar power plants is significantly augmented by operating at temperatures exceeding 1000 °C if durable, compact, high-temperature heat exchangers (HXs) are developed. Whereas metal 3D-printed HXs fail to operate under extreme temperature conditions, ceramic 3D-printed HXs are deemed promising candidates for high-temperature, highly oxidizing environments. Ceramic 3D-printed HXs, however, exhibit a high gas permeability issue through hot and cold separating walls due to an excessive non-volatile photopolymer content utilized. Here, a novel leak-free lung-inspired ceramic 3D-printed HX employing a highly complex heat transfer topology is introduced for high-temperature energy-efficient systems. The high permeability issue of ceramic 3D-printed HXs is fully eliminated through a uniform zinc-based coating. It was determined that the effective thermal conductivity of the sintered ceramic heat exchangers is affected by the coated layer. The high-temperature thermal and hydraulic characteristics of the lung-inspired ceramic 3D-printed HX are comprehensively examined and compared against a millichannel 3D-printed HX module. Experimental results indicated that the lung-inspired 3D-printed ceramic HX significantly outperforms its millichannel counterpart. The lung-inspired HX shows a volume-based power density of 8.2 MW/m3 at a hot-side inlet air temperature of 700 °C, a 71 % improvement compared with that of the millichannel HX. More importantly, the higher thermal duty of the lung-inspired 3D-printed HX is realized at a lower normalized pressure drop penalty of 510 Pa/W, 22 % lower than that of the millichannel ceramic HX. This study accelerates the advancement of high-performance ceramic 3D-printed HXs with complex topologies for high-temperature energy-efficient systems and/or corrosive environments.

The high-temperature mechanical properties of HS110S steel and its corrosion behaviors in harsh downhole environment

PurposeThis paper aims to investigate the high-temperature mechanical properties of HS110S steel and its corrosion behaviors in harsh downhole environment.Design/methodology/approachIn this work, mechanical property measurements were carried out from 25°C to 350°C and the scanning electron microscopy was used to observe the fracture morphology. The weight-loss measurements and surface characterization were used to evaluate the corrosion resistance of HS110S steel in harsh downhole environment.FindingsResults show that the yield strength and tensile strength of HS110S steel at 350 °C are 779 and 861 MPa, respectively. Compared with room temperature, the reduction rate values are both less than 20%. At the high-temperature corrosion environment (350 °C), the static and dynamic corrosion rates are 0.9668 and 1.9236 mm/a, respectively. The generated corrosion products are mainly composed of FeSx, FeCO3 and Fe3O4. Therefore, the HS110S steel applied under such conditions needs to take suitable protective measures.Originality/valueIn general, the HS110 steel has widely used in conventional development conditions (e.g. low H2S or high CO2 environments). However, to the best of the authors’ knowledge, no studies have reported on its application at more than 250°C. Therefore, this work can be a reference to the application of HS110S steel in high-temperature corrosion conditions.

Influence of Spheroidizing Treatment on the Graphite and Mechanical Properties of Cast High-nickel Austenitic Ductile Iron

Cast high-nickel austenitic ductile iron (CHNADI) is widely used in high temperature, low temperature and strong corrosive environments due to its good comprehensive mechanical properties. However, its low yield strength limits its application in nuclear power equipment. Herein, the influence of nodularizer and processing temperature on the graphite and mechanical properties of CHNADI was investigated to significantly promote its properties. With the increase of nodularizer dosage, the mechanical properties of tensile strength, yield strength, elongation, and impact toughness first increase and then decrease. When the dosage of nodularizer is 0.9 mass% and the processing temperature is 1480°C, CHNADI exhibits excellent mechanical properties with the hardness of 132 HB, tensile strength of 450 MPa, yield strength of 224 MPa, elongation rate of 46% and impact toughness of 91 J. In this work, a new strategy of preparing CHNADI without heat treatment is proposed to meet the strict requirements of this material, which is significant for the development of CHNADI.

