Zn-Co Alloy Coatings Research Articles

Corrosion behaviour of Zn-Co alloy and multilayered metallic coatings: a comparative insight

ABSTRACT Zinc layers are widely used for the protection of various classes of steel in atmospheric conditions. An improvement in corrosion performance of these layers may qualify them to be used in harsh marine conditions. In this study, two practical approaches, including alloying Zn with Co at different current densities and electrolyte temperatures, as well as deposition of multilayered Zn-Co films with various numbers of interlayers, were applied to fabricate corrosion barrier coatings on St 37 steel. The surface and cross-sectional morphologies of the coatings were studied using scanning electron microscopy (SEM). Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation tests were employed to compare the corrosion resistance of the alloy and multilayered coatings. Results illustrated that the best protection performance is obtained for multilayered coatings with top Co layer showing promise to be used in marine applications.

Colourful thin passive films on a Zn-Co alloy formed by anodic oxidation

Passive layers were formed on electrolytic Zn-Co alloy coatings in an ethanolic solution with sodium hydroxide by anodic oxidation. The obtained thin films were characterized by their morphology (OM, SEM), chemical analysis (XPS), thickness and deep-profile chemical analysis (GDOES) and corrosion properties (NSS test, EIS). Because of the appearance of interference colours with all of the samples, they were also characterized by UV-Vis absorption spectroscopy. The colours varied and were related to the thicknesses of the obtained thin films, which ranged from 91 to 220 nm. The passive films grew linearly with the terminal voltage. Based on the XPS results, mixed (oxide-alkoxide) compositions were found to have a beneficial influence towards providing improved corrosion resistance. According to the conducted corrosion studies, the best protection was ensured when the coatings were anodized at terminal voltages between 30 and 50 V.

Corrosion performance of Zinc Based Binary and Ternary Alloy Coatings

Abstract The binary (Zn-Ni, Zn-Co, Zn-Fe) and ternary (Zn-Ni-Co, Zn-Co-Fe) coatings have been developed with sulphamic acid (SA) as an additive using an electrolytic method of sulfate bath. The bath composition, pH, temperature, etc have been optimized using regular Hull cell technique. The effect of current density (C.D) on wt. % of metal, thickness (t), hardness (VHN), and cathode current efficiency (C.C.E) of the plating have been investigated. The potentiodynamic polarization technique has been used to determine corrosion resistances of binary and ternary alloy coatings. The corrosion rate (CR) of ternary Zn-Ni-Co alloy coating is about 18 times and 20 times higher as compared to binary Zn-Ni and Zn-Co alloy coatings and ternary (Zn-Co-Fe) alloy coating is also approximately 27 times and 28 times higher than Zn-Co and Zn-Fe coating at optimized conditions. Thus, ternary coatings can replace conventional binary alloy coatings for many technological applications including aerospace, automobiles etc.

The advantage of ultrasound during electrodeposition on morphology and corrosion stability of Zn-Co alloy coatings

ABSTRACTThe influence of ultrasound agitation on the properties of electrodeposited Zn-Co coatings was analysed and compared with coatings deposited by using magnetic stirring. The morphology and chemical composition of the deposits were analysed by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy. X-ray diffraction was used in examining the crystallographic orientation and phase composition of the coatings. It was found that utilising ultrasound during deposition enhanced the compactness, homogeneity and corrosion resistance of the coatings compared to deposition with magnetic stirring of the plating solution. Magnetically stirred coatings were dendritic and porous, whereas coatings produced under ultrasound agitation were more homogeneous and dense. The higher the ultrasound intensity applied the higher the cobalt content in the coating. The corrosion resistance of Zn-Co coatings was improved by ultrasound agitation, with the best results obtained at 20 W cm−2 of ultrasound intensity.

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

ABSTRACTZn-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.

