Corrosion protection of electrodeposited multilayer nanocomposite Zn-Ni-SiO2 coatings

Multilayer nanocomposite coatings of Zn-Ni-SiO₂ were deposited galvanostatically on mild steel (MS) from Zn-Ni bath, having Zn⁺² and Ni⁺² ions and uniformly dispersed nano-SiO₂ particles. The corrosion characteristics and properties of multilayered nanocomposite (MNC) coatings were evaluated by electrochemical polarization and impedance methods. Such deposition conditions as, bath composition, cyclic cathode current densities (CCCD’s) and number of layers were optimized for peak performance of coatings against corrosion. A significant improvement in the corrosion performance of MNC coatings was observed when a coating was changed from a monolayer to multilayer type. Corrosion rate (CR) of MNC coating decreased progressively with number of layers up to an optimal level, and then started increasing. The increase of CR at a higher degree of layering is attributed to diffusion of layers due to a very short deposition time, failing to give the enhanced corrosion protection. The formation of layers, inclusion of silica particle in MNC coating matrix were confirmed by SEM and XRD study. At optimal current densities, i.e. at 3.0–5.0 A/cm², the Zn-Ni-SiO₂ coating having 300 layers, represented as (Zn-Ni-SiO₂)_30/5.0/300 is found to be about 107 times more corrosion resistant than a monolayer Zn-Ni-SiO₂ coating, developed from the same bath for the same time. The reasons responsible for the extended corrosion protection of MNC Zn-Ni-SiO₂ coatings, compared to corresponding monolayer Zn-Ni and (Zn-Ni-SiO₂) coatings were analyzed, and results were discussed.     

Electrodeposition of Zn–Ni coatings as Cd replacement for corrosion protection of high strength steel

Electrodeposition of Zn–Ni coatings performed in acidic baths are not suitable for high strength steels due to their high susceptibility to hydrogen embrittlement.In this work, Zn–Ni coatings were deposited on a high strength steel (4340) upon stirring conditions from an alkaline bath. A complete characterisation of the coatings (corrosion, morphology and composition) has been accomplished, correlating the electrodeposition conditions with these features. The best protective properties of the grown coatings were achieved for the alloys with a single phase structure of γ-Ni₅Zn₂₁ and a denser morphology. Additionally, the hydrogen content incorporated is lower than even cadmium-coated 4340 steel which has undergone a postbaking dehydrogenation treatment.     

Electrochemical studies on the electrodeposited Zn-Ni-Co ternary alloy in different media

The electroplating of ternary Zn-Ni-Co alloy, surface morphology and corrosion resistance were investigated and contrasted with the characteristics of Zn-Ni electrodeposits. The investigation of electrodeposition was carried out using cyclic voltammetry and galvanostatic techniques, while potentiodynamic polarization resistance and anodic linear sweeping voltammetry techniques were used for corrosion study. Under the examined conditions, the electrodeposition of the alloy was of anomalous type. It was found that the obtained Zn-Ni-Co alloy exhibited more preferred surface appearance and better corrosion resistance compared to Zn-Ni alloy that electrodeposited at similar conditions. During the cathodic scan of cyclic voltammetry, a cathodic peak at − 574 mV is appeared and correlated with the deposition of sulfur liberated from the reduction of sulphate group in the presence of H⁺. Up to four anodic peaks were obtained by cyclic voltammetry technique, two correlated with zinc oxidation from pure deposited Zn and γ-Ni₅Zn₂₁ phases and two correlated with oxidation of cobalt and nickel, were observed. The phase structure, surface morphology and chemical composition of the deposits were characterized by means of X-ray diffraction analysis, scanning electron microscopy and atomic absorption spectroscopy, respectively.     

