Corrosion stability of electrodeposited cyclic multilayer Zn–Ni alloy coatings

Abstract

This paper reports on a study of electrodeposition and characterisation of cyclic multilayer coatings of Zn–Ni alloy from a sulphate bath. Cyclic multilayer alloy coatings were deposited on mild steel through the single bath technique by appropriate manipulation of cathode current densities. The thickness and composition of the individual layers of the CMA deposits were altered precisely and conveniently by cyclic modulation of the cathode current during electrodeposition. Multilayer deposits with sharp change in composition were developed using square current pulses, using thiamine hydrochloride and citric acid as additives. Laminar deposits with different configurations were produced and their corrosion behaviours were studied by AC and DC methods in 5%NaCl solution. It was observed that the corrosion resistance of the CMA coating increased progressively with the number of layers (up to certain optimal numbers) and then decreased. The decrease in corrosion resistance at high degree of layering was attributed to interlayer diffusion due to less relaxation time for redistribution of metal ions at cathode during deposition. The coating configurations have been optimised for peak performance of the coatings against corrosion. It was found that CMA coating developed at cyclic cathode current densities of 3·0/5·0 A dm^?2 with 300 layers showed the lowest corrosion rate (0·112×10^?2 mm/year) which is ～54 times better than that of monolithic Zn–Ni alloy, deposited from the same bath. The protection efficacy of CMA coatings is attributed to the difference in phase structure of the alloys in successive layers, deposited at different current densities, evidenced by X-ray diffraction analysis. The formation of multilayers and corrosion mechanism were examined by scanning electron microscopy.     

Evaluation of corrosion resistance of electrodeposited nanocrystalline Ni–Fe alloy coatings

Nickel–iron alloys with a compositional range of 24–80?wt-% iron were electrodeposited on a copper substrate from a sulphate-based bath and using a stirring rate of 100?rev?min^?1. The effect of applied current density and Ni²⁺/Fe²⁺ metal ion ratio of plating bath on the properties of alloy coatings was examined. Crystal structure and grain size of Ni–Fe alloy coatings were investigated using X-ray diffraction technique. Field emission scanning electron microscopy and energy dispersive X-ray spectroscopy were used to analyse the surface morphology and chemical composition of coatings. Microhardness test was applied to evaluate the hardness of the coatings. Finally, the electrochemical behaviour of the Ni–Fe alloy coatings was studied by a polarisation test in 10?wt-% H₂SO₄ solution. Results revealed that current density and plating bath composition had a strong effect on the characteristics of coatings. As the iron content of alloys produced increased, their corrosion resistance improved with the best corrosion resistivity being achieved at a metal ion ratio of 0.5 and applied current density of 2.5?A?dm^?2.     

Nanosized structural features of electrodeposited Au–Co and Au–Ni alloy coatings

The structure of Au–Co and Au–Ni alloy coatings deposited at low current density (2–20?mA?cm^?2) from weakly acidic additive-free electrolyte with higher (16–20?g?L^?1 Co or Ni) than usually employed (0.1–1.0?g?L^?1) concentration of the alloying element was investigated. Under these conditions, structural effects in the coatings were observed representing nanoscale, porous (in the case of Au–Co coatings) or hollow (in Au–Ni alloy coatings) formations, passing through the coating and ending at the surface as craters. They could be associated with the influence of the accompanying hydrogen evolution. On one hand, hydrogen bubbles are firmly adsorbed on the surface, and on the other hand, the electrolyte has very good penetrating and covering ability. As a result, depending on the dynamics of the deposition process, porous structures with different configurations are formed. The formation of the structures begins in the early stages of the electrocrystallisation and the substrate affects the number, size and distribution of the features.     

Microstructure and tribological properties of carburized 95W–3.5Ni–1.0Fe–0.5Co heavy alloy

Tungsten heavy alloys(WHAs) produced by powder technology are widely used for the mechanical manufacturing, electronic and defense components, etc.Tribological properties of these alloys need to be improved to meet the severe service conditions demanded. Carburization is a promising way to resolve this problem. In this work, microstructure and tribological properties of the carburized 95W–3.5Ni–1.0Fe–0.5Co heavy alloy were investigated in comparison with those of the untreated alloy. Results show that the carburized layer consists of a porous, outer WC layer and a modified W grain layer surrounded by Fe_6W_6C and Co_6W_6C at 970℃, regardless of the carburizing time. The depth of the carburized layer linearly increases in a relatively short time and slightly increases during the subsequent period. Surface roughness increases with carburizing time. Carburization can stabilize friction coefficient and effectively improve the wear resistance of the tungsten heavy alloy due to its significantly increased hardness and non-deformability, but the porous structure in the WC layer has a negative influence on its wear resistance. The carburized layer is damaged in the porous WC layer in the form of the spalling of WC particles where there are some microcracks and micropores, accompanied with peeling due to the solid tribofilm being pushed away.     

