The aim of this research work was to codeposit nano-SiO2 particles into Zn-Ni alloy coatings in order to improve some surface properties. It had been investigated the effect of loading the plating bath with nanoparticles on composition, morphology, phase structure of deposits, and their subsequent influence on the corrosion process in corrosive solution of 3% NaCl and the thermal stability of deposits at
The need for coatings with improved resistance to highly aggressive environments is high as a result of a growing demand for extended safe service life of industrial objects. Composite electrodeposition is a valuable new surface intensification technology to obtain composite coatings allowing to codeposit inorganic and organic particles into the coatings in order to improve the surface properties. Several studies showed that electrochemically embedded particles impart special properties to composite coatings produced, which can meet the industrial request such as wear resistance, self-lubrication, and corrosion resistance [
Zn-Ni-SiO2 coating has recently been used to replace cadmium plating which is widely used in the aerospace industries (i.e., landing gear, hydraulic actuators, high-performance fasteners, break, and fuel lines) because of its excellent corrosion resistance. The relation between our work and existing works on Zn-Ni-SiO2 composite coating is to replace the coating with Zn-Ni-SiO2 alloy as this coating shows great promise from the corrosion resistance point of view, representing a real challenge. Zn-Ni-SiO2 coating system may also find a wide range of advanced applications in marine industry, so we found it interesting to study this type of composite coating in 3% NaCl.
The particles of SiO2 are hydrophilic and hence incorporate hardly into the surface of cathode. The main purpose of this research work was to codeposit nano-SiO2 particles into the Zn-Ni alloy coatings under direct current in order to improve the surface properties and the corrosion resistance in aggressive media.
Then, the composite coatings had been characterized from compositional (EDX), morphological (SEM), structural (XRD), and depth profile (GDOES) points of view. In the meantime, electrochemical properties of the composite coatings had been studied by potentiodynamic polarization and electrochemical impedance spectroscopy.
The chemical compositions of the basic electrolyte of Zn-Ni alloys deposition is given in Table
Solution composition and conditions for alloy electroplating [
Electrolyte ingredients | Concentration (g · L−1) | Plating parameters | |
---|---|---|---|
ZnSO4 · 7H2O | 57.5 | pH = 2, 5 Temperature (°C): 30 ± 0.5 | |
NiSO4 · 6H2O | 52.5 | ||
H3BO3 | 9.3 | 1.1 | |
Na2SO4 | 56.8 | ||
H2SO4 | 0.53 |
Galvanostatic measurements were performed on steel rod cathode which was placed in a PTFE mount for obtaining cross-sectional area of 0.2 cm2 in contact with solution. The deposition was done on steel (STUB 100CR6) substrates. The chemical composition of STUB 100CR6 steel was 98.6% Fe, 0.95% C, 0.2%, Mn 0.15% Si, and 0.015% S (wt%). Initially, the steel samples were mechanically polished with successively finer grades of emery paper. They were then washed in deionized water. This procedure was repeated until a clear and smooth surface was obtained. To obtain fixed hydrodynamic conditions, the Zn-Ni coatings were deposited on a rotating disc electrode (RDE) with a constant rotation velocity of 1000 rpm. The deposition experiments were performed at a current density of 25 mA/cm2, and the plating time was 15 min. The SiO2 powder (AEROSIL 200) with a mean diameter of 12 nm was used as received without any pretreatment.
Before electrodeposition, the nanoparticles were dispersed in the bath by ultrasonic wave for 30 min. Particles with concentration of 5 g/L were maintained in an electrolytic bath in suspension by continuous magnetic stirring of 200 rpm for at least 24 hours before deposition.
For electrochemical measurements (anodic polarization curves and impedance spectra), the coated samples were exposed to 3% NaCl solution (
An X-ray diffraction investigation of Zn-Ni electrodeposits was carried out using an X-ray diffractometer D8 (Advance Bruker) equipped with a copper anode generating Ni-filtered CuK
Glow discharge optical emission spectroscopy (GDOES); the distribution of species in the deposit, was determined by depth profiling using a Jobin Yvon GD-Profiler instrument equipped with a 4 mm diameter anode and operating after optimization at a pressure of 650 Pa and a power of 30 W in an argon atmosphere. This low power was retained to decrease the speed of abrasion of the deposits with low thickness and to obtain maximum information at the surface. Quantified compositional results were evaluated automatically utilizing the standard Jobin Yvon quantum Intelligent Quantification software. The instrument was calibrated with standard known composition. Depths were calculated using relative sputter rates, obtained from the sputter yields of each major element with corrections for composition and discharge conditions.
In Table
Electrophoretic mobility and Zeta potential measurements by “Rank Brother II”.
pH of bath solution | Effective charge of SiO2 particles | Electrophoretic mobility (cm2 · V−1 · s−1) | Zeta potential (mV) |
---|---|---|---|
6.7 | — | 1.45 × 10−8 | 18.6 |
We noted negatively charged particles of SiO2. This type of oxide is hydrophilic, so it always interacts with the electrolyte, and therefore chemical and physical adsorption of electrolyte ions onto the particle occurs. This adsorption and the initial particle surface composition determine the particle surface charge, which induces a double layer of electrolyte ions around the particle. In electrolytes, double layers play a major role in the interactions between particles and also between particles and the electrode [
The absolute value of the zeta potential is a very important factor for the degree of particles incorporation. In fact, zeta potential determined the static electricity force among nano-SiO2 particles and influenced greatly the character either agglomeration or dispersion among the nanoparticles [
Table
Composition analysis by X-ray fluorescence of Zn-Ni deposits electroplated at 25 mA/cm2 for 15 min onto steel substrate.
Coating systems | Ni | Zn | |
---|---|---|---|
Zn-Ni alloy coating | 6.3 | 93.7 | 14.9 |
Zn-Ni/nano-SiO2 alloy composite | 12.3 | 87.7 | 7.1 |
The ratio of the less noble metal Zn to the more noble metal Ni in the deposit (
The surface morphology of Zn-Ni alloy coating showed branched acicular crystallite structure (Figure
SEM micrographs and EDS of the surfaces of Zn-Ni-coated steel at 25 mA/cm2 for 15 min in plating bath: (a) Zn-Ni alloy coating, (b) Zn-Ni alloy composite (with 5 g/L SiO2).
Figure
GDOES depth quantified profile of the Zn-Ni alloy composite (with 5 g/L SiO2) electrodeposited with (25 mA/cm2, 15 min) onto steel substrate.
Figure
X-ray diffractograms for Zn-Ni alloy coating and Zn-Ni alloy composite (with 5 g/L SiO2) electroplated at 25 mA/cm2 for 15 min.
Crystallite sizes of the coatings were calculated from the X-ray peak broadening of the (330) diffraction peak using Scherrer’s formula [
Zn-Ni alloy deposit exhibited a crystallite size of 50 nm, whereas the crystallite size of Zn-Ni composite coating decreased to 26 nm. We could explain this evolution by the fact that the nanoparticles of silica might perturb the crystal growth by increasing the number of nucleation sites and consequently a reduction in the grain size occurs as reported by Pavlatