Corrosion Testing of Ni Alloy HVOF Coatings in High Temperature Environments for Biomass Applications

This paper reports the corrosion behavior of Ni alloy coatings deposited by high velocity oxyfuel spraying, and representative boiler substrate alloys in simulated high temperature biomass combustion conditions. Four commercially available oxidation resistant Ni alloy coating materials were selected: NiCrBSiFe, alloy 718, alloy 625, and alloy C-276. These were sprayed onto P91 substrates using a JP5000 spray system. The corrosion performance of the coatings varied when tested at ~525, 625, and 725 °C in K₂SO₄-KCl mixture and gaseous HCl-H₂O-O₂ containing environments. Alloy 625, NiCrBSiFe, and alloy 718 coatings performed better than alloy C-276 coating at 725 °C, which had very little corrosion resistance resulting in degradation similar to uncoated P91. Alloy 625 coatings provided good protection from corrosion at 725 °C, with the performance being comparable to wrought alloy 625, with significantly less attack of the substrate than uncoated P91. Alloy 625 performs best of these coating materials, with an overall ranking at 725 °C as follows: alloy 625 > NiCrBSiFe > alloy 718 ? alloy C-276. Although alloy C-276 coatings performed poorly in the corrosion test environment at 725 °C, at lower temperatures (i.e., below the eutectic temperature of the salt mixture) it outperformed the other coating types studied.     

Fireside Corrosion Behavior of HVOF and Plasma-Sprayed Coatings in Advanced Coal/Biomass Co-Fired Power Plants

This article presents a systematic evaluation of coatings for advanced fossil fuel plants and addresses fireside corrosion in coal/biomass-derived flue gases. A selection of four candidate coatings: alloy 625, NiCr, FeCrAl and NiCrAlY were deposited onto superheaters/reheaters alloy (T91) using high-velocity oxy-fuel (HVOF) and plasma spraying. A series of laboratory-based fireside corrosion exposures were carried out on these coated samples in furnaces under controlled atmosphere for 1000 h at 650 °C. The tests were carried out using the “deposit-recoat” test method to simulate the environment that was anticipated from air-firing 20 wt.% cereal co-product mixed with a UK coal. The exposures were carried out using a deposit containing Na₂SO₄, K₂SO₄, and Fe₂O₃ to produce alkali-iron tri-sulfates, which had been identified as the principal cause of fireside corrosion on superheaters/reheaters in pulverized coal-fired power plants. The exposed samples were examined in an ESEM with EDX analysis to characterize the damage. Pre- and post-exposure dimensional metrologies were used to quantify the metal damage in terms of metal loss distributions. The thermally sprayed coatings suffered significant corrosion attack from a combination of aggressive combustion gases and deposit mixtures. In this study, all the four plasma-sprayed coatings studied performed better than the HVOF-sprayed coatings because of a lower level of porosity. NiCr was found to be the best performing coating material with a median metal loss of ~87 μm (HVOF sprayed) and ~13 μm (plasma sprayed). In general, the median metal damage for coatings had the following ranking (in the descending order: most to the least damage): NiCrAlY > alloy 625 > FeCrAl > NiCr.     

Microbially influenced corrosion and inhibition of nickel–zinc and nickel–copper coatings by Pseudomonas aeruginosa

The present study describes the effect of Pseudomonas aeruginosa on the corrosion rate of nickel–zinc and nickel–copper alloy coatings. The presence of bacteria was associated with decreases in R_ct values, suggesting that P. aeruginosa promoted the corrosion of nickel–copper alloy coatings. However, R_ct values of nickel–zinc coatings increased in response to bacterial inoculation, corresponding to a decrease in corrosion rate for nickel–zinc alloy coatings. Our results suggest that the activity of P. aeruginosa facilitated the corrosion of nickel–copper alloy, while serving a protective function for the nickel–zinc alloy.     

Hot corrosion resistance of high-velocity oxyfuel sprayed coatings on a nickel-base superalloy in molten salt environment

No alloy is immune to hot corrosion attack indefinitely. Coatings can extend the lives of substrate materials used at higher temperatures in corrosive environments by forming protective oxides layers that are reasonably effective for long-term applications. This article is concerned with studying the performance of high-velocity oxyfuel (HVOF) sprayed NiCrBSi, Cr₃C₂−NiCr, Ni−20Cr, and Stellite-6 coatings on a nickel-base superalloy at 900 °C in the molten salt (Na₂SO₄-60% V₂O₅) environment under cyclic oxidation conditions. The thermogravimetric technique was used to establish kinetics of corrosion. Optical microscope, x-ray diffraction, scanning electron microscopy/electron dispersive analysis by x-ray (SEM/EDAX), and electron probe microanalysis (EPMA) techniques were used to characterize the as-sprayed coatings and corrosion products. The bare superalloy suffered somewhat accelerated corrosion in the given environmental conditions. whereas hot corrosion resistance of all the coated superalloys was found to be better. Among the coating studied, Ni−20Cr coated superalloy imparted maximum hot corrosion resistance, whereas Stellite-6 coated indicated minimum resistance. The hot corrosion resistance of all the coatings may be attributed to the formation of oxides and spinels of nickel, chromium, or cobalt.     

Corrosion Behavior of Thermally Sprayed NiCrBSi Coating on 16MnR Low-Alloy Steel in KOH Solution

NiCrBSi coatings were selected as protective material and air plasma-sprayed on 16MnR low-alloy steel substrates. Corrosion behavior of 16MnR substrates and NiCrBSi coatings in KOH solution were evaluated by polarization resistance (R _p), potentiodynamic polarization curves, electrochemical impedance spectroscopy, and immersion corrosion tests. Electrolytes were solutions with different KOH concentrations. NiCrBSi coating showed superior corrosion resistance in KOH solution compared with the 16MnR. Corrosion current density of 16MnR substrate was 1.7-13.0 times that of NiCrBSi coating in the given concentration of KOH solution. By contrast, R _p of NiCrBSi coating was 1.2-8.0 times that of the substrate, indicating that the corrosion rate of NiCrBSi coating was much lower than that of 16MnR substrate. Capacitance and total impedance value of NiCrBSi coating were much higher than those of 16MnR substrate in the same condition. This result indicates that corrosion resistance of NiCrBSi coating was better than that of 16MnR substrate, in accordance with polarization results. NiCrBSi coatings provided good protection for 16MnR substrate in KOH solution. Corrosion products were mainly Ni/Fe/Cr oxides.     

Performance of high-velocity oxyfuel-sprayed coatings on an Fe-based superalloy in Na2SO4-60%V2O5 environment at 900 °C part II: Hot corrosion behavior of the coatings

NiCrBSi, Cr₃C₂-NiCr, Ni-20Cr, and Stellite-6 coatings were deposited on an Fe-based superalloy by the high-velocity oxyfuel (HVOF) thermal spray process. The hot corrosion behavior of the coatings in an aggressive environment of Na₂SO₄-60%V₂O₅ at 900 °C under cyclic conditions was studied. The thermogravimetric technique was used to establish the kinetics of corrosion. X-ray diffraction, scanning electron microscopy/energy-dispersive x-ray and electron probe microanalysis techniques were used to analyze the corrosion products. Hot corrosion resistances of all the coatings were found to be better than the uncoated superalloy. The Ni-20Cr coating was found to be the most protective, followed by Cr₃C₂-NiCr coatings. The Ni-20Cr coating had reduced the mass gain by 90% of that gained by the uncoated superalloy. The hot corrosion resistance shown by the Cr₃C₂-NiCr coating was slightly better compared with the NiCrBSi coating; however, both of the coatings performed better than the Stellite-6 coating. The Stellite-6 coating was the least effective among the coatings studied, but it was still successful in decreasing the mass gain to about one fourth compared with the uncoated superalloy. The formation of oxides and spinels of nickel, chromium, or cobalt may be contributing to the development of hot corrosion resistance in the coatings. This article focuses on the hot corrosion behavior of HVOF coatings. The characterization of these coatings has been presented in part I included in this issue.     