Influence of Zn-Ni alloy electrodeposition techniques on the coating corrosion behaviour in chloride solution

The aim of this research work is to optimize the plating conditions during electrodeposition of Zn-Ni alloys. Electrodeposits of Zn-Ni alloys have been synthesized from sulphate bath using cyclic voltammetry and chronopotentiometry techniques under different conditions. X-ray diffraction measurements reveal that the alloys consisted of <gamma>-Ni₅Zn₂₁ and pure zinc phases. The composition and morphology of the deposits have been also studied and discussed. The surface analyses indicate that the deposition took place with the formation of Zn-Ni alloy coatings, containing at least 10 wt.% Ni. In order to obtain better barrier properties and corrosion resistance, coated steel samples have been immersed in 3% NaCl solution and studied using potentiodynamic stripping and electrochemical impedance spectroscopy. The process of dezincification is reduced when the coated steel is electroplated by chronopotentiometry (5 mA and 10 mA). In addition, these samples exhibit an improved morphology and fine grain size as compared with deposits electroplated by cyclic voltammetry.     

Development of compositionally modulated multilayer Zn-Ni deposits as replacement for cadmium

Compositionally modulated multilayer (CMM) Zn-Ni deposits were electrodeposited from single acidic bath (pH = 4.7) by using a potentiostatic sequence. The Zn and Ni composition in the alloy was tailored as a function of distance from the steel substrate. X-ray diffraction studies of the deposit showed the presence of γ-phase with a composition of Ni₅Zn₂₁. The corrosion properties of modulated multilayer coatings were studied in 5% NaCl solution using electrochemical corrosion techniques. The polarization resistance of the deposits varied as a function of Ni content between 1700 and 3440 Ω. CMM Zn-Ni with 20 wt% Ni exposed in ASTM B117 salt spray test did not show any red rust formation after 400 h.     

Role of cadmium on corrosion resistance of Zn-Ni alloy coatings

Cadmium (Cd) catalyzed Zn-Ni alloy plating has been accomplished galvanostatically on mild steel (MS) using gelatin and glycerol as additives. The effect of addition of Cd into Zn-Ni bath has been examined in terms of nickel (Ni) content and corrosion resistance of Zn-Ni-Cd ternary alloy coatings. The process and product of electrolysis under different concentrations of additives and Cd have been investigated by cyclic voltammetry (CV). The effects of current density (c.d.) on Ni content of the alloy have been studied by spectrophotometric method, supported by EDX analysis. The deposition has been carried out under different concentrations of Cd ranging from 0.004 to 0.1 M. The corrosion rates (CR) of Zn-Ni alloy coatings have been found to decrease drastically with addition of Cd. It has been also revealed that the CR of binary Zn-Ni alloy coatings decreased with the increase of Cd concentration only up to a certain optimal concentration, i.e., up to 0.02 M, and then remained unchanged. An effort to change the anomalous type of codeposition into normal one by changing the molar ratios of the metal ions, i.e. [Cd²⁺]/[Ni²⁺] as 0.01, 0.05 and 0.25 has remained futile. CV study demonstrated an important role of Cd in mutual depositions of Zn²⁺ and Ni²⁺ ions by its preferential adsorption, thus leading to the increased Ni content of the alloy. The bath composition and operating parameters have been optimized for deposition of bright and uniform Zn-Ni-Cd alloy coatings. Changes in the surface morphology and phase structure of Zn-Ni alloy coatings due to addition of Cd has been confirmed by Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) study respectively. Experimental investigations so as to identify the role of Cd in codeposition Zn-Ni alloy coatings have been carried out and the results are discussed.     

