Heat treatment effects on the corrosion resistance of some HVOF-sprayed metal alloy coatings

The present study evaluates the effects of a 600 °C, 1 h heat treatment on the corrosion resistance of three High Velocity Oxygen Fuel (HVOF) flame-sprayed alloy coatings: a Co-28Mo-17Cr-3Si (similar to Tribaloy-800) coating, a Ni-20Cr-10W-9Mo-4Cu-1C-1B-1Fe (Diamalloy-4006) coating and a Ni-32Mo-16Cr-3Si-2Co (similar to Tribaloy-700) coating. Electrochemical polarization tests and free corrosion tests were performed in 0.1 M HCl aqueous solution. The corrodkote test (ASTM B380-97R02) was also performed, to evaluate the coatings qualitatively. The heat treatment improves the corrosion resistance of the Co-28Mo-17Cr-3Si coating and of the Ni-20Cr-10W-9Mo-4Cu-1C-1B-1Fe coating by enhancing their passivation ability. The precipitation of sub-micron sized secondary phases after the treatment may produce galvanic microcells at intralamellar scale, but the beneficial contribution provided by the healing of the very small but dangerous interlamellar defects (normally present in thermal spray coatings but not detectable using ordinary scanning electron microscopy) prevails. The effect on Ni-32Mo-16Cr-3Si-2Co coatings is more ambiguous: its sensitivity to crevice corrosion is worsened by the heat treatment.     

Residual stresses in HVOF-sprayed ceramic coatings

In this paper, the residual stress state of thermally sprayed ceramic coatings was examined by combining different experimental and analytical techniques, in order to provide a thorough characterisation of through-thickness stress profiles and a cross-verification of results. HVOF-sprayed ceramics, manufactured using commercial and nanostructured Al₂O₃ powders and commercial Cr₂O₃ powders, and atmospheric plasma-sprayed (APS) ceramics, manufactured using commercial Al₂O₃ and Cr₂O₃ powders, were investigated.The near-surface stress was measured by X-ray diffraction. The through-thickness profile and the intrinsic quenching stress were analytically computed by the Tsui-Clyne iterative model, using the X-ray measurement result as input, and results were validated by the substrate chemical removal method. Further verification was achieved by applying the in-situ curvature technique to the deposition of HVOF-sprayed Al₂O₃ coating.HVOF-sprayed Al₂O₃ coatings deposited using both conventional and nanostructured powders feature a similar, almost equibiaxial tensile stress on the top surface (116.5 MPa and 136.5 MPa, respectively) and a moderate through-thickness gradient (about 12 MPa and 20 MPa, respectively). Their intrinsic quenching stresses were analytically estimated to be 184 MPa and 205 MPa, respectively. APS Al₂O₃ possesses higher top surface stress (220 MPa) and quenching stress (311 MPa). However, it shows a less pronounced stress gradient (≈ 3 MPa) than HVOF-sprayed Al₂O₃-based coatings, because cracks, pores and weak lamella boundaries in the APS coating can accommodate the deformations induced by the bending moments arising both during coating deposition and during cooling.The model-derived quenching stress of the conventional HVOF Al₂O₃ coating was validated by the in-situ curvature measurement technique.Cr₂O₃-based coatings are significantly different. They display a lower residual stress in the near-surface region: 20 MPa in the APS coating, 27.5 MPa in the HVOF one. The HVOF coating also exhibits a very large stress gradient of ≈ 77 MPa. Machining and sliding processes (like polishing and dry sliding tribological testing) change their surface residual stresses to compressive ones.     

Corrosion protection of AZ31 magnesium alloy by a TiO2 coating prepared by LPD method

TiO₂ layer was prepared as a protective coating for AZ31 magnesium alloy by the liquid phase deposition (LPD) method followed by an annealing treatment. The structural evolution and crystallization of coating brought by annealing were investigated by field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD), respectively. The corrosion protection performance was evaluated in a three-electrode electrochemical examination system. The anatase TiO₂ layer shows evident corrosion resistance. With the increase of the annealing temperature and prolongation of annealing time, the anticorrosion property was improved. The improvements of the anticorrosion properties were related with the structural evolution of the coating brought by the annealing treatment.     