The Effect of Bond Coat Roughness on the CMAS Hot Corrosion Resistance of EB-PVD Thermal Barrier Coatings

In a high-temperature, high-flame-velocity, and high-pressure gas corrosion environment, the intercolumnar pores and gaps of electron beam–physical vapor deposition (EB-PVD) thermal barrier coatings (TBCs) may serve as infiltration channels for molten calcium–magnesium–alumino–silicate (CMAS), leading to the severe degradation of TBCs. In order to clarify the relationship between the roughness of the bond coat and the CMAS corrosion resistance of the EB-PVD TBCs, 7 wt.% yttria-stabilized zirconia (7YSZ) TBCs were prepared on the surfaces of four different roughness-treated bond coats. The effect of the bond coat roughness on the columnar microstructure of the EB-PVD YSZ was investigated. The effect of the change of the bond coat’s microstructure on the CMAS corrosion resistance of the EB-PVD YSZ was studied in detail. The results showed that the reduction in the roughness of the bond coat contributes to the improved formation of the EB-PVD YSZ columns. The small and dense columns are similar to a lotus leaf-like structure, which could reduce the wettability of CMAS and minimize the spread area between the coating and the CMAS melt. Thus, the CMAS corrosion resistance of the coating can be greatly improved. This preparation process also provides a reference for the preparation of other TBC materials, improving the resistance to CMAS hot corrosion.

Fractal coatings of Ni and NiYSZ for high-temperature corrosion mitigation in solar salt

This paper introduces a novel approach to corrosion mitigation using electrodeposited, fractal Ni and NiYSZ coatings. Highly adherent, durable multiscale coatings with fractal dimensions above 1.80 were fabricated. Corrosion characteristics of coated plain and etched surfaces of In800H, SS310, SS316 and SS347, in comparison to Ha230, were studied in molten 60% NaNO3 + 40% KNO3 at 600 °C. The corrosion rate of coated Fe-based alloys reduced by up to 68% compared to uncoated alloys and was on par with or lower than the corrosion rate of Ha230. The corrosion-resistant coatings provide for use of low-cost alloys in high temperature corrosive environments.

High Temperature Corrosion Behavior of High Velocity Oxy Fuel Sprayed NiCrMoFeCoAl-30%SiO2 and NiCrMoFeCoAl-30%Cr2O3 Composite Coatings on ASTM SA213-T22 Steel in a Coal-fired Boiler Environment

High-velocity oxy fuel (HVOF) sprayed coatings can improve the corrosion resistance of bare ASTM SA213-T22 boiler steel. In this report, we have investigated the NiCrMoFeCoAl-30%SiO2 and NiCrMoFeCoAl-30%Cr2O3 composite coatings were deposited on bare ASTM SA213-T22 boiler steel for corrosion protection. High-temperature corrosion studies were conducted in a molten salt (Na2SO4-60%V2O5) environment at 700ºC under thermo-cyclic conditions. The as-sprayed composite coatings are characterized for microstructure and mechanical properties. The thermo-gravimetric method was utilized to understand the kinetics of corrosion. Characterization of the corrosion products was examined by using scanning electron microscope (SEM)/ Energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) techniques. The obtained results suggest both the composite coatings are favorable to corrosion resistance over the bare ASTM SA213-T22 boiler steel. The NiCrMoFeCoAl-30%Cr2O3 composite coating was concluded to present a superior corrosion resistance in the high-temperature corrosion environment because of the uniform distribution of the composite coating matrix and the development of protective protection Cr2O3 in the scale. The molten salt heat-treated chromium oxide containing coating shows good corrosion stability than the silica composite. This could be attributed to the high temperature assisted formation metal chromates, chromites and oxide layers.

Study on sintering behaviour of ytterbium disilicate and ytterbium monosilicate for environmental barrier coating applications

Yb2Si2O7 (YbDS) and Yb2SiO5 (YbMS) are two promising materials being developed as environmental barrier coatings (EBC) for the protection of Ceramic Matrix Composites (CMC) applied to gas-turbine engines operating in high-temperature corrosive environments. In this work, sintering behaviours of YbDS, YbMS, and YbDS/YbMS composites compacts were investigated. The effect of the thermal ageing at 1350 °C on microstructural characteristics as well as crack healing and elastic properties were examined. It was found that YbDS had a lower critical sintering temperature and higher grain growth rate than that of YbMS due to lower activation energy. The sintering behaviour of the YbDS/YbMS composites showed that the addition of YbMS retarded the grain growth rates and contributed to the stabilisation of the elastic properties. In addition to this, composite containing 22 wt %YbMS displayed a crack healing behaviour during high-temperature exposure, which was attributed to the generation of compressive stress, consequently accelerated diffusion in the YbDS matrix.