Tensile deformation and microstructure of a nanocrystalline Ni–W alloy produced by electrodeposition

Deformation and fracture characteristics of the electrodeposited nanocrystalline Ni–W alloy with a grain size of 8.1 nm were investigated. Tensile tests were carried out at room temperature with specimen of 25–30 μm in thickness. The fractured surface was examined using SEM and high-resolution TEM was used to study the microstructure of deformed specimens. Based on these observations we propose a deformation mechanism and fracture process for nanocrystalline Ni–W during tensile deformation are initiated by grain boundary sliding.     

Increased adhesion of diamond-like carbon–Si coatings and its tribological properties

A pre-activation process on substrate surface has remarkably improved the poor adhesion strength of diamond-like carbon (DLC)–Si coatings on steels which is the largest obstacle in achieving a widespread application of the coatings onto machine components. The activation process consists of preliminary nitriding followed by ion etching under the selected condition. Very fine protrusions formed by the processes provide large adhesion strength to the coatings that were made continuously within the same DC-plasma-assisted chemical vapor deposition (PACVD). No intermediate layers are necessary. The critical load of DLC–Si coatings thus treated reached over 50 N in the scratch tests. The coatings with the critical load over 50 N showed much improved rolling fatigue life. The DLC–Si coating with over 50 N critical load endured a rotationa1 stress of 10⁸ cycles at a contact pressure of 3.4 GPa, whereas the DLC–Si coatings with 10 N spalled at 10⁶ cycles.     

Instrumented indentation properties of electrodeposited Ni–W alloys with different microstructures

The microstructures and mechanical properties electrodeposited Ni–W alloys synthesized at two plating bath temperatures of 353 and 348 K were investigated. Whereas the 353 K sample is amorphous, the 348 K sample has a mixed amorphous-nanocrystalline structure. As a result, the strength of the 348 K sample exhibits strong strain rate dependence during nanoindentation.     

Preparation and oxidation behaviour of electrodeposited Ni–CeO2 nanocomposite coatings

Ni–CeO₂ nanocomposite coatings with different CeO₂ contents were prepared by codeposition of Ni and CeO₂ nanoparticles with an average particle size of 7 nm onto pure Ni surfaces from a nickel sulfate. The CeO₂ nanoparticles were dispersed in the electrodeposited nanocrystalline Ni grains (with a size range of 10–30 nm). The isothermal oxidation behaviours of Ni–CeO₂ nanocomposite coatings with two different CeO₂ particles contents and the electrodeposited pure Ni coating were comparatively investigated in order to elucidate the effect of CeO₂ at different temperatures and also CeO₂ contents on the oxidation behaviour of Ni–CeO₂ nanocomposite coatings. The results show that the as-codeposited Ni–CeO₂ nanocomposite coatings have a superior oxidation resistance compared with the electrodeposited pure Ni coating at 800 °C due to the codeposited CeO₂ nanoparticles blocking the outward diffusion of nickel along the grain boundaries. However, the effects of CeO₂ particles on the oxidation resistance significantly decrease at 1050 °C and 1150 °C due to the outward-volume diffusion of nickel controlling the oxidation growth mechanism, and the content of CeO₂ has little influence on the oxidation.     

Microstructure evolution and mechanical properties of spark plasma sintered W–Ni–Mn alloy

Tungsten heavy alloys (90W–6Ni–4Mn) were prepared through spark plasma sintering (SPS) using micron-sized W, Ni, and Mn powders without ball milling as raw materials. The effects of sintering temperature on the microstructure and mechanical properties of the 90W–6Ni–4Mn alloys were investigated. SPS technology was used to prepare 90W–6Ni–4Mn alloys with relatively high density and excellent comprehensive performance at 1150–1250 °C for 3 min. The 90W–6Ni–4Mn alloys consisted of the W phase and the γ-(Ni, Mn, and W) binding phase, and the aγerage grain size was less than 10 µm. The Rockwell hardness and bending strength of alloys first increased and then decreased with increasing sintering temperature. The best comprehensiγe performance was obtained at 1200 °C, its hardness and bending strength were HRA 68.7 and 1162.72 MPa, respectiγely.     