Corrosion resistances of stressed building structures made of aluminum alloys

Propagation of various types of localized corrosion on real stressed building structures was examined. The naturally aged 1915T alloy used in such structures was found to be highly resistant under various atmospheric conditions. It was insensitive to corrosion cracking and exfoliation corrosion, and the pitting depth did not exceed 0.21 mm in both the base metal and weld joints after 20-year tests. A mathematical analysis of experimental data allowed the prediction that even on the Main Exhibition pavilion near the foundry, the maximum corrosion depth should not exceed 0.5 mm after 50 years of operation. Postwelding artificial aging reduced the resistance to corrosion cracking. Because of this, the Sochi Concert hall was attacked by numerous corrosion cracks. However, a decrease in the actual stresses below a threshold level for corrosion cracking prevents further crack propagation. The effects of corrosive components of the atmosphere were studied. Pitting corrosion was found to be mainly promoted by the presence of chlorides and acid discharges from production departments. Various factors that stimulate and inhibit the corrosion of stressed aluminum building structures were revealed. Original Russian Text ? V.S. Sinyavskii, V.D. Kalinin, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 6, pp. 635–646.     

Properties of the TiN coatings on previously Ti ion-implanted magnesium alloy substrate

To enhance the mechanical properties of TiN coating on magnesium alloy, metal vapor vacuum arc (MEVVA) ion implantation was performed to modify magnesium alloy substrate before TiN film deposition. Implantation energy was fixed at 45 keV and dose was at 9 × 10¹⁷ cm^− 2. TiN coatings were deposited by magnetically filtered vacuum-arc plasma source on unimplanted and implanted substrate. The microstructure composition distribution and phase structure were analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The chemical states of some typical elements of the TiN coating were analyzed by means of X-ray photoelectron spectroscopy (XPS). The properties of corrosion resistance of TiN coatings were studied by CS300P electrochemical-corrosion workstation, and the mechanism of the corrosion resistance was also discussed.     

Microstructure and corrosion properties of plasma-sprayed NiCr-Cr3C2 coatings comparison with different post treatment

The effects of laser and plasma arc remelting on the microstructure and properties of plasma-sprayed NiCr-Cr₃C₂ coatings on steel substrates have been investigated. The microstructure of the coatings has been analyzed by X-ray diffraction (XRD) and scanning electron microscope (SEM). It is found that the Cr₃C₂, δ-(Cr,Ni), Cr₇C₃ and Cr₂₃C₆ phases were obtained for both coatings, before and after remelting treatment. The laser remelting was operated in a continuous way with 800 W power in different scan speed, while the plasma arc remelting was operated with a plasma cladding machine under different scan currents. However, the denser microstructure of both remelted coatings can be obtained, especially for the plasma arc remelted coating. The Vickers microhardness measurement showed certain enhancement values for both remelted coatings. The corrosion behavior was evaluated through salt spray corrosion (SSC) method. Energy-dispersive spectroscopy (EDS) showed that the chloride was produced during SSC process. The higher corrosion resistance for plasma arc remelted coating may be due to the more compact microstructure, less porosity rate and tensile residual stress. Compared with laser remelting method, plasma arc remelting is a cheap, convenient and effective remelting method.     

Plasma nitriding behavior of Ti6Al4V orthopedic alloy

The influence of plasma nitriding on mechanical, corrosion and tribological properties of Ti6Al4V has been investigated using X-ray diffraction, microhardness tester, scanning electron microscopy, pin-on-disc tribotester, electrochemical polarization and impedance spectroscopy. Plasma nitriding treatment of Ti6Al4V has been performed in 25%Ar-75%N₂ gas mixture, for treatment times of 1-4 h at the temperatures of 650-750 °C. The corrosion tests were carried out in Ringer solution at 37 °C, and the wear tests were performed in dry sliding conditions. XRD analyses confirm the formation of δ-TiN and tetragonal ?-Ti₂N phases in the modified layer. It was observed that the surface hardness and wear resistance increase as the treatment time and temperature increase. The electrochemical impedance measurements indicate a decrease in double layer capacitance value and increase in charge transfer resistance for the nitrided specimens compared to the untreated ones.     

