Zn Ni Coatings Research Articles

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-Ni 5Zn 21 and a denser morphology. Additionally, the hydrogen content incorporated is lower than even cadmium-coated 4340 steel which has undergone a postbaking dehydrogenation treatment.

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 26μm year−1 for Zn–Ni, Zn–Fe and Zn–Ni–Fe, respectively) was characteristic of coatings deposited at 30, 30 and 40mAcm−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.

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 (Ni 5Zn 21 and Ni 3Zn 22 intermetallides, a solid solution of Zn in Ni, or a solid solution of Ni in Ni 5Zn 21), whereas the weak acid chloride electrolyte produces two-phase coatings consisting of Ni 5Zn 21 with the admixture of polycrystalline Zn or Ni. The Zn–Ni coating with a nickel content of 19 at.% consisting of Ni 5Zn 21 intermetallic phase exhibits the highest corrosion resistance.

Pulse plating effect on composition and corrosion properties of zinc alloy coatings

The influence of pulse plating parameters on the chemical and phase composition, surface topography and corrosion resistance of Zn–Ni (∼12%Ni), Zn–Co (<1%Co) and Zn–Fe (<1%Fe) alloy coatings has been studied. Pulse plating of Zn alloys resulted in different outcomes for Zn–Ni and low alloyed (Zn–Co and Zn–Fe) coatings. Non-stationary crystallisation resulted in formation of single phase γ-Zn21Ni5 coatings, while two different phases of Zn were produced during Zn–Co and Zn–Fe deposition. Pulse plated Zn–Ni coatings possessed from two to three times smaller grain size and at the same time, corrosion currents half those of the direct current deposits. However, the reduction in the grain size of Zn–Co and Zn–Fe coatings was not accompanied by any reduction in the corrosion current of these alloys. It is suggested that the dual phase formation was the principal reason why pulse plated low alloyed Zn coatings did not exhibit improvement in their corrosion resistance.

Surface characterization of the Zn–Ni–Al 2O 3 nanocomposite coating fabricated under ultrasound condition

Zn–Ni–Al 2O 3 nanocomposite coating, which was fabricated by eletrodeposition technique with the aid of ultrasound, was investigated by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis and X-ray photoelectron spectroscopy (XPS). The results reveal that 7.2 wt.% nano-alumina particles uniformly dispersed in the matrix of the composite coating. The XPS analyses demonstrate that the outermost layer of Zn–Ni–Al 2O 3 coating was composed of nano-alumina and Zn(OH) 2, while the transition layer between the outermost layer and the Zn–Ni matrix consisted of nano-alumina, metallic Zn, ZnO and metallic Ni. In order to investigate the influences of ultrasonic agitation and the incorporation of nano-alumina on the composition and surface structure of Zn–Ni matrix, the comparison studies of Zn–Ni–Al 2O 3 nanocomposite coating with Zn–Ni coatings fabricated with and without ultrasound were conducted. The results indicate that ultrasonic agitation resulted in a decrease of Ni content in the Zn–Ni matrix and an increase of the thickness of surface oxide layer; while the incorporation of nano-α-Al 2O 3 increased the Ni content in the Zn–Ni matrix.

Protective coating for magnesium alloy

A phosphate–permanganate conversion coating was applied as the pretreatment process for AZ91D magnesium alloy substrate. Zn–Ni alloys were electrodeposited onto the treated AZ91D magnesium alloy from sulfate bath. The morphology and phase composition of the coatings were determined with X-ray diffraction (XRD) and Scanning Electron Microscope (SEM). The results reveal that the conversion rate depends on pH of solution and treatment time. Salt spray and the electrochemical polarization testing were applied to evaluate the corrosion performance of phosphate–permanganate and Zn–Ni coated alloys. It was found that Ni content in deposit is a function of current density and bath composition. Zn–13 wt.% Ni coating provides very good corrosion protective function to inner AZ91D magnesium alloy. Phosphate–permanganate treatment enhances the corrosion resistance of Zn–Ni coatings.