Electrodeposition of Zn-Ni, Zn-Fe and Zn-Ni-Fe alloys

Zn-Fe, Zn-Ni and Zn-Ni-Fe coatings were electrodeposited galvanostatically on mild steel from acidic baths (pH 3.5) consisted of ZnCl₂, NiCl₂, FeCl₂, gelatin, sulfanilic (p-aminobenzenesulfonic) acid and ascorbic acid. Cyclic voltammetry showed that the effect of gelatin was more pronounced than that of sulfanilic acid, and that the deposition of the ternary alloy behaved differently from the deposition of the binary alloys. In all three systems, the Faradaic efficiency was higher than 88%, the rate of Zn deposition was heavily influenced by mass-transport limitation at high applied current densities, and the deposition was of anomalous type. For each applied current density, the concentrations of Ni and Fe in the ternary alloy were higher than the corresponding concentrations in the binary alloys. The hardness of Zn-Ni coatings was the highest, while that of Zn-Fe coatings was the lowest. The Zn-Ni-Fe coatings were the smoothest, had distinguished surface morphology, and contained ZnO in the bulk, not just on the surface. The lowest corrosion rate in each alloy system (214, 325 and 26 μm year⁻¹ for Zn-Ni, Zn-Fe and Zn-Ni-Fe, respectively) was characteristic of coatings deposited at 30, 30 and 40 mA cm^− 2, respectively. The higher corrosion resistance of the ternary alloy was also reflected by a higher corrosion potential, a higher impedance and a higher slope of the Mott-Schottky line. The enhanced corrosion behavior of the ternary alloy was thus attributed to its chemical composition, phase content, roughness and the synergistic effect of Ni and Fe on the n-type semiconductor surface film.     

Comparative Isothermal Oxidation Behaviour of New Aluminide Coatings from Slurries Containing Al Particles and Conventional Out-of-Pack Aluminide Coatings

This paper investigates the microstructural differences of an out-of-pack (SVPA) aluminide coating with a TBC/aluminide coating deposited by slurry (PARTICOAT) onto René N5 superalloy material. Their isothermal oxidation behaviour in air at 1,100 °C up to 1,000 h is compared with the support of Thermocalc^® modelling. Whereas the kinetics appears lower in the SVPA than in the slurry for short oxidation term (<100 h), the mass gains are similar after long exposure time (1,000 h). Both coatings develop a NiAl₂O₄/Al₂O₃ duplex scale, the slurry showing a thicker spinel layer than the SVPA. Interdiffusion brings about the transformation of β-NiAl into γ′-Ni₃Al in both systems which extends more in the slurry coatings, thereby inducing coarser precipitation of refractory elements. However, in contrast to the out-of-pack coatings, both the precipitates and the top coat of alumina hollow spheres developed in the slurry coatings significantly limit rumpling of the oxide scale.     

Effect of electrodeposition conditions on the composition, microstructure, and corrosion resistance of Zn-Ni alloy coatings

The formation, composition, and structure of electrodeposited zinc-nickel alloys were investigated. It has been shown that both anomalous and normal codeposition of zinc and nickel can be realized by changing the bath composition and deposition conditions, with the nickel content in the resultant deposit being varied in a wide range (from 2 to 90 at.%). It has been also shown that the ammonical diphosphate electrolyte allows deposition of Zn-Ni coatings with a homogeneous phase structure (Ni₅Zn₂₁ and Ni₃Zn₂₂ intermetallides, a solid solution of Zn in Ni, or a solid solution of Ni in Ni₅Zn₂₁), whereas the weak acid chloride electrolyte produces two-phase coatings consisting of Ni₅Zn₂₁ with the admixture of polycrystalline Zn or Ni. The Zn-Ni coating with a nickel content of 19 at.% consisting of Ni₅Zn₂₁ intermetallic phase exhibits the highest corrosion resistance.     

Comparison of corrosion-resistance and hydrogen permeation properties of Zn-Ni, Zn-Ni-Cd and Cd coatings on low-carbon steel

Zn-Ni-Cd alloy was electroplated from an alkaline sulfate bath under potentiostatic conditions. The corrosion and hydrogen permeation characteristics of Zn-Ni-Cd alloy coatings electrodeposited from alkaline bath were studied and compared with those of Cd and Zn-Ni coatings obtained using commercial baths. Zn-Ni-Cd alloy was electroplated from an alkaline sulfate bath under potentiostatic conditions. The corrosion potential of this Zn-Ni-Cd coating was −0.62 V vs. SCE, which is still negative potential compared to iron. The corrosion rate of Zn-Ni-Cd coated steel was 0.073 mm y⁻¹, which is estimated in a solution at a pH of 7. This value is much lower than the corrosion rate of Zn-Ni alloy (0.502 mm y⁻¹) and Cd (0.306 mm y⁻¹) coatings deposited from commercial baths. Zn-Ni-Cd alloys are also demonstrated to have superior hydrogen permeation inhibition properties compared to Cd and Zn-Ni coatings. Kinetic parameters of hydrogen permeation such as the transfer coefficient, α, the modified exchange current density, i₀^′, thickness dependent adsorption-absorption rate constant, k^″, recombination rate constant, k₃, surface hydrogen coverage, θ_s, were evaluated by applying a mathematical model to analyze experimental results.     