Heat-resistant glass-ceramic coatings protecting low-alloyed steels against high-temperature gas corrosion

Glass-ceramic coating aimed at protecting low-alloyed and low-carbon steels under technological heating is developed; it decreases the metal losses to scale by a factor of 12, as well as substantially decreases the decarbonizing and dealloying of the metal surface. The coating is characterized by an optimum ratio of the refractory to glasslike phases, which provides a reliable adhesion of the coating to the metal. As a result of sintering solid and liquid phases, a barrier layer forms that prevents the oxygen penetration to the metal and, hence, decelerates the corrosion processes at the coating-metal interface under heating conditions.     

Effect of plasma surface treatment on mechanical and corrosion protection properties of UV-curable sol-gel based GPTS-ZrO2 coatings on mild steel

Hybrid sol-gel based nanocomposite coatings derived from hydrolysis and condensation of a photopolymerizable silane precursor 3-Glycidoxypropyltrimethoxy silane in combination with zirconium-n-propoxide were deposited on mild steel substrates by a dip coating technique. In some cases, substrates were subjected to an atmospheric air-plasma surface pre-treatment prior to coating deposition. The coatings were subsequently densified by exposure to ultraviolet radiation followed by a thermal treatment at 250 °C. Characterization of the coatings with respect to thickness, water contact angle, pencil scratch hardness, adhesion and abrasion resistance was carried out. Corrosion testing was carried out on the coatings for a 1 h exposure to a 3.5% NaCl solution by electrochemical polarization and impedance measurements. The hybrid sol-gel coatings were found to improve the mechanical properties and corrosion resistance of mild steel. Plasma surface pre-treatment was found to improve the adhesion of coatings significantly and decreased the corrosion rate from 0.2652 mpy obtained for coatings without any surface pre-treatment to 0.0015 mpy, which was nearly 600 times lower than that of bare mild steel.     

Characterization and properties of the MgF2/ZrO2 composite coatings on magnesium prepared by micro-arc oxidation

Through micro-arc oxidation, the MgF₂/ZrO₂ composite coatings were prepared on magnesium at the different applied voltages (in the range of 400-550 V) in a zirconate electrolytic solution. The morphologies, phase components, microhardness, bond strengths, and corrosion resistances of the composite coatings were investigated. The effect of the applied voltages on the characteristics and properties of the composite coatings and the basic formation mechanism of the coatings were also discussed. The results indicate that the composite coatings are relatively dense and uniform in thickness, and predominantly composed of MgF₂, tetragonal ZrO₂ (t-ZrO₂) and monoclinic ZrO₂ (m-ZrO₂). The composite coatings exhibit a gradient distribution in phase component from the surface to the inner part. It is found that the applied voltage plays an important role in the characteristics and properties of the composite coatings. With the increase of the applied voltage, the thickness and the t-ZrO₂ content of the composite coatings increase, while the m-ZrO₂ content decreases and no significant variation is observed in the MgF₂ content. Moreover, the surface microhardness and bond strength of the coatings increases with the applied voltage increasing. The microhardness values display a gradient distribution in the cross sections of the coatings, and the maximum microhardness value and its corresponding position in the cross sections are related to the applied voltage. In addition, the corrosion resistances of the composite coatings on magnesium surface are obviously superior to the magnesium substrate in the NaCl solutions, and the effect is more remarkable at higher voltage.     

Enhanced tribological properties of PECVD DLC coated thermally sprayed coatings

This research investigates the enhancement of the tribological properties of various thermally-sprayed coatings (APS Ni-50Cr, APS Al₂O₃-13%TiO₂ and HVOF WC-17Co) on steel substrate, achieved through the deposition of a thin DLC-based film. Higher adhesive strength between thin films and thermally-sprayed coatings compared to the simple thin film/carbon steel system was found by scratch testing. Dry sliding ball-on-disk tests performed under lower contact pressure conditions (5 N normal load, 6 mm diameter alumina ball) indicated a significant decrease in wear rates and friction coefficients of thermally-sprayed coatings when the thin DLC-based film is employed; little differences exist between the tribological behaviour of the various thin film/thermal spray coating systems and that of DLC-based film on carbon steel. Under higher contact pressure conditions (10 N normal load, 3 mm diameter alumina ball), the thin film/WC-Co system exhibited the best wear performance. These results indicate the superior tribological performance of DLC/thermal spray coating systems, especially under severe contact conditions.     