Investigations on role of HVOF sprayed Co and Ni based coatings to combat hot corrosion

Abstract

This paper aims to investigate the hot corrosion resistance of high velocity oxy-fuel (HVOF) sprayed cobalt based (Stellite-6) and nickel based (Ni–20Cr) coatings deposited on the superalloy Superni-718 (Ni–19Cr–18˙5Fe–5˙13Ta–3˙05Mo–0˙9Ti–0˙5AI–0˙18Mn–0˙18Si–0˙15Cu–0˙04C) in the Na₂SO₄–60%V₂O₅ salt environment at 900°C under cyclic conditions. The X-ray diffractometry, scanning electron microscopy/energy dispersive analysis and electron probe microanalyser techniques were used to study the corrosion products with respect to their morphology, phase composition and element concentration. The thermogravimetric technique was used to establish the kinetics of corrosion. The bare alloy underwent severe hot corrosion attack. The Ni–20Cr coating shows excellent hot corrosion resistance with negligible spallation, whereas Stellite-6 coating reveals less hot corrosion resistance and more spallation. The hot corrosion resistance of Ni–20Cr coating has been attributed to the formation of oxides of chromium, nickel and spinel of nickel chromium. The oxides of silicon, chromium, cobalt and spinels of cobalt–chromium and nickel–chromium have contributed for hot corrosion resistance of Stellite-6 coatings.     

Erprobung verschiedener metallischer Legierungen für die Abwasseraufbereitung hinter Rauchgasentschwefelungsanlagen

Evaluation of different metal alloys for waste water treatment of flue gas desulfurization plants

1 Vortrag anläßlich der ACHEME '94 Frankfurt/Main, 5.-11. Juni 1994.

Desulfurization of flue gases from coal-fired power plants is carried out by wet scrubbing processes in 90% of the cases. Using milk of lime or lime stone suspensions as a sorbent, said processes not only remove sulfur dioxide but also all other flue gas pollutants. The sulfur compounds are mostly separated in the form of calcium sulfate (gypsum) in the solid-liquid separation process whereas the water-soluble components, mainly calcium chloride are discharged with the scrubber solutions. Since the salt load of effluents to be discharged into municipal sewer systems or surface waters is limited by legislation, the FGD scrubber solutions must be treated by evaporation, salt modification or resource recovery. For evaporation crystallization of these extremely corrosive FGD scrubber solutions which – depending on the process – can lead all the way to a product suitable for depositing, an economic equipment concept based on metallic construction materials was to be investigated. In a parameter study, electrochemical test methods were used to determine the influence of the pH value, the Fe³⁺, F^? and SO₂ contents in a solution containing 200 g/l CaCl₂ + 30 g/l NaCl at 80°C on the resistance to pitting corrosion of a stainless steel and various NiCrMo alloys. In addition, electrochemical and wet corrosion tests (e.g. crevice corrosion) were conducted using original FGD scrubber solutions with 43–50% dry substance from the power plants Reuter-West, Oberhavel and Rudow of the BEWAG, Berlin. Of the materials tested, NiCrMo alloys 2.4605 (NiCr 23 Mo 16 Al, Alloy 59), 2.4602 (NiCr 21 Mo 14 W Alloy 22) and 2.4819 (NiMo 16 Cr 15 W, Alloy C-276) proved to be resistant under the conditions of the third evaporation crystallization stage, with the corrosion resistance decreasing in the above order. The stainless steel 1.4529 (X 2 NiCrMoCu 25 20 6) and the NiCrMo alloy 2.4856 (NiCr 22 Mo 9 Nb, Alloy 625) experienced pitting corrosion in nearly all test solutions at redox potential. Also the alloy 2.4610 (NiMo 16 Cr 16 Ti, Alloy C-4) was not always sufficiently resistant. In the evaporation crystallization process, care should be taken to ensure that the pH does not fall below 5, that the redox potential of the FGD scrubber solutions remains below the transpassive dilution or pitting potential and that crevices are avoided, if possible.     