Characterization of thin Zn–Ni alloy coatings electrodeposited on low carbon steel

The characteristics of initial layer formation in alkaline bath for Zn–Ni (12–15%) alloy electrodeposition on low carbon steel plates are detected in a nanometric thickness range by electron probe microanalysis (EPMA), with both bulk sample and thin film on substrate correction procedure, glow discharge optical emission spectroscopy (GDOES) and gracing incidence X-ray diffraction (GIXRD). The Zn–Ni coatings were elaborated using either intensiostatic or potentiostatic mode. A preferential deposition of Ni, in the initial thin layer, is detected by these analyses; according to EPMA and GDOES measurements, a layer rich in nickel at the interface substrate/deposit is observed (90 wt.% Ni) and approved by GIXRD; the thin layer of Ni formed in the first moments of electrolysis greatly inhibits the Zn deposition. The initial layer depends upon the relative ease of hydrogen and metal discharge and on the different substrate surfaces involved. The electrodeposition of zinc–nickel alloys in the first stage is a normal phenomenon of codeposition, whereby nickel – the more noble metal – is deposited preferentially.

Development of Zn–Ni–Cd coatings by pulse electrodeposition process

A pulse electrodeposition process was developed for the synthesis of Zn–Ni–Cd alloys with cadmium contents between 2 and 7 wt.%. The alloy was deposited from an alkaline bath (pH=9.3). The results indicated that the average current density and pulse current on time (Ton) control the corrosion potential and the composition of the resultant alloys. An increase in average current density resulted in a decrease in both nickel and cadmium content in the alloy. At lower Ton time (0.5 ms), higher percentages of zinc and cadmium were deposited. The barrier properties of Zn–Ni–Cd alloys plated using different average current densities were evaluated by using mechanical, electrochemical and ASTM B117 salt spray tests and compared with commercial Zn–Ni and Cd deposits. The results indicated that Zn–Ni–Cd alloys exhibit superior barrier properties when compared to Cd or Zn–Ni coatings.

Hydrogen evolution and corrosion performance of NiZn coatings

Abstract The hydrogen evolution reaction (HER) was studied on bare brass (B), nickel coated brass (B/Ni), nickel–zinc alloy coated brass (B/NiZn, leached Zn), nickel (under layer)/nickel–zinc alloy coated (top layer) brass electrodes (B/Ni/NiZn, leached Zn) in 1 M NaOH solution using electrolysis, cathodic polarization and electrochemical impedance spectroscopy (EIS) techniques. Long term tests were performed in order to evaluate the electrocatalytic activity and corrosion performance of the prepared electrodes with operation time. The results showed that leached electrodes, especially B/Ni/NiZn electrode, have a smaller overpotential than the others and a good electrocatalytic activity for the HER. Its electrocatalytic activity is also stable over electrolysis time. Contrary to the HER performance, B/NiZn cannot provide an adequate corrosion protection. However, when a thin Ni layer was coated under the NiZn alloy coating (B/Ni/NiZn), a greater electrocatalytic stability and corrosion protection was obtained.

Pulse plating effect on microstructure and corrosion properties of Zn–Ni alloy coatings

The influence of pulse plating parameters on the surface morphology, grain size, lattice imperfection and corrosion properties of Zn–Ni alloy has been studied. The coatings were electrodeposited in an alkaline cyanide-free solution. AFM was applied for surface morphology examination, XRD measurements were carried out for phase composition and texture analysis, electron probe microanalysis was used for alloy chemical composition studies, while electrochemical techniques were applied for corrosion performance evaluation. The pulse plated Zn–Ni coatings appeared to consist of the γ-Zn21Ni5 phase and the composition of the alloy depended on the plating parameters. The grain size, lattice imperfection and homogeneity of grain distribution were established to be the main factors determining corrosion behaviour of the coating.

Corrosion behavior of pulse-plated Zn–Ni alloy coatings on AZ91 magnesium alloy in alkaline solutions

Zn–Ni alloy coatings were plated on AZ91 magnesium alloy using a pulse potential electrodeposition method after zinc immersion and Zn–Cu plating. The density and frequency of electric current, deposition time and t on / t off have large effects on chemical composition, thickness of the Zn–Ni coatings. The Zn–Ni coatings provide very good corrosion protective function to inner AZ91 magnesium alloy. The good corrosion resistance of Zn–Ni coatings is attributed to the existence of the γ-phase and the homogeneous and compact structure of the coatings. Chromating treatment can further improve the corrosion resistance of Zn–Ni coatings.