Microstructural analysis of low Ni content Zn alloy electrodeposited under applied magnetic field

The codeposition behaviour of Zn and Ni has been studied in sulphate electrolytes in presence of a superimposed magnetic field up to 1 T parallel to the surface. Structural analysis by X-Ray diffraction (XRD) method revealed that the alloys consisted of a mixture of zinc, η-phase and γ-phase. The results showed that the magnetic field B was responsible for variations of the alloy structural parameters (lattice imperfection and texture) for low pH. When the pH of electrolyte was increased, the effect of the magnetic field was erased. It was suggested that the preferential growth direction of Zn-Ni alloy was induced by the mass-transport enhancement of H⁺ ions promoted by B which induced an increase of the pH near the cathode. Then, for high value of pH up to 3.5, the crystallographic orientation (101) of Zn was always favoured to the prejudice of the Ni₅Zn₂₁ phase.     

Development and tribological characterisation of nanostructured Zn-Ni and Zn-Co coatings: a comparative study

Zn-Ni and Zn-Co alloy coatings were electrodeposited on mild steel from sulphate-based baths. The morphology, microstructure, microhardness and tribological behaviours of the coatings have been studied and discussed. While the Zn-5wt-% Co layers presented a nanocrystalline simple nodular structure (45?±?5?nm), the Zn-14wt-% Ni showed a particular structure called cauliflower morphology (30?±?7?nm). The X-ray diffraction analysis showed that each of the electrodeposits was formed from zinc solid solution with a uniform zinc-cobalt intermetallic phase γ₂ (CoZn₁₃) for Zn-5wt-% Co alloy. However, a single γ-phase (intermetallic compound Ni₅Zn₂₁) was presented for the Zn-14wt-% Ni alloys. The Zn-14wt-% Ni films were found to be harder and rougher than the Zn-5wt-% Co layers. Plastic deformation and oxide layers production were the main wear mechanisms for the investigated coatings. The Zn-14wt-% Ni coatings were found to have the best wear resistance due to their microhardness and particular structure.     

Stepwise Depletion of Coating Elements as a Result of Hot Corrosion of NiCrAlY Coatings

Present investigation deals with the hot corrosion behaviour of the NiCrAlY coatings deposited by HVOF technique on Superni76 under cyclic conditions at 900  °C in the presence of Na₂SO₄ + 60% V₂O₅ salt. The weight change behaviour of the coatings was followed with time up to 200 cycles and K _p value was calculated for the hot corrosion process. Surface and cross-section of the corroded samples were examined by FESEM/EDS and XRD to follow the progress of corrosion up to 200 cycles. In earlier cycles, the corrosive species oxidised top surface of the coatings. With increasing number of cycles, oxidation of the coatings occurred up to 40-μm depth. A Cr-depleted band was seen below the oxide scale. Further increase in number of cycles led to migration and oxidation of Al to form Al₂O₃ sublayer at coating/scale interface, thereby leading to formation of Al-depleted zone in the coating below the Al₂O₃ sublayer. The corrosion resistance of the NiCrAlY coatings is attributed to the formation of the continuous and dense Al₂O₃ sublayer at the coating/scale interface, which acts as barrier to the migration of Cr to the surface. The appearance of Al₃Y after 100 and 200 cycles also contributes to the increased corrosion resistance of coatings after 100 and 200 cycles.     