Hot Corrosion Studies of Detonation-Gun-Sprayed NiCrAlY?+?0.4?wt.% CeO2 Coated Superalloys in Molten Salt Environment

Rare earth oxide (CeO₂) has been incorporated in NiCrAlY alloy and hot corrosion resistance of detonation-gun-sprayed NiCrAlY + 0.4 wt.% CeO₂ coatings on superalloys, namely, superni 75, superni 718, and superfer 800H in molten 40% Na₂SO₄-60% V₂O₅ salt environment were investigated at 900 °C for 100 cycles. The coatings exhibited characteristic splat globular dendritic structure with diameter similar to the original powder particles. The weight change technique was used to establish corrosion kinetics. X-ray diffraction (XRD), field emission scanning electron microscopy/energy-dispersive analysis (FE-SEM/EDAX), and x-ray mapping techniques were used to analyze the corrosion products. Coated superfer 800H alloy showed the highest corrosion resistance among the examined superalloys. CeO₂ was found to be distributed in the coating along the splat boundaries, whereas Al streaks distributed non-uniformly. The main phases observed for the coated superalloys are oxides of Ni, Cr, Al, and spinels, which are suggested to be responsible for developing corrosion resistance.     

Untersuchungen zur Prüfung geschweißter NiCrMo-Legierungen für Rauchgasentschwefelungsanlagen auf selektive Korrosion

Investigation on testing of welded NiCrMo-alloys for flue gas desulfurization plants on selective corrosion The use of NiCrMo-alloys is mandatory in places where a very high corrosion resistance is required, as well in oxidizing as in socalled reducing media. All standardized tests for examination on intercrystalline corrosion, however, are providing strongly oxidizing conditions only. Therefore test methods have to he found for examination on selective corrosion in less oxidizing media as they are prevailing in aqueous flue gas desulfurization media of fossil fired power plants. Having this object in mind the anodic polarization behaviour of the NiCrMo alloys C-276, C-4 and 59 has been examined in sulfuric acid solutions with additions of SCN^? and Cl^? as structure sensitive ions. The material conditions selected for being examined were the solution annealed and sensitized wrought alloys as well as weldments made with matching filler and varied heat-input during welding. In the active-passive transition range the various alloy, conditions display in fact a different behaviour with respect to selective and intercrystalline corrosion but only under test conditions which have to be closely controlled and which are differing from alloy to alloy. These tests, on the one hand, provide the less oxidizing character as prevailing in aqueous flue gas desulfurization media but are much more corrosive than in practice on the other. The different sensitivity of the various materials and material conditions to selective corrosion as shown in this type of test corresponds with practical experience. In case of a heat-input during, welding which exceeds 8 kJ/cm a tendency to increased selective corrosion is most pronounced with Alloy C-276 followed by Alloy C-4. With Alloy 59 an influence of heat-input during welding can be seen only at extremely low or extremely high heat-input rates of 6.2 or 15.1 kJ/cm respectively. Alloy 59 proves to be the less sensitive NiCrMo-alloy also with respect to welding parameters. Though the test conditions experienced so far represent a first approach to a test method which conforms with real world conditions in flue gas desulfurization plants the test results do not allow a forecast of the materials' corrosion behaviour during such type of service. However, an evaluation in terms of a ranking should he possible.     