Effects of Electroplated Zinc-Nickel Alloy Coatings on the Fatigue Strength of AISI 4340 High-Strength Steel

Recovered substrates have been extensively used in the aerospace field. Cadmium electroplating has been widely applied to promote protective coatings in aeronautical components, resulting in excellent corrosion protection combined with a good performance in cyclic loading. Ecological considerations allied to the increasing demands for corrosion resistance have resulted in the search for possible alternatives. Zinc-nickel (Zn-Ni) alloys have received considerable interest recently, because these coatings show advantages such as a good resistance to white and red rust, high plating rates, and acceptance in the market. In this study, the effect of electroplated Zn-Ni coatings on AISI 4340 high-strength steel was analyzed for rotating bending fatigue strength, corrosion, and adhesion resistance. The compressive residual stress field was measured by x-ray diffraction prior to fatigue tests. Optical microscopy documented coating thickness, adhesion characteristics, and coverage extent for nearly all substrates. Fractured fatigue specimens were investigated using scanning electron microscopy (SEM). Three different Zn-Ni coating thicknesses were tested, and comparisons with the rotating bending fatigue data from electroplated Cd specimens were performed. Experimental results differentiated the effects of the various coatings on the AISI 4340 steel behavior when submitted to fatigue testing and the influence of coating thickness on the fatigue strength.

Zn–Ni alloy coatings pulse-plated on magnesium alloy

Zn–Ni alloy coatings are plated on AZ91 magnesium alloy using a pulse potential after an appropriate pretreatment. The current density, frequency, t on/ t off and time have effects on thickness, morphology, Ni content, microhardness and anticorrosion of Zn–Ni coatings. Corrosion can reach above 200 h in salt spray test according to ASTM B117; the corrosion behavior of coatings is interfacial corrosion of particles and results in particle degradation as observed by scanning electron microscopy. Weight loss method and polarization curves are also used to investigate anticorrosion properties. Bonding strength is above 14.8 MPa in adhesive tensile tests.

Adherence of Electrodeposited Zn–Ni Coatings on EN AW2024 T3 Aluminium Alloy

The use of hexavalent chromium in surface treatments will be reduced in the future, as it is suspected to be carcinogenic. Electrodeposition of Zn–Ni, which is currently used on steel, represents a non-chromate alternative surface treatment for the corrosion protection of aluminium alloys. Zn–Ni coatings were electrodeposited onto an EN AW2024 T3 aluminium alloy sheet in a laboratory flow cell. To obtain several percentages of Ni in the coatings, solutions with different Ni2+ concentrations were used. The influence of a specific pretreatment to promote adherence, such as zincate immersion (109 g L–1 ZnO, 525 g L–1 NaOH) or phosphoric anodisation (36 wt-%H3PO4, 20°C) prior to electrodeposition was also investigated. The smoothness of the coating, measured on a three-dimensional roughness tester, increased when the percentage of Ni increased. This can be explained by a microstructural refinement observed using SEM and AFM. These microstructural changes are due to the evolution of the crystal structure of the coatings and can be observed by X-ray diffraction. The mechanical properties of the coatings (hardness and Young's modulus) were measured by microindentation and nanoindentation. The adherence of the coatings was tested by a scratch test and a three point bending test dedicated to coatings. The scratch behaviour of the coatings was a function of the percentage of Ni. The scratches observed indicate a ductile fracture for coatings with a low percentage of Ni and a brittle fracture for a high percentage of Ni. Bending tests demonstrated the favourable effect of pretreatments such as zincate immersion or phosphoric anodisation as well as adhesion enhancement as a function of increasing percentage of Ni. SE/503

Comparison of Mechanical, Corrosion, and Hydrogen Permeation Properties of Electroless Ni-Zn-P Alloys with Electrolytic Zn-Ni and Cd Coatings

Abstract Novel Ni-Zn-P alloys containing 74 wt% Ni was prepared from a sulfate electrolyte using an electroless process. The mechanical, corrosion, and hydrogen permeation resistance characteristics of the developed Ni-Zn-P alloys were studied and compared with those of electrodeposited Cd and Zn-Ni coatings. Comparison of the mechanical properties shows that the developed alloy possesses engineering attributes comparable to those of Cd coatings. Corrosion studies in a 0.5-M sodium sulfate (Na2SO4) + 0.5-M boric acid (H3BO3) solution at pH 7.0 reveals that the dissolution rate of Ni-Zn-P alloys is five times lower than that of Zn-Ni and Cd coatings. In addition to the corrosion and engineering properties, comparison of the hydrogen permeation characteristics of the developed coating was made with that of Cd and Zn-Ni coatings. The exchange current density, i0; thickness dependent adsorption-absorption rate constant, k″; recombination rate constant, k3; and the surface hydrogen coverage, θs, that character...