Corrosion Investigations of Black Chromite Films on Zn and Zn-Co Coatings with Low Cobalt Content

The corrosion resistance and protective ability of black-colored chromite (Cr3+ based) and chromium-free conversion films (CFs) on electrodeposited zinc and Zn-Co alloy coatings having low cobalt content in a neutral model medium of 3% NaCl solution have been investigated and characterized. Test methods such as polarization resistance measurements, scanning electron microscopy, EDS, x-ray diffraction analysis, and x-ray photoelectron spectroscopy analyses have been applied in order to scrutinize the actual protective characteristics of these films as well as to determine the composition of the corrosion products appearing as a result of the treatment in the corrosion medium. The experimental results revealed better protective characteristics, consequently superior performance of the electrodeposits with chromite films compared to these with chromium free or without any additional CF. The processes occurring on the sample’s surface during the immersion in the chemical conversion solutions as well as the influence of the H3PO4 in the course of the treatment are also commented on and discussed.

Zinc-Cobalt Alloy Deposited on the Titanium Surface as Electrocatalysts for Borohydride Oxidation

The present work is focused on the investigation of the oxidation of BH4− ions in an alkaline solution on a low-cost ZnCo alloy deposited on the Ti surface. The Co and ZnCo alloy coatings were deposited on the Ti surface via electrodeposition. The morphology, structure and composition of the prepared catalysts were examined by FESEM and EDX. The activity of the fabricated catalysts was investigated toward the oxidation of BH4− ions by means of cyclic voltammetry and chronoamperometry. It has been determined that Co/Ti and ZnCo/Ti catalyze the oxidation of H2 generated by the catalytic hydrolysis of BH4− and direct oxidation of BH4− ions. The H2 oxidation current densities measured on both the catalysts are rather close and reach up to 60 mA cm−2, whereas the borohydride oxidation current densities are ∼3–4 times higher at −0.2 V on ZnCo/Ti than those obtained on Co/Ti and Zn/Ti.

Zinc-Cobalt Alloy Deposited on the Titanium Surface as Electrocatalysts for Borohydride Oxidation

The present work is focused on the investigation of the electrochemical oxidation of BH4 – ions in an alkaline solution on a low-cost ZnCo alloy deposited on the titanium surface. The Co and ZnCo alloy coatings were deposited on the titanium surface via electrodeposition. The morphology, structure and composition of the prepared catalysts were examined by Field Emission Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy. The activity of the fabricated catalysts was investigated towards the oxidation of BH4 – ions by means of cyclic voltammetry. It has been determined that Co/Ti and ZnCo/Ti catalyze the oxidation of H2 generated by the catalytic hydrolysis of BH4 – and direct oxidation of BH4 – ions. The H2 oxidation current densities measured on both the catalysts are rather close and reach up to 60 mA cm–2, whereas, the borohydride oxidation current densities are ~3 times higher at -0.2 V on the ZnCo/Ti catalyst than those obtained on the Co/Ti catalyst.

Gold-Zinc-Cobalt Deposited on Titanium as Electrocatalysts for Borohydride Oxidation

This work presents the investigation of the electrochemical oxidation of borohydride in alkaline solutions on the ZnCo alloy deposited on the titanium surface and modified with gold nanoparticles (denoted as AuZnCo/Ti). The ZnCo alloy coatings with the thickness of ~5 mm were deposited on the titanium surface (1 x1 cm) via electrodeposition. Au crystallites were deposited on the ZnCo/Ti surface via the galvanic displacement technique. The morphology, structure and composition of the prepared catalysts were examined by Field Emission Scanning Electron Microscopy, Energy Dispersive X-ray Analysis and Inductively Coupled Plasma Optical Emission Spectroscopy. Electrocatalytic activity of the prepared catalysts was investigated towards the oxidation of borohydride by means of cyclic voltammetry, chronoamperometry and chronopotentiometry. It was found that the AuZnCo/Ti catalysts with the Au loadings of 31, 63 and 306 µg cm-2 show enhanced catalytic activity towards electro-oxidation of BH4 – ions as compared to that of ZnCo/Ti, Zn/Ti or Co/Ti catalysts.