Excimer laser surface alloying of titanium with nickel and palladium for increased corrosion resistance

In the electron beam treatment of flue gases, titanium foil is employed as an electron-transparent window. Due to its degradation in the flue gas environment and eventual failure, extension of the life of the window is being sought. Previous studies have indicated significant improvements of corrosion resistance from surface alloying with nickel or palladium, using high intensity pulsed plasma beams, but restricted size of vacuum systems prevents treatment of large surfaces. In the present work, an excimer laser was employed to surface alloy titanium foil with nickel or palladium, using fluences in the range 0.4-1.1 J cm⁻² and either nitrogen or argon as the cover gas. The resultant surfaces provided high resistance to corrosion in 0.1 M H₂SO₄ solution at 80 °C that simulates, under accelerated conditions, the degradation of titanium by the flue gas. The improved behaviour is associated with the corrosion potential being shifted to the region of passivity. Treatments at increased fluences reduced losses of nickel and increased alloying of palladium during processing of the foils. Palladium was largely retained during the subsequent immersion tests, contrasting with the depletion of nickel by corrosion that limits the durability of the treated foils. The corrosion rates of the optimum palladium-alloyed surfaces were about two orders of magnitude lower than that of untreated titanium.     

Corrosion of alloys and their diffusion aluminide coatings by KCl:K2SO4 deposits at 650 °C in air

P91 ferritic‐martensitic steel, 17Cr–13Ni and alloy 800 austenitic stainless steels and Inconel 617 alloy have been aluminised to form Fe₂Al₅, (Fe,Ni)Al and Ni₂Al₃ aluminide coatings. These alloys and their corresponding coatings were subjected to corrosion in air by 50:50 mol/mol K₂SO₄/KCl deposits at 650 °C for 300 h. With the exception of the Inconel 617 alloy, significant metal losses (>180 µm) were recorded. These losses were planar for P91 alloy but involved internal corrosion for the two austenitic steels. The (Fe,Ni)Al and NiAl coatings on the austenitic steels and the Inconel 617 alloy were significantly corroded via intergranular and internal chloridation–sulphidation–oxidation. In contrast, the Fe₂Al₅ coating on the P91 alloy coating was virtually unattacked. For the alloys, the relative extents of corrosion damage can be explained in terms of the stability and volatility of metal chlorides formed. For the coatings, STEM/EDS analyses enable clear linkages to be made between the presence and number of Cr‐rich particles on coating grain boundaries and the corrosion damage observed for the coatings.     

Pulsed Electrodeposition of Composite Coatings Based on Zinc–Nickel Alloy

Composite electrochemical coatings (CECs) based on zinc–nickel alloy and modified by carbon nanotubes (CNTs) are obtained by pulsed electrolysis. The microstructure and tribological properties of these CECs are studied. It is found that addition of dispersed CNTs into the electrolyte for zinc–nickel alloy deposition results in a 1.35- to 1.65-fold decrease in the friction coefficient of the formed coatings. The electrochemical corrosion behavior of the zinc–nickel–CNT CECs is studied in 0.5 M H₂SO₄ solution.     

Korrosionsverhalten des Werkstoffs 1.4539 und von Nickelbasis-Legierungen in Gaswässern

Corrosion behaviour of material no. 1.4539 and nickel based alloys in gas waters Laboratory tests with synthetic gas waters containing the gases ammonia, carbon dioxide, hydrogen sulphide and hydrogen cyanide were carried out in order to examine the influence of medium components on the corrosion of material No. 1.4539 and nickel based alloys Hastelloy C-4, C-22 and C-276. Hydrogen sulfide was identified as the decisive component for corrosion. For stainless steel corrosion rates of about 2 mm ° a⁻¹ were already found at 50°C in a critical pH-range with sulfide concentrations > 2%. As cyanide stimulates corrosion by dissolving sulfide surface layers by complexation of the iron ions, an increased material loss rate per unit area was found in the critical range with increasing cyanide concentration. The much more stable nickel based alloys only revealed considerable weight losses after being exposed in the autoclave at 100°C. The graduation of the loss rates C-22 > C-4 > C-276 can be explained by the different contents of high grade alloy elements. The testing of nickel based alloys of the Hastelloy type and of material No. 1.4539 and 1.4571 by means of the dynamic tensile test (CERT-method) revealed no risks of stress corrosion cracking in the tested media. The mechanical data, such as energy at break, elongation at rupture and reduction of area when breaking, did not indicate any change in comparison to the data found in a neutral medium (glycerine). As a function of the aggressiveness of the liquid phase, the metallographic analysis showed some traces of local corrosion in the highly deformed area, ranging from pitting to limited surface cracking. These phenomena were found to nearly the same extent for all materials.     