The use of Al-Al2O3 cold spray coatings to improve the surface properties of magnesium alloys

Pure Al and 6061 aluminium alloy based Al₂O₃ particle-reinforced composite coatings were produced on AZ91E substrates using cold spray. The strength of the coating/substrate interface in tension was found to be stronger than the coating itself. The coatings have corrosion resistance similar to that of bulk pure aluminium in both salt spray and electrochemical tests. The wear resistance of the coatings is significantly better than that of the AZ91 Mg substrate, but the significant result is that the wear rate of the coatings is several decades lower than that of various bulk Al alloys tested for comparison. The effect of post-spray heat treatment, the volume fraction of Al₂O₃ within the coating and of the type of Al powder used in the coatings on the corrosion and wear resistance was also discussed.     

Influence of ZnO content and annealing temperature on the dielectric properties of ZnO/Al2O3 composite coatings

ZnO/Al₂O₃ coatings were prepared by atmospheric plasma spraying (APS) using ZnO powders and Al₂O₃ powders as starting materials. The dielectric properties of these coatings were discussed. Both the real part of permittivity and the energy loss increase greatly with increasing ZnO content over the frequency range 8.2-12.4 GHz, which can be ascribed to orientation polarization and relaxation polarization due to a higher ZnO content. The frequency-dependent maximum of the loss tangent is found to obey Debye theory. In addition, annealing temperature which leads to the change of ZnO content also plays an important role in the dielectric performance.     

Development and characterization of single wire-arc-plasma sprayed coatings of nickel on carbon blocks and alumina tube substrates

A single wire-arc-plasma spray torch has been used to develop metal coatings on carbon and alumina substrates under argon atmosphere for various applications. Nickel coatings of around 1 mm thickness have been deposited on selected area (60 mm × 200 mm on each side) of large size carbon blocks by intermittent arc spraying and cooling to reduce thermal stresses and possibility of coating de-lamination from the base substrate. The same process is also used for depositing about 3 mm thick nickel metal coatings (8 mm dia. × 40 mm long) on alumina tubes for developing electrical feed throughs. The nickel coated alumina tubes were tested for the vacuum compatibility of the coated material with the base tube. The coated assemblies could withstand vacuum of the order of 1 × 10^− 6 Torr and the leak rate was found to be less than 1 × 10^− 9 Std. cc/s for Helium gas, indicating excellent bonding of the coated metal with alumina ceramics and no connected open porosity in the coatings. X-ray diffraction studies were conducted for identifying the phases and the optical microscope with image analysis technique was used for studying the microstructure and porosity in the coatings.     

Ion beam assisted deposition and characterization of Zr-based multilayered coatings

ZrN/W multilayered coatings with different nanoscale modulation periods have been synthesized at different deposition time using ion beam assisted deposition. XRD, AES, Nanoindenter and profiler were employed to investigate the influence of modulation period on microstructure and mechanical properties of the coatings. The results showed that all superlattice coatings almost revealed higher mechanical property than the monolithic ZrN and W coatings. At modulation period of 8.6 nm, XRD pattern showed a significant mixture of strong ZrN (111), W (110), as well as weak ZrN (220) textures. It possessed the highest hardness (∼ 26 GPa), elastic modulus (∼ 310 GPa), and fracture resistance (∼ 80 mN), compared with the ones with other modulation period.     

Influence of parameters of the HVOF thermal spray process on the properties of multicomponent white cast iron coatings

High velocity oxi-fuel (HVOF) thermal spray process has been used in order to deposit a new alloy known as multicomponent white cast iron. The coatings were characterized in terms of macrostructure, phase composition, porosity and hardness. Coating characteristics and properties were found to be dependent on the particles size range, spray distance, gases flow rate and oxygen to propane ratio. For set of parameters utilized in this job a narrow particle size range between 20 and 45 μm with a spray distance of 200 mm and oxygen to propane ratio of 4.6 are the preferred coating parameters. Coating porosity of 0.9% and hardness of 766 HV were obtained under these conditions.     

