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.     

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 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.     

Effect of Induced Magnetic Field on Electrocrystallization of Zn-Ni Alloy and Their Corrosion Study

Zn-Ni alloy coatings have been deposited galvanostatically on mild steel under the effect of induced magnetic field (B), using gelatin and glycerol as additives. The effect of field intensity (from 0.05 to 0.4 T) and direction (both parallel and perpendicular) on electrocrystallization process has been studied considering the magnetohydrodynamic effect. The corrosion behaviors of coatings, deposited under different conditions of B, were evaluated by electrochemical AC and DC methods. Under optimal condition of B (perpendicular), Zn-Ni coatings showed about 3 times less corrosion rate (CR) than the one developed under natural convection (B = 0 T), deposited from same bath for same duration. The significant decrease of CR was attributed to unique electrocrystallization process during deposition, favoring increased γ-Ni₅Zn₂₁ (321) and decreased γ-Ni₅Zn₂₁ (330) phase. Progressive decrease of CR with increase of B showed that corrosion protection efficacy of the coatings bears close relation with their crystallographic orientations and surface topography, evidenced by XRD study and SEM analysis. The effect of B on thickness, microhardness, surface morphology, phase structure, and the corrosion resistance of coatings was analyzed and results were discussed.     

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.     

Zn-Ni合金镀层中添加第三种元素和纳米颗粒的研究新进展

介绍了在汽车、航空航天等行业中得到广泛应用的钢铁零件电镀Zn-Ni合金镀层,以及往碱性、氯化物等锌镍合金镀液中加入Fe、Co、Mn、Ce、P等第三种元素所获得的锌镍三元合金镀层,具有更优良的耐腐蚀性、催化性等性能的情况。介绍了往Zn-Ni合金镀液里加入氧化硅、氧化铈、氧化钛、氧化铝、碳化硅等纳米颗粒的进展情况,发现含有纳米颗粒的锌镍复合镀层具有耐腐蚀性、耐磨损性、热稳定性更好,硬度更高等优点。梳理了2016年以来在Zn-Ni合金电镀中添加第三种元素和纳米颗粒的多层镀层研究新进展。从Zn-Ni单一镀液中沉积Ni-P和Zn-Ni合金多层镀层时,在低电流密度下沉积出Ni-P层;在较高电流密度下,沉积出含3.2%P的Zn-Ni-P合金镀层,这种多层镀层可以大幅度提高钢铁零件的防腐蚀性能。介绍了在含12%Ni的Zn-Ni镀层上镀覆Ni-Co-SiC纳米复合镀层的情况,这种多层结构既可以提高镀层的结合力,又可提高其在3.5%NaCl溶液中的耐腐蚀性能。该复合镀层是一种硬度高、磨损量低的新型Zn-Ni合金复合镀层。     

Permeation measurements to study hydrogen uptake by steel electroplated with zinc-cobalt alloys

Hydrogen permeation measurements were performed to investigate hydrogen uptake by a steel substrate when electroplated with zinc-cobalt alloys. The influence of plating bath composition and pH were studied and the effect of absorbed hydrogen on embrittlement of high strength steel was measured in slow strain rate tests. It was shown that the majority of the hydrogen generated during electroplating was either evolved from the surface or trapped in the coating. Only a very small proportion, equivalent to 0.1-0.6% of the total charged passed, was absorbed by the steel and was responsible for causing embrittlement. The average amount of hydrogen required to cause hydrogen embrittlement failure of quenched and tempered AISI 4340 steel was 1.5 × 10⁻⁷ g H atoms cm⁻². There was a linear relationship between the rates of hydrogen uptake from the different coatings and the extent of hydrogen embrittlement produced in the high strength steel. In certain conditions, a cobalt-rich layer formed at the steel/coating interface and this layer was shown to be beneficial in restricting hydrogen uptake and embrittlement.     