Determination of the corrosion rate of Zn and Zn–Ni layers by the EDS technique

A method of corrosion rate determination based on X-ray energy-dispersive spectroscopy (EDS) has been developed. The study of corrosion in test solutions showed different behavior of Zn and Zn–Ni coatings during the corrosion process. The corrosion rate of the zinc coatings was nearly constant during all experiments. The courses of corrosion processes for Zn–Ni coatings showed always the same three successive stages. Dezincification of the alloys in the first and the second stages caused an increase of the Ni content in the layer during the corrosion process.

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 0.5 M Na 2 SO 4+0.5 M H 3 BO 3 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, i 0 ′, thickness dependent adsorption–absorption rate constant, k ″, recombination rate constant, k 3, surface hydrogen coverage, θ s, were evaluated by applying a mathematical model to analyze experimental results.

Influence of structural factor on corrosion rate of functional Zn–Ni coatings

The electrocrystallization processes of Zn–Ni coatings from weakly acid sulfate solutions with the addition of citrate and acetate complexing compounds have been compared. The deposition kinetics and the influence of electrolyte composition on the Zn–Ni alloy composition and on some physicochemical properties (corrosion resistance, texture, phase composition, morphology) were determined. It was found that the properties of coatings obtained under the same operating conditions in electrolytes with the same [Ni(II)]/[Zn(II)] concentration ratio and pH but containing different complexing agents were different. The composition of the deposits and their texture as well as phase structure were found to depend on the complexing species present. Coatings deposited from acetate solutions are richer in Ni than those from citrate solutions. The corrosion resistance of a Zn–Ni alloy depends in practice on the percentage of Ni, but in sulfate–citrate electrolytes, layers containing less Ni were obtained with the same corrosion resistance as Ni-rich coatings deposited from acetate solution. The corrosion current decreases more rapidly with increasing Ni content. This effect is connected mainly with phase composition. In citrate solutions practically single-phase (hexagonal η) deposits were obtained which provide better protection with a lower corrosion rate in comparison with alloys consisting of three phases (hexagonal η, monoclinic δ, regular γ) deposited from acetate baths.

Electrodeposition of Zn–Ni protective coatings from sulfate–acetate baths

The electrodeposition conditions for Zn–Ni alloys from sulfate–acetate electrolytes have been studied with the view of preparing protective coatings. The influence of electrolyte composition (different [Ni(II)]/[Zn(II)] ratios, pH, buffer), cathode current density, cathode potential and hydrodynamic conditions on the composition of coatings, cathode current efficiency and corrosion resistance were determined. For all of the conditions examined, strong inhibition of nickel reduction with simultaneous increase in the rate of zinc discharge characteristic of an anomalous system, have been observed. The Zn(II) discharge becomes diffusion-controlled at more negative cathode potentials, whereas the partial nickel current densities are independent of electrode rotation speed. Consequently, nickel content and current efficiency are reduced with decreasing thickness of the diffusive layer. An increase in pH above 3.3 causes a significant catalysis of Zn–Ni deposition with a simultaneous decrease of the nickel in coatings. This effect may be related to the formation and increasing concentration of Zn(II) and Ni(II) acetate complexes in this condition. The Zn–Ni coatings obtained (5–18% Ni) characterise improved corrosion resistance in comparison to Zn layers deposited under the same conditions.

Corrosion Behaviour of Zn and Zn Alloy Coatings in Alkaline Media

SUMMARYThe corrosion behaviour of electrodeposited Zn, Zn-Co, Zn-Fe and Zn-Ni coatings without and with Chromate films was studied in alkaline solutions and mortar probes. DC polarization measurements were used for electrochemical characterization of the investigated samples, while AES XPS techniques were applied for the Chromate film analysis. The stability of the anodically formed oxide films on unchromated coatings appeared to be dependent on their composition. The oxide layer stability in Cl free NaOH solution was higher for Zn-Ni and Zn-Co alloys, while introduction of Cl ions caused lowering of stability especially for Zn-Ni coating. The detected corrosion current values implied that unchromated Zn alloys did not possess higher corrosion resistance in alkaline media containing Cl ions. Meanwhile, chromated Zn-Co coatings exhibited the beneficial effect of alloying for corrosion in alkaline solutions and concrete. A higher concentration of Cr(Vl) compounds in the passive Chromate layer of the alloyed sample may be the reason for its superior corrosion resistance.