Properties of High Velocity Suspension Flame Sprayed (HVSFS) TiO2 coatings

TiO₂ coatings were manufactured by the High Velocity Suspension Flame Spraying (HVSFS) technique using a nanopowder suspension. Their microstructure, nanohardness, tribological properties and photocatalytic activity were studied and compared to conventional atmospheric plasma sprayed (APS) and HVOF-sprayed TiO₂ coatings manufactured using commercially available feedstock. The HVSFS process leaves a fairly large freedom to adjust coating properties (thickness, porosity, anatase content, hardness, etc…) according to the desired objective. Layers with higher anatase content and higher porosity can be produced to achieve higher photocatalytic efficiency, better than conventional APS and HVOF TiO₂. Alternatively, dense protective layers can be deposited, possessing lower porosity and pore interconnectivity and better wear resistance than as-deposited APS and HVOF layers. In all cases, HVSFS-deposited layers are thinner (20 µm-60 µm) than those which can be obtained by conventional spraying processes.     

Synthesis of nanoscale multilayered ZrN/W2N multilayered coatings by magnetron sputtering

ZrN/W₂N multilayered coatings with nanoscale modulation period in an ultra-high vacuum rf magnetron sputter chamber. XRD, SEM, Nano Indenter and profiler were employed to investigate the influence of modulation periods and working pressures on structural and mechanical properties of the coatings. The low-angle XRD pattern and cross-sectional SEM indicated a well-defined composition modulation and layer structure of the multilayered coating. All multilayered coatings revealed higher hardness than the rule-of-mixtures value of monolithic ZrN and W₂N coatings at different working pressures. The maximum hardness was up to 34 GPa. The multilayers obtained mixed polycrystalline textures of ZrN(111), W₂N(111), W₂N(200) and W₂N(311). 0.8 Pa was an optimum working pressure for mechanical property enhancement.     

Sliding wear behavior of the supersonic plasma sprayed WC-Co coating in oil containing sand

To improve the wear resistance of the cylinder liner of the engines operating in desert area, the tungsten carbide-based cermet coating (WC-12wt.%Co) was produced using the supersonic plasma spraying technique. The microstructure and phase composition of the as-sprayed coating were evaluated. The effect of sand size in lubricant on the friction and wear behavior of the coating under the lubrication of oil containing sand was investigated on a ball-on-disk tribometer. Characterization of the coating shows that the coating possesses dense microstructure with low level of porosity. The decarburization of WC is limited for the high particle velocity of the supersonic plasma spraying and only a few of W₂C phases are observed in the coating. With the higher hardness and the good adhesion with the substrate the sprayed coating exhibits a better wear resistance compared with the gray casting iron used in the current cylinder liner. Micro-structural examination shows that the wear of coating is dominated by micro-cutting, WC particles pull-out and sub-surface indentation-induced sub-surface cracking, while the main wear mechanism of the gray casting iron is micro-cutting and plastic deformation.     

Effect of the suspension composition on the microstructural properties of high velocity suspension flame sprayed (HVSFS) Al2O3 coatings

Seven different Al₂O₃-based suspensions were prepared by dispersing two nano-sized Al₂O₃ powders (having analogous size distribution and chemical composition but different surface chemistry), one micron-sized powder and their mixtures in a water + isopropanol solution. High velocity suspension flame sprayed (HVSFS) coatings were deposited using these suspensions as feedstock and adopting two different sets of spray parameters.The characteristics of the suspension, particularly its agglomeration behaviour, have a significant influence on the coating deposition mechanism and, hence, on its properties (microstructure, hardness, elastic modulus). Dense and very smooth (Ra ~ 1.3 μm) coatings, consisting of well-flattened lamellae having a homogeneous size distribution, are obtained when micron-sized (~ 1-2 μm) powders with low tendency to agglomeration are employed. Spray parameters favouring the break-up of the few agglomerates present in the suspension enhance the deposition efficiency (up to > 50%), as no particle or agglomerate larger than ~ 2.5 μm can be fully melted. Nano-sized powders, by contrast, generally form stronger agglomerates, which cannot be significantly disrupted by adjusting the spray parameters. If the chosen nanopowder forms small agglomerates (up to a few microns), the deposition efficiency is satisfactory and the coating porosity is limited, although the lamellae generally have a wider size distribution, so that roughness is somewhat higher. If the nanopowder forms large agglomerates (on account of its surface chemistry), poor deposition efficiencies and porous layers are obtained.Although suspensions containing the pure micron-sized powder produce the densest coatings, the highest deposition efficiency (~ 70%) is obtained by suitable mixtures of micron- and nano-sized powders, on account of synergistic effects.