Film properties and stability influence on impedance distribution during the dissolution process of low-carbon steel exposed to modified alkaline sour environment

In this work, we characterized the anodic dissolution and the hydrogen transport within carbon steel (SAE 1018) samples immersed in alkaline sour solutions (CN⁻, polysulfide-base inhibitor and H₂S(aq)). The evolution of interfacial and transport processes could be quantified by Electrochemical Impedance Spectroscopy (EIS) and hydrogen permeation measurements. EIS experimental data were analyzed and fitted by using Transmission Line Model (TLM); this latter helped to propose the mechanisms through the porous layer of the corrosion products formed. The area influencing the dissolution and the mass transfer process was quantified by the pores number, pores thickness and the interfacial passive electrical elements describing the mechanisms in different regions within the pores of the corrosion product layer. The TLM was used to analyze the active-mass transport processes occurred at different spatial positions of the porous layer, such as the mass transfer at the wall and the active-mass transfer at the base of the cylindrical pore of the non-stoichiometric Fe_xS_y.     

Pulsed microwave plasma polymerization of silicon oxide ?lms: Application of efficient permeation barriers on polyethylene terephthalate

A microwave driven low pressure plasma reactor is developed based on a modi?ed Plasmaline antenna for plasma processing of polyethylene terephthalate (PET) foils and bottles. It allows for the treatment of thermolabile packaging materials, e.g. plasma sterilization and permeation barrier coating. Silicon oxide ?lms are deposited on PET foils as a permeation barrier coating. A pulsed hexamethyldisiloxane:oxygen plasma is ignited under various conditions and the oxygen permeation is investigated. A criterion for the homogeneous deposition of SiO_x coatings is described depending on the residence time of process gases. Additionally, the composition of the coatings is analyzed by means of Fourier transform infrared spectroscopy regarding carbon and hydrogen content. A strong relation between barrier properties and ?lm composition is found: good oxygen barriers are observed as carbon content is reduced and ?lms become inorganic, quartz-like. A residual permeation as low as J = 1.0 ± 0.3 cm³ m^− 2 day^− 1bar^− 1 for SiO_x coated PET foils is achieved. The dependencies of important plasma parameters, such as gas mixture, process pressure, power and pulse conditions on oxygen permeation through packaging foil are shown to optimize the coating process.     

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.     

Initial corrosion protection of Zn–Mn alloys electrodeposited from alkaline solution

The effects of a deposition current density (c.d.) on the corrosion behaviour of Zn–Mn alloy coatings, deposited from alkaline pyrophosphate solution, were investigated by atomic absorption spectrophotometry (AAS), X-ray diffraction (XRD), atomic force microscopy (AFM), optical microscopy, electrochemical impedance spectroscopy (EIS) and measurement of corrosion potential (E_corr). XRD analysis disclosed that zinc hydroxide chloride was the main corrosion product on Zn–Mn coatings immersed in 0.5 mol dm⁻³ NaCl solution. EIS investigations revealed that less porous protective layer was produced on the alloy coating deposited at c.d. of 30 mA cm⁻² as compared to that deposited at 80 mA cm⁻².     

A SERS investigation of the electrodeposition of Au in a phosphate solution

The spectroelectrochemical behaviour of cyanide and phosphate ions adsorbed on a Au electrode during electrodeposition and corrosion in a KAu(CN)₂ phosphate bath was studied by in situ SERS (Surface Enhanced Raman Spectroscopy) and cyclic voltammetry. SERS spectra exhibiting features corresponding to CN⁻ and PO₄⁻ vibrations were recorded under potentiostatic control in an interval from − 1600 to + 1500 mV vs. Ag/AgCl. Surface CN⁻ species are the dominating ones under both cathodic and anodic polarisations. As far as the phosphate vibrations are concerned, bands related to PO₄⁻ are visible in both cathodic and anodic conditions. A fluorescence background was evident in the potential range − 1300 ÷ + 1300 mV. At cathodic potentials phosphate ion adsorption on electrode surface prevails over cyanide for potentials more cathodic than − 1000 mV. At − 700 mV a discontinuity is noticed in the Stark tuning of CN⁻, probably due to a change of Au(CN)₂⁻ adsorption mechanism. At anodic potentials bands due to cyanide oxidation could be observed.     