Performance of air plasma sprayed Cr3C2–25NiCr and NiCrMoNb coated X8CrNiMoVNb16–13 alloy subjected to high temperature corrosion environment

Thermal barrier coating plays a vital role in protecting materials’ surfaces from high-temperature environment conditions. This work compares the demeanour of uncoated and air plasma sprayed Cr3C2–25NiCr and NiCrMoNb coated X8CrNiMoVNb16–13 substrates subjected to air oxidation and molten salt (Na2SO4 + 60%V2O5) environment condition at 900 °C for 50 cycles. Coating characteristics have been analyzed through microstructure, thickness, porosity, hardness, and bond strength. SEM, EDS and XRD analysis were used to analyze corrosion’s product at the end of the 50th cycle. Coating microstructures showed a uniform laminar structure that is adherent and denser with a coating thickness of 150 ± 20 μm and porosity less than 3.5%. The Microhardness of both the coated substrates were higher than that of the bare substrate. Cr3C2–25NiCr and NiCrMoNb coating bond strength was 38.9 MPa and 42.5 MPa. Thermogravimetric analysis showed the parabolic rate law of oxidation for all the substrates in both environments. In the molten salt environment, all the substrates exhibited higher weight gain compared to the air oxidation environment. In both environmental conditions, the uncoated X8CrNiMoVNb16–13 alloy exhibited higher weight gain than the coated substrates. The formation of Cr2O3, NiO and spinel oxide NiCr2O4 offers good resistance to corrosion to all the substrates in both the environmental condition. However, the presence of Mo and Nb significantly accelerated the corrosion of the substrate, thereby increasing the weight of the NiCrMoNb substrate. It is observed that Cr3C2–25NiCr and NiCrMoNb coating over the X8CrNiMoVNb16–13 substrate significantly protected the substrate against the hot corrosion than the bare alloy exposed to similar environmental conditions.

The study of Cr3C2-25NiCr and 35WC-Co/65NiCrBSi-based HVOF coatings for high-temperature erosion resistance application

ABSTRACT This work concerns a technique to produce thermal sprayed coatings using HVOF (high-velocity oxy-fuel) method as it is a widely used application in high-temperature erosion and corrosion environment. Two types of hard coatings Cr3C2-25NiCr and 35WC-Co/65NiCrBSi were sprayed on an SS-347 steel substrate. The erosion tests were done using high-temperature jet erosion set up to study the effect of velocity, temperature and impact angles on the coatings produced. The surface morphology of the samples was characterized using 3D laser confocal microscopy, light microscopy, scanning electron microscopy with EDAX and X-ray diffraction. Microhardness, porosity level and roughness parameters were evaluated to infer a probable mechanism of material removal. The Cr3C2-25NiCr coating exhibited ∼3.3 times higher erosion resistance than 35WC-Co/65NiCrBSi coating at 90° and 30° impact angles. SEM and 3D laser confocal images on the eroded surface of the coatings reveal the combination of ductile and brittle fracture of coating.

Silicon nitride modified enamel coatings enable high thermal shock and corrosion resistances for steel protection

Vitreous enamel is a durable inorganic material that has been extensively used in our daily life for decoration and protection of the metallic substrates. It guarantees optimal corrosion protection to the covered substrates, but, on the other side, the brittle nature of its matrix highly limits its application. Here new enamel coatings were synthesized by adding silicon nitride with the contents of 0, 1.5, 2.5, 3.5, 5, and 7.5 wt%. Scanning electron microscope (SEM), X-ray diffraction (XRD), micro indentation, impact, thermal shock, and potentiodynamic polarization tests were conducted to evaluate the microstructure and engineering properties of the studied enamel coatings. Results showed that the addition of silicon nitride leads to a high internal porosity of the enamel coating, which consequently improves the resistances against cracking under indentation and impact loads. Especially, the silicon nitride modified enamel coatings were found to survive more than 100 cycles of thermal shocks. Despite the porosity rising, the addition of silicon nitride enhanced the corrosion resistance of enamel coatings at early immersion age, and all enamel coatings kept around 10 times lower corrosion rate than that of the uncoated steel at later immersion age. The findings of this work may provide a solution to steel protection against harsh high-temperature exposure and corrosive environments.