Corrosion Resistance of APS- and HVOF-Sprayed Coatings in the Al2O3-TiO2 System

In this article, the results of corrosion investigations performed on thermally sprayed ceramic coatings with different compositions in the Al₂O₃-TiO₂ system (Al₂O₃, Al₂O₃-3%TiO₂, Al₂O₃-40%TiO₂, and TiO _x ) are presented. The coatings were deposited on corrosion-resistant steel substrates using atmospheric plasma spraying (APS) and high-velocity oxy-fuel (HVOF) spraying processes and characterized by means of optical microscopy, scanning electron microscopy (SEM), and x-ray diffraction (XRD). The corrosion properties were investigated in 1 N solutions of NaOH and H₂SO₄, at room temperature, 60 °C, and 85 °C, as well as in hydrothermal conditions with deionized water at 100 °C and 200 °C. The corrosion stability of the coatings depended on coating characteristics (spraying method, microstructure, and crystalline phase composition) and the corrosive environment (media, test temperature, and duration). In contrast to expectations, APS-sprayed coatings were found to be more corrosion-resistant than the HVOF-sprayed coatings. Addition of TiO₂ to Al₂O₃ increased the corrosion stability, especially for the HVOF-sprayed coatings. In this work, TiO _x coatings were found to be more corrosion-resistant than the Al₂O₃-based coatings.     

Development of nano-structured cyclic multilayer Zn-Ni alloy coatings using triangular current pulses

Cyclic multilayer alloy (CMA) deposits of Zn-Ni were developed on mild steel from sulphate bath having thiamine hydrochloride (THC) and citric acid (CA) as additives. CMA coatings were developed galvanostatically using triangular current pulses, under different conditions of cyclic cathode current density (CCCD’s) and number of layers. The corrosion behaviors of the coatings were evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy methods, and were compared with that of monolayer Zn-Ni alloy of same thickness. At optimal configuration, CMA coating represented as, (Zn-Ni)_2.0/5.0/300 was found to exhibit ∼40 times better corrosion resistance compared to monolayer alloy, (Zn-Ni)_3.0. Cyclic voltammetry study demonstrated that THC and CA have improved the appearance of the deposit by complexation with metal ions. The corrosion protection efficacy of CMA coatings was attributed to the difference in phase structure of the alloy in successive layers, evidenced by XRD analysis. The formation of multilayer and corrosion mechanism was analyzed by Scanning Electron Microscopy (SEM) study.     

Phase Equilibrium of the Fe-Cr-Ni-Al Quaternary System at 900 °C

The 900 °C isothermal sections of the Fe-Cr-Ni-Al quaternary with Fe fixed at 70 at.% and that with Ni fixed at 60 at.% have been determined by scanning electron microcopy coupled with energy dispersive spectroscopy and x-ray diffraction. Only one three-phase region marked as α-Fe + β-(Fe,Ni)Al + γ-Fe has been found in the 70Fe-Cr-Ni-Al section. With regard to the Fe-Cr-60Ni-Al section, there are three three-phase regions, i.e., α-Cr + γ-Ni + γ′-Ni₃Al, γ-Ni + γ′-Ni₃Al + β-(Fe,Ni)Al and α-Cr + β-(Fe,Ni)Al + γ′-Ni₃Al and one four-phase region, namely α-Cr + β-(Fe,Ni)Al + γ-Ni + γ′-Ni₃Al. No quaternary compound is found in the present work.     