Preparation and properties of W–Cu–Zn alloy with low W–W contiguity

The W–Cu–Zn alloy with a-brass matrix and low W–W contiguity was prepared by method of electroless copper plating combined with spark plasma sintering(SPS) method.The effects of process and parameters on the microstructure and mechanical properties of the alloy were investigated.The W–Cu–Zn alloy with a relative density of 96 % and a W–W contiguity of about 10 % was prepared by original fine tungsten particles combined with wet mixing method and SPS solid-state sintering method at 800 °C for 10 min.The microstructure analysis shows that Cu–Zn matrix consists of nano-sized a-brass grains,and the main composition is Cu_3Zn electride.The nano-sized Cu was coated on the surface of tungsten particles by electroless copper plating method,and the fairly low consolidation temperature and short solid-state sintering time result in the nano-sized matrix phase.The dynamic compressive strength of the W–Cu–Zn alloy achieves to1000 MPa,but the alloy shows poor ductility due to the formation of the hard and brittle Cu_3Zn electrides.The fine-grain strengthening and the solution strengthening of the Cu–Zn matrix phase are responsible for the high Vickers microhardness of about 300 MPa for W–Cu–Zn alloy.     

Comparison of tribological properties of industrial low friction coatings

MX2(M= Mo, W; X=S, Se) and DLC (a-C: H and WC/C) are the two kinds of typical low friction coatings widely used in industry. The friction and wear properties of these two kinds of coatings marked as MOVIC,MOST, MoSez/Ni, WSez, a-C: H and WC/C coatings were determined by fretting tests in ambient air of different humidity. The results show that the coefficient of friction of MXz coatings increases when the relative humidity of air increases whereas the coefficient of friction DLC coatings decreases with the increasing of relative humidity. MOVIC and WSe2 coatings have a poor friction and wear resistance because of non-basal planes (100) and ( 101 ) parallel to the surface in the MOVIC coating, or the rough and porous surface of WSe2 coatings. Among these six coatings, MoSe2/Ni and WC/C eoatinas have the highest wear resistance which seems to be unaffected by the relative humidity.     

A comparative study on the electrochemical behaviour of amorphous and nanocrystalline cobalt–tungsten electrodeposited coatings

Amorphous and nanocrystalline cobalt–tungsten coatings were electrodeposited from a citrate-ammonia bath on copper substrates. Both coatings showed a nodular surface morphology, but a microcrack network was observed in the amorphous coating. The cyclic voltammograms of both deposits revealed anodic and cathodic low-current plateaus around the open circuit potential, exhibiting a passive behaviour. Mott–Schottky analysis showed that the passive films exhibit n-type semiconductivity behaviour and that formed on the amorphous coating showed higher crystal defects. Electrochemical impedance spectroscopy revealed that the amorphous coating has higher corrosion resistance than the nanocrystalline one at both open circuit and anodic potentials. This was attributed to the higher pore resistance of passive film formed at the open circuit potential and more chemical stability of the amorphous coating which reduces its dissolution at the anodic potential. The plugging of the microcrack network in the amorphous coating by corrosion products eliminated the negative effect of microcracks.     

Formation and study of the properties of finishing coatings of a circuit board with tin–zinc alloy instead of tin–lead alloy coatings

A formation technique of electroplating coatings of tin–zinc alloy (50) based on using low-toxicity lactic acid as surfactant, ligand, and buffer addition was developed. The dependences of alloy composition and cathodic yield on the alloy current and the coating quality on the concentration of tin and zinc ions in the solution, the content of lactic acid, the solution acidity (pH), the temperature, and the current cathodic density were investigated. The optimum conditions of the process carrying out were determined. The performance characteristics of obtained coatings were investigated. It was proven that the coating with this alloy meets the requirements of GOST (State Standard) 23752–79 “Circuit Boards. General Technical Specifications” and can be used for the circuit boards of products of the instrument-making industry instead of tin–lead alloy, which will make it possible to reduce the environmental risk of production, as well as increase the efficiency and maintainability of devices and systems.     

The electrodeposition of nanocrystalline Cobalt–Nickel–Phosphorus alloy coatings: a review

The importance of Co–Ni–P alloy deposits is summarised and recent developments are highlighted. Electroplating and electroless deposition of nanocrystalline Co–Ni–P ternary coatings are considered. Nanostructure, physical and mechanical properties (including corrosion resistance) of various bath types and compositions (including pH and electrolyte additives) as well as plating conditions (including current density, temperature and agitation) are summarised. Applications range from wear and corrosion resistant coatings, particularly as a hard chromium replacement to speciality hydrogen evolution electrodes in water electrolysis. Following this concise review, future research needs are briefly listed.     