Organic additive-copper(II) complexes as plating precursors

Cu(II) ions previously coordinated with typical electroplating organic additives were investigated as an alternative source of metal for plating bath. The coordination complexes were isolated from reaction between CuSO₄ and organic additives as ligands (oxalate ion, ethylenediamine or imidazole). Deposits over 1010 steel were successfully obtained from electroplated baths using the complexes without any addition of free additives, at pH = 4.5 (H₂SO₄/Na₂SO₄). These deposits showed better morphologies than deposits obtained from CuSO₄ solution either in the absence or presence of oxalate ion as additive (40 mmol L^− 1), at pH = 4.5 (H₂SO₄/Na₂SO₄). It is suggestive that the starting metal plating coordinated with additives influences the electrodeposition processes, providing deposits with corrosion potentials shifted over + 200 mV in 0.5 mol L^− 1 NaCl (1 mV s^− 1). The resistance against corrosion is sensitive to the type of additive-complex used as precursor. The complex with ethylenediamine presented the best deposit results with the lowest pitting potential (− 0.27 V vs 3.0 mol L^− 1 CE). It was concluded that the addition of free additives to the electrodeposition baths is not necessary when working with previously coordinated additives. Thus, the complexes generated in ex-situ are good alternatives as plating precursors for electrodeposition bath.     

Microstructure and Corrosion Resistance of Electrodeposited Ni-Cu-Mo Alloy Coatings

This paper deals with the electrodeposition of Ni-Cu-Mo ternary alloy coatings on low-carbon steel substrate from an aqueous citrate sulfate bath. The structures and microstructure of coatings were characterized by scanning electron microscopy and x-ray diffractometry. The corrosion resistance of coatings was investigated by potentiodynamic polarization (Tafel) and electrochemical impedance spectroscopy techniques. The results show that the Ni-Cu-Mo coatings are mainly composed of fcc-Ni phase and a small amount of NiCu phase. Ni-Cu-Mo coatings exhibit a nodular surface morphology, and the roughness of electroplated coating increases with the increasing of Na₂MoO₄·2H₂O in the bath. The corrosion performance of the coatings is significantly affected by the Mo content of the alloy coating and their surface morphology. The coating prepared in bath containing 40 g/L Na₂MoO₄·2H₂O has the highest corrosion resistance in 3.5 wt.% NaCl solution, while that prepared in bath containing 60 g/L (or more) Na₂MoO₄·2H₂O shows a lower corrosion resistance due to the presence of microcracks on the coating surface.     

Corrosion behavior of glassy Ni55Co5Nb20Ti10Zr10 alloy in 1 N HCl solution studied by potentiostatic polarization and XPS

Corrosion resistance of glassy Ni₅₅Co₅Nb₂₀Ti₁₀Zr₁₀ (at.%) alloy in 1 N HCl solution was investigated with respect to the electrochemical behavior and the compositions of the passive film and the underlying alloy surface just below the passive film. The potentiostatic polarization curve indicated that the alloy was spontaneously passivated with a low passive current density of the order of 10⁻³ A m⁻². The quantitative X-ray photo-electron spectroscopy (XPS) analysis revealed that the thickness of the surface film increased linearly with an anodizing ratio of 1.5 nm V⁻¹. The high corrosion resistance of the glassy alloy was due to the formation of niobium, titanium and zirconium-enriched passive film. The growth mechanism of the passive films is also discussed.     