Gold-Zinc-Cobalt Deposited on Titanium as Electrocatalysts for Borohydride Oxidation

Investigation of the electrochemical oxidation of borohydride in alkaline solutions on the ZnCo alloy deposited on the titanium surface and modified with gold nanoparticles (denoted as AuZnCo/Ti) is presented. The ZnCo alloy coatings with the thickness of ~5 µm were deposited on the titanium surface (1 x1 cm) via electrodeposition from the electrolyte described in Ref. [1]. Au crystallites were deposited on the ZnCo/Ti surface via the galvanic displacement technique. The ZnCo/Ti catalysts were immersed into the 1 mM HAuCl4 + 0.1 M HCl solution at 25 oC for 0.5, 1 and 5 min, respectively [2].The morphology, structure and composition of the prepared catalysts were examined by Field Emission Scanning Electron Microscopy, Energy Dispersive X-ray Analysis and Inductively Coupled Plasma Optical Emission Spectroscopy. Electrocatalytic activity of the prepared catalysts was investigated towards the oxidation of borohydride by means of cyclic voltammetry, chronoamperometry and chronopotentiometry.It has been determined that the AuZnCo/Ti catalysts with Au loadings of 31, 63 and 306 µg cm-2 show enhanced catalytic activity towards electro-oxidation of BH4 – ions as compared to that of ZnCo/Ti or Co/Ti catalysts. The chronoamperometry and chronopotentiometry studies also confirmed a high electrocatalytic activity of the AuZnCo/Ti catalysts for the direct electro-oxidation of BH4 – ions as compared to that of ZnCo/Ti and Co/Ti catalysts.

Electrodeposition of Cobalt Rich Zn-Co alloy Coatings from Citrate Bath

Zn-Co alloy coatings were produced on carbon steel, at room temperature, from citrate baths (0.100 mol L-1) containing [Zn2+]= [Co2+] = 0.05 mol L-1. Coatings with %m/m Co > %m/m Zn were observed under all deposition conditions, except for I = 10 A m-2. As the current density (I) was increased, a significant decrease (p<0.05) in %m/m Zn was noted, and the highest value of % m/m Co was observed at 80 A m-2. The Co-rich coatings presented refined microstructure, with small grain sizes. The smallest Icorr of the studied substrate/coatings systems, 2.4 µA cm-2, was obtained at I = 10 A m-2. Under these conditions, the coatings contained ~ 30 % m/m Co and 70 % m/m Zn and only the γ-ZnCo phase was obtained. The Icorr found in the present work is smaller than those usually found in the literature for Zn-Co coatings with small Co content.

Zinc-Cobalt Alloy Deposited on the Titanium Surface As Electrocatalysts for Borohydride Oxidation

The present work is focused on the investigation of the electrochemical oxidation of borohydride in alkaline solution on low-cost ZnCo alloy deposited on the titanium surface (denoted as ZnCo/Ti). The ZnCo alloy coatings with the thickness of ~5 µm were deposited on the titanium surface (1 x1 cm) via electrodeposition from the electrolyte described in Ref. [1]. Prior to ZnCo alloy deposition, the titanium plates were degreased with acetone and then pretreated in diluted H2SO4 (1:1 vol) at 90 oC for 10 s. Cobalt coatings with the thickness of ~3 µm were deposited from the same electrolyte [1] with only difference that zinc ions were excluded from the plating bath.The morphology, structure and composition of the prepared catalysts were examined by FESEM, EDS and ICP-OES. Activity of the fabricated catalysts was investigated towards the oxidation of borohydride by means of cyclic voltammetry and chronoamperometry.It was found that the ZnCo alloy coating, electrodeposited on the titanium surface, consists of ~81.3 at. % of Zn and ~16.1 at. % of Co.It has been determined that the Co/Ti and ZnCo/Ti catalysts catalyzes oxidation of H2 generated by catalytic hydrolysis of BH4 – and direct BH4 – ions oxidation. Notably, the H2 oxidation current densities measured on both catalysts are similar and reach up to 60 mA cm–2, whereas, the borohydride oxidation current densities are ~3-6 times higher on the ZnCo/Ti catalysts than those obtained on the Co/Ti catalyst. The chronoamperometry and chronopotentiometry studies also confirmed a high electrocatalytic activity of the ZnCo/Ti catalyst for the direct electro-oxidation of BH4 – ions as compared to that of Co/Ti electrode.The ZnCo/Ti catalysts are promising low-cost anode material for direct borohydride fuel cells.