Electrodeposition and corrosion behaviour of nanocrystalline Fe–P coatings

Electrodeposition of alloy coatings is a proven route to corrosion protection of metal substrates but has met some restrictions due to changed European regulations. Iron-phosphorus codeposits appear to be eco-friendly candidates for substitution of hard chromium, and alloys such as Ni–P, Co–Ni etc. The main task in formulating proper electrolytes for Fe–P deposition is the stabilisation of the Fe²⁺ ion to hinder the oxidation to Fe³⁺ by air. In this work glycine was selected as complexing agent for Fe²⁺. Fe–P layers were deposited from a glycine-containing electrolyte at various glycine concentrations, current densities and temperatures. The properties of the layers vary greatly with a change of each applied parameter. Interaction of complex equilibria and pH change due to cathodic hydrogen evolution can lead to electrochemical oscillations which result in multilayer structures of the deposits. The corrosion resistance of Fe–P layers obtained in this work is comparable to industrially used Ni–P coatings.     

Corrosion performance and tribological behavior of diamond-like carbon based coating applied on Ni−Al−bronze alloy

The effect of diamond-like carbon (DLC) coating (fabricated by cathodic arc deposition) on mechanical properties, tribological behavior and corrosion performance of the Ni?Al?bronze (NAB) alloy was investigated. Nano-hardness and pin-on-plate test showed that DLC coating had a greater hardness compared with NAB alloy. Besides, the decrease in friction coefficient from 0.2 for NAB substrate to 0.13 for the DLC-coated sample was observed. Potentiodynamic polarization and EIS results showed that the corrosion current density decreased from 2.5 μA/cm² for bare NAB alloy to 0.14 μA/cm² for DLC-coated sample in 3.5 wt.% NaCl solution. Moreover, the charge transfer resistance at the substrate–electrolyte interface increased from 3.3 kΩ·cm² for NAB alloy to 120.8 kΩ·cm² for DLC-coated alloy, which indicated an increase in corrosion resistance due to the DLC coating.     

Microstructure and superelastic properties of free forged Ti–Ni shape-memory alloy

Elemental titanium (Ti) and nickel (Ni) powders were consolidated by spark plasma sintering (SPS) to fabricate Ti–51%Ni (mole fraction) shape-memory alloys (SMAs). The objective of this study is to enhance the superelasticity of SPS produced Ti–Ni alloy using free forging as a secondary process. Products from two processes (with and without free forging) were compared in terms of microstructure, transformation temperature and superelasticity. The results showed that, free forging effectively improved the tensile and shape-memory properties. Ductility increased from 6.8% to 9.2% after forging. The maximum strain during superelasticity increased from 5% to 7.5% and the strain recovery rate increased from 72% to 92%. The microstructure of produced Ti–51%Ni SMA consists of the cubic austenite (B2) matrix, monoclinic martensite (B19′), secondary phases (Ti₃Ni₄, Ti₂Ni and TiNi₃) and oxides (Ti₄Ni₂O and Ti₃O₅). There was a shift towards higher temperatures in the martensitic transformation of free forged specimen (aged at 500 °C) due to the decrease in Ni content of B2 matrix. This is related to the presence of Ti₃Ni₄ precipitates, which were observed using transmission electron microscope (TEM). In conclusion, free forging could improve superelasticity and mechanical properties of Ti–51%Ni SMA.     

Preparation and properties of borate glass coatings on Ti-based alloy substrates

A series of BaO-La2O3-B2O3 （BLB） glass coats on the Ti-based alloy substrates were developed at different temperatures for different times. The BLB glasses were analyzed by differential thermal analysis （DTA） and thermal mechanical analysis （TMA） to determine the crystallization temperature and coefficients of thermal expansion （CETs） of the glass. The tensile strength and microstructure of the glass coats were analyzed and the effects of the coating condition on the tensile strength and microstructure were discussed. The results show that the CETs of the borate glass at different temperatures match with those of Ti-based alloy, and the difference between the borate glass and Ti-based alloy at each temperature is below 5%. The spreading area in N2 atmosphere is much larger than that in air atmosphere, indicating that N2 atmosphere is helpful for the wetting of borate glass to Ti-based alloy. The tensile strength of the glass coats can reach as high as 28.42 MPa, meeting the requirements for the coat binder. With the increase of coating time, the tensile strength of coats increases firstly while then decreases. The coat prepared at 730 ℃ for 30 min is fairly smooth and complete, while the other coats contain lots of defects such as large or small uncoated region. It is believed that the coating temperature of 730℃ and coating time of 30 min are the proper coating conditions to prepare BLB glass coats.