热喷涂Zn-Ni复合涂层在海水中的腐蚀行为研究

利用喷雾干燥法制备了不同Ni含量的团聚型Zn-Ni复合粉末,并在此基础上用氧乙炔火焰喷涂工艺制备了Zn-Ni复合涂层。通过动电位极化和电化学阻抗测试,并结合SEM、EDS和XRD分析,研究涂层在海水介质中的防护性能和腐蚀机理。结果表明：涂层的自腐蚀电位稳定值在-0.98~-0.95 V,Ni可起到稳定Zn(OH)₂,抑制其向疏松ZnO转化的作用,腐蚀产物的堆积使得涂层电阻R_c和电荷转移电阻R_t不断增大,腐蚀电流不断减小;不同Ni含量涂层的耐蚀性存在明显差异,其中Ni含量为20 mass%的涂层耐蚀性能最好。     

Hot corrosion and oxidation behavior of a novel Pt + Hf-modified γ′-Ni3Al + γ-Ni-based coating

A new type of Pt + Hf-modified γ′-Ni₃Al + γ-Ni-based coating has been developed in which deposition involves Pt electroplating followed by combined aluminizing and hafnizing using a pack cementation process. Cyclic oxidation testing of both Pt + Hf-modified γ′ + γ and Pt-modified β-NiAl coatings at 1150 °C (2102 °F), in air, resulted in the formation of a continuous and adherent α-Al₂O₃ scale; however, the latter developed unwanted surface undulations after thermal cycling. Type I (i.e. 900 °C/1652 °F) and Type II (i.e. 705 °C/1300 °F) hot corrosion behavior of the Pt + Hf-modified γ′ + γ coating were studied and compared to Pt-modified β and γ + β-CoCrAlY coatings. Both types of hot corrosion conditions were simulated by depositing Na₂SO₄ salt on the coated samples and then exposing the samples to a laboratory-based furnace rig. It was found that the Pt + Hf-modified γ′ + γ and Pt-modified β coatings exhibited superior Type II hot corrosion resistance compared to the γ + β-CoCrAlY coating; while the Pt + Hf-modified γ′ + γ and γ + β-CoCrAlY coatings showed improved Type I hot corrosion performance than the Pt-modified β.     

Microstructural Characterization and Cyclic Hot Corrosion Behaviour of Sputtered Co�CAl Nanostructured Coatings on Superalloy

Nanostructured Co?CAl coatings on Superni-718 superalloy substrate were deposited by DC/RF magnetron sputtering in the present work. The microstructure and cyclic hot-corrosion behavior of nanostructured Co?CAl coatings on Superni-718 superalloy were investigated in molten salt of 40 wt% Na₂SO₄ + 60 wt% V₂O₅ at 900 °C. The results showed that a dense scale formed on the coated samples exposed to corrosive environment during thermal cycling. The spinel phases of CoCr₂O₄, CoAl₂O₄ and NiCr₂O₄ were found in the corroded scale of the coatings, resulting in an effective inhibition of O and S diffusion. The sputtered Co?CAl coatings exhibited high hot corrosion resistance due to the formation of ??-CoAl phases in the coating. The relevant corrosion mechanisms substantiating the role of coatings are discussed.     

A Comparative Study on the Effect of MWCNT as Reinforcement on the Corrosion Parameters of Different Ni–W/MWCNTs Nanocomposite Coatings in Various Corrosive Media

Nickel–tungsten multi-walled carbon nanotubes (Ni–W/MWCNTs) nanocomposite coatings were co-electrodeposited in the ammonium-free bath by means of constant direct current coulometry. The results indicate that the amount of MWCNTs incorporated into the nanocomposite coatings has a key role in the improvement of their microhardness and corrosion resistance. The corrosion behavior of the coatings was evaluated using potentiodynamic polarization and electrochemical impedance spectroscopy methods in three corrosive media of 3.5 wt% NaCl, 1.0 M NaOH, and 0.5 M H₂SO₄. The experimental data of the corrosion current density (j_corr), corrosion rate (CR), the polarization resistance (R_p), and microhardness indicate that the presence of MWCNTs in coatings improves the quality of those coatings. The surface morphology of the coatings and the elemental analysis data were obtained by scanning electron microscopy and energy dispersive X-ray microanalysis respectively. As the results showed, the coatings were uniform and crack-free in the presence of 5.3 wt% carbon. Also, a microhardness test revealed that the nanocomposite coating containing 5.3 wt% carbon obtained in an ammonium-free bath which provided the higher content of tungsten had the highest hardness value among others.