The study of Zn–Co alloy coatings electrochemically deposited by pulse current

The electrochemical deposition by pulse current of Zn-Co alloy coatings on steel was examined, with the aim to find out whether pulse plating could produce alloys that could offer a better corrosion protection. The influence of on-time and the average current density on the cathodic current efficiency, coating morphology, surface roughness and corrosion stability in 3% NaCl was examined. At the same Ton/Toff ratio the current efficiency was insignificantly smaller for deposition at higher average current density. It was shown that, depending on the on-time, pulse plating could produce more homogenous alloy coatings with finer morphology, as compared to deposits obtained by direct current. The surface roughness was the greatest for Zn-Co alloy coatings deposited with direct current, as compared with alloy coatings deposited with pulse current, for both examined average current densities. It was also shown that Zn-Co alloy coatings deposited by pulse current could increase the corrosion stability of Zn-Co alloy coatings on steel. Namely, alloy coatings deposited with pulse current showed higher corrosion stability, as compared with alloy coatings deposited with direct current, for almost all examined cathodic times, Ton. Alloy coatings deposited at higher average current density showed greater corrosion stability as compared with coatings deposited by pulse current at smaller average current density. It was shown that deposits obtained with pulse current and cathodic time of 10 ms had the poorest corrosion stability, for both investigated average deposition current density. Among all investigated alloy coatings the highest corrosion stability was obtained for Zn-Co alloy coatings deposited with pulsed current at higher average current density (jav = 4 A dm-2).

Corrosion resistant Zn-Co alloy coatings deposited using saw-tooth current pulse

Micro/nanostructured multilayer coatings of Zn-Co alloy were developed periodically on mild steel from acid chloride bath. Composition modulated multilayer alloy (CMMA) coatings, having gradual change in composition (in each layer) were developed galvanostatically using saw-tooth pulses through single bath technique (SBT). CMMA coatings were developed under different conditions of cyclic cathode current densities (CCCDs) and number of layers, and their corrosion resistances were evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) method. Optimal configuration, represented as (Zn-Co)2·0/4.0/300 was found to exhibit ∼ 89 times better corrosion resistance compared to monolithic (Zn-Co)3·0 alloy deposited for same time, from same bath. The better corrosion resistance of CMMA coatings was attributed to changed interfacial dielectric properties, evidenced by dielectric spectroscopy. Improved corrosion resistance was attributed to formation of n-type semiconductor film at the interface, supported by the Mott-Schottky plot. Further, the formation of multilayer and corrosion mechanism was analysed using scanning electron microscopy (SEM).

Effect of temperature and pH of ammonium galvanic bath on the properties of Zn-Co alloy coatings

The results of investigations on Zn-Co coatings obtained electrochemically in acidic and neutral ammonium baths are presented. The chemical composition, morphology and surface roughness of received coatings were determined together with the current efficiency. It was found that the coatings morphology depends on the process parameters. In the case of neutral bath the increase of bath temperature resulted in decrease of cobalt contents in the coatings, whereas it was not practically observed for acidic baths. It was also observed that surface roughness of the Zn-Co coatings decreased with the increase of temperature for both types of baths. It was also found that the grain size of coatings obtained in neutral bath depends on pH of bath.

Microstructure and corrosion behavior of Zn-Co alloys deposited from three different plating baths

The effects of plating baths of different composition on the microstructure and corrosion stability of Zn-Co alloy coatings were studied. Zn-Co alloys with the same Co content were deposited from chloride plating baths containing different amounts of Co2+ ions, as well as from a sulphate-chloride plating bath. The surface morphology and crystallite size were investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The corrosion stability of the Zn- Co alloys was determined by following the change of the open circuit potential with time of immersion in a 3 % NaCl solution and by polarization measurements. The results showed a significant influence of the plating bath on the morphology and corrosion stability of the Zn-Co alloys. The surface of the alloy coatings deposited from the chloride baths were uniform and homogenous, whereas the deposit obtained from the sulphate-chloride bath was quite inhomogeneous. The corrosion stability of the homogenous Zn-Co deposits obtained by deposition from both chloride baths was higher than that of the deposit obtained from the sulphate-chloride bath. An increase in the Co content in the chlorideplating bath resulted in a reduction of the alloy crystallite size and it was shown that the alloy with the smaller crystallites of the two alloy deposits, although having the same chemical content, exhibited a lower corrosion rate.