Nickel–boron plating on magnesium and AZ91D alloy by a chromium-free electroless process

Ni–B coatings have been deposited directly on commercial purity magnesium and AZ91D magnesium alloy by a chromium-free electroless process and characterized using scanning electron microscopy, glow discharge optical emission spectroscopy, gravimetric measurements and, for corrosion evaluation, hydrogen evolution. The surfaces of the coatings reveal a typical cauliflower-type appearance and form at a rate of ~ 8 to 13 μm/h. The nickel and boron contents of the coatings are relatively uniform across the whole thickness. The addition of NH₄HF₂ to the electroless bath improves the corrosion resistance provided by the coatings during immersion of coated substrates in sodium chloride solution.     

Tribological behavior and mechanism of NiCrBSi–Y2O3 composite coatings

The NiCrBSi–Y₂O₃ composite coatings were prepared on the surface of 45 carbon steel by plasma spray, the microstructure and tribological properties of the coatings were investigated. The results show that the NiCrBSi–Y₂O₃ composite coatings are mainly composed of γ-Ni, CrB, Cr₇C₃ and Y₂O₃. With addition of Y₂O₃, hard phases such as CrB, Cr₇C₃ emerge in composite coating, and the density of the composite coatings also increases. The NiCrBSi–0.5Y₂O₃ composite coating presents excellent tribological properties. Its friction coefficient is 0.175, which is about 37% of that of the pure NiCrBSi coating. The mass wear loss is 1.2 mg, which is reduced by 43% compared with the pure NiCrBSi coating. When the loads are 6–10 N, the NiCrBSi–0.5Y₂O₃ composite coating suffers from slight wear and the wear mechanisms are mainly adhesive wear accompany with slight micro-cutting wear and micro-fracture wear. As the load increases to 12 N, the wear mechanisms are adhesive wear and severe micro-cutting wear.     

Manufacturing and Properties of High-Velocity Oxygen Fuel (HVOF)-Sprayed FeVCrC Coatings

This paper studies the microstructure, sliding wear behavior and corrosion resistance of high-velocity oxygen fuel (HVOF)-sprayed FeVCrC-based coatings. Various process parameters were tested to evaluate their effects on the coating properties, which were also compared to those of HVOF-sprayed NiCrBSi and Stellite-6 coatings. The Fe alloy coatings are composed of flattened splats, originating from molten droplets and consisting of a super-saturated solid solution, together with rounded particles, coming from partially unmolten material and containing V- and Fe-based carbide precipitates. All process parameters, apart from “extreme” settings with excess comburent in the flame, produce dense coatings, indicating that the feedstock powder is quite easily processable by HVOF. These coatings, with a microhardness of 650-750 HV_0.3, exhibit wear rates of ≈2 × 10^?6 mm³/(Nm) in ball-on-disk tests against sintered Al₂O₃ spheres. They perform far better than the reference coatings, and better than other Fe- and Ni-based alloy coatings tested in previous research. On the other hand, the corrosion resistance of the coating material (tested by electrochemical polarization in 0.1 M HCl solution) is quite low. Even in the absence of interconnected porosity, this results in extensive, selective damage to the Fe-based matrix. This coating material is therefore unadvisable for severely corrosive environments.     