碱性电镀锌镍合金的研究

采用恒电流沉积方法和X射线能谱(EDS)技术,研究了碱性Zn-Ni合金的主要电沉积工艺参数对镀层组成的影响规律,获得了Ni含量稳定为(11~13)mass%的Zn-Ni合金镀层,证实了Zn-Ni合金的共沉积过程遵循异常共沉积机制。采用扫描电镜(SEM)、原子力显微镜(AFM)和X射线衍射仪(XRD)等对优化的Zn-Ni合金镀层进行了表征,发现镀层主要具有γ相(NiZn₃)结构,其表面平整、致密、光亮;腐蚀测试表明Zn-Ni合金镀层具有优良的耐蚀性能。     

Effects of pH and chloride concentration on pitting corrosion of AA6061 aluminum alloy

Effects of pH solution and chloride (Cl⁻) ion concentration on the corrosion behaviour of alloy AA6061 immersed in aqueous solutions of NaCl have been investigated using measurements of weight loss, potentiodynamic polarisation, linear polarisation, cyclic polarisation experiment combined with open circuit potential transient technique and optical or scanning electron microscopy.The corrosion behaviour of the AA6061 aluminum alloy was found to be dependant on the pH and chloride concentration [NaCl] of solution. In acidic or slightly neutral solutions, general and pitting corrosion occurred simultaneously. In contrast, exposure to alkaline solutions results in general corrosion. Experience revealed that the alloy AA6061 was susceptible to pitting corrosion in all chloride solution of concentration ranging between 0.003 wt% and 5.5 wt% NaCl and an increase in the chloride concentration slightly shifted both the pitting E_pit and corrosion E_cor potentials to more active values. In function of the conditions of treatment, the sheets of the alloy AA6061 undergo two types of localised corrosion process, leading to the formation of hemispherical and crystallographic pits.Polarisation resistance measurements in acidic (pH = 2) and alkaline chloride solutions (pH = 12) which are in good agreement with those of weight loss, show that the corrosion kinetic is minimised in slightly neutral solutions (pH = 6).     

Inhibition of mild steel corrosion in acidic medium using synthetic and naturally occurring polymers and synergistic halide additives

The corrosion inhibition of mild steel in H₂SO₄ in the presence of gum arabic (GA) (naturally occurring polymer) and polyethylene glycol (PEG) (synthetic polymer) was studied using weight loss, hydrogen evolution and thermometric methods at 30-60 °C. PEG was found to be a better inhibitor for mild steel corrosion in acidic medium than GA. The effect of addition of halides (KCl, KBr and KI) was also studied. Results obtained showed that inhibition efficiency (I%) increased with increase in GA and PEG concentration, addition of halides and with increase in temperature. Increase in inhibition efficiency (I%) and degree of surface coverage (θ) was found to follow the trend Cl⁻ < Br⁻ < I⁻ which indicates that the radii and electronegativity of the halide ions play a significant role in the adsorption process. GA and PEG alone and in combination with halides were found to obey Temkin adsorption isotherm. Phenomenon of chemical adsorption is proposed from the trend of inhibition efficiency with temperature and values obtained. The synergism parameter, S_I evaluated is found to be greater than unity indicating that the enhanced inhibition efficiency caused by the addition of halides is only due to synergism.     

Increasing the wear and corrosion resistance of magnesium alloy (AZ91D) with electrodeposition from eco-friendly copper- and trivalent chromium-plating baths

Mg alloy, AZ91D, which has a two-phase structure, was successfully electroplated in an alkaline Cu-plating bath. The Cu-coated Mg alloy specimen was further electroplated in eco-friendly acidic Cu and then trivalent Cr baths to obtain an anti-wear and anti-corrosion Cr/Cu coating. Experimental results show that the wear and corrosion resistance of the Mg alloy specimen was considerably improved by trivalent Cr electrodeposition. The hardness of the as-plated Cr deposit was drastically increased by using reduction-flame heating for 0.5 s. The above-mentioned results were measured via bonding strength, hardness, wear and corrosion tests. A superior wear and corrosion resistance was obtained when a Cu-coated Mg alloy specimen was electroplated with a trivalent Cr deposit, followed by heating with reduction-flame heating for 0.5 s.