Composition and morphology of Zn-Co alloy coatings deposited by means of pulse plating containing reverse current

The influence of bath constituents and pulse parameters on cobalt content, surface morphologies and grain size of Zn-Co alloy deposits was studied using a pulse plating technique with a squarewave current containing reverse pulse. It is found that Zn-Co alloy coatings obtained from the bath with the cobalt ion concentration over 60 wt% have a higher cobalt content in deposits. The results of pulse plating show that the average current density and reverse anodic current density amongst the variables investigated have very strong effects on the cobalt content in the Zn-Co alloy deposits. It is possible to electrodeposit Zn-Co alloy coatings with 10–90 wt% cobalt by modulating pulse parameters. The grain size, surface appearance and internal stress in the deposit were improved significantly by introducing the reverse current.

Electrochemical corrosion behaviour and surface morphology of electrodeposited zinc, zinc–cobalt and their composite coatings

The electrochemical behaviour as well as the surface morphology characteristics of electrodeposited Zn and Zn-Co (≈3 wt-%) alloy coatings on low carbon steel samples, with and without included nanoparticles of organic co-polymers in their structure, are discussed. Their corrosion resistance and protective properties were studied in a model corrosion medium, 5 wt-% NaCl, using electrochemical methods such as polarisation resistance (Rp) measurements and potentiodynamic (PD) polarisation curves. Scanning electron microscopy (SEM) was performed to examine the surface morphology changes of the samples before and after the corrosion treatment. Some conclusions are drawn about the influence and importance of the organic nanoparticles on the corrosion behaviour of the coatings presented.

Microstructure, Morphology and Corrosion Resistance of Electrodeposited Zinc-Cobalt Compositionally Modulated Alloy Multilayer Coatings

SUMMARYThe morphologies and corrosion resistance of electrodeposited zinc-cobalt compositionally modulated alloy multilayer coatings produced from a sulphate bath have been investigated and reported in this paper. Various current density waveforms were designed using a computer-aided pulse plater unit to deposit zinc-cobalt alloy layers of alternate compositions from 10% cobalt to 70% cobalt. Different numbers of layers with different thicknesses were deposited by designing a variety of current density waveforms. It was shown that designing waveforms with steps to produce a gradual increase or decrease in current densities can prevent crack formation and improves the quality of Zn/Co CMAM coatings. Neutral salt spray corrosion tests showed that Zn/Co CMAM coatings have better corrosion resistance than single layer Zn-Co alloy coatings. Effects of bath temperature and agitation on the corrosion resistance of these CMAM coatings were also investigated. It was found that the best corrosion protection could be achieved for a two layered Co/Zn CMAM coating deposited at 25–30 °C from a bath with a high rate of agitation.

Development of a Bath for Electrodeposition of Zinc-Cobalt Compositionally Modulated Alloy Multilayered Coatings

The production of compositionally modulated alloy multilayers (CMAM) of Zn-Co alloys from an acid sulfate bath has been extensively studied and the electrodeposition parameters and conditions optimized to yield layers of high and low cobalt contents. It is shown that a change of deposition mechanism (from anomalous to equilibrium codeposition) occurs and is associated with the formation of ions in solution and not to the formation of an electrode film of as traditionally believed. Spectroscopic studies (UV-visible) showed that ions are formed on the anode surface due to oxidation of ions in the bath, and these trivalent ions prevent high-cobalt-containing Zn-Co alloy coatings. Using a two-compartment cell was found to be very effective in preventing the formation of trivalent cobalt ions in the catholyte portion of the bath and thus changing the codeposition mechanism from anomalous to equilibrium and consequently promoting high levels of cobalt in the deposits. This technique also increased the cathode current efficiency to 98%. By using a computer-aided pulse plating unit and designing different waveforms, various types of Zn-Co CMAM coatings of different thickness and number of layers were electrodeposited. © 2003 The Electrochemical Society. All rights reserved.