Hot corrosion behavior of detonation gun sprayed Cr3C2–NiCr coatings on Ni and Fe-based superalloys in Na2SO4–60% V2O5 environment at 900 °C

The present work investigates the hot corrosion resistance of detonation gun sprayed (D-gun) Cr₃C₂–NiCr coatings on Superni 75, Superni 718 and Superfer 800 H superalloys. The deposited coatings on these superalloy substrates exhibit nearly uniform, adherent and dense microstructure with porosity less than 0.8%. Thermogravimetry technique is used to study the high temperature hot corrosion behavior of bare and Cr₃C₂–NiCr coated superalloys in molten salt environment (Na₂SO₄–60% V₂O₅) at high temperature 900 °C for 100 cycles. The corrosion products of the detonation gun sprayed Cr₃C₂–NiCr coatings on superalloys are analyzed by using XRD, SEM, and FE-SEM/EDAX to reveal their microstructural and compositional features for elucidating the corrosion mechanisms. It is shown that the Cr₃C₂–NiCr coatings on Ni- and Fe-based superalloy substrates are found to be very effective in decreasing the corrosion rate in the given molten salt environment at 900 °C. Particularly, the coating deposited on Superfer 800 H showed a better hot corrosion protection as compared to Superni 75 and Superni 718. The coatings serve as an effective diffusion barrier to preclude the diffusion of oxygen from the environment into the substrate superalloys. It is concluded that the hot corrosion resistance of the D-gun sprayed Cr₃C₂–NiCr coating is due to the formation of desirable microstructural features such as very low porosity, uniform fine grains, and the flat splat structures in the coating.     

Comparative study on corrosion protection properties of electroless Ni‐P‐ZrO2 and Ni‐P coatings on AZ91D magnesium alloy

Electroless Ni‐P‐ZrO₂ and Ni‐P coatings on AZ91D magnesium alloy were prepared, and their corrosion protection properties were compared in this paper. The potentiodynamic curves and electrochemical impedance spectroscopy (EIS) of the coated magnesium alloy in 3.5% NaCl solution showed that the corrosion performance of Ni‐P‐ZrO₂ composite coating was superior to that of Ni‐P coating. The same conclusion was obtained with salt spray and immersion tests. The corrosion morphologies of two kinds of coatings with various immersion time intervals in 3.5% NaCl solution indicated that most corrosion products concentrated on the nodules boundaries of Ni‐P coating and blocked corrosion pit was the main corrosion form. For the Ni‐P‐ZrO₂ coating, tortuous nodules boundaries were not the weak sites of the coating and corrosion initiated from the nickel phosphor alloy around the nanometer powders. Open corrosion pits occurred on the composite coating surface, and the coating was corroded gradually. Thus, the Ni‐P‐ZrO₂ coating exhibited better corrosion protection property to magnesium alloy substrate than Ni‐P coating.     

Electrochemical behaviour of Ni-base alloys exposed under oil/gas field environments

The electrochemical behaviour of Ni-base alloys (Inconel 625, Inconel 718, G3 and Incoloy 825) is carried out at 80 °C in CO₂/H₂S corrosion environments using cyclic potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) techniques. The passivity mechanisms are analysed and discussed. In addition, some significant characterisation parameters such as E_corr, I_pass, E_pit, E_pp, ΔE and I_pass in cyclic polarisation curves are analysed and compared to reveal the corrosion resistance of various Ni-base alloys. The equivalent circuit model and ZsimpWin software are utilised to discuss the Nyquist plots of various Ni-base alloys. The diffusion mechanism in EIS measurement is discussed. The result shows that the corrosion resistance of the Ni-base alloys to CO₂ corrosion or CO₂/H₂S corrosion follows the sequence: Inconel 625 > G3 > Inconel 718 > Incoloy 825. H₂S works as a cathodic depolariser with accelerating initiation of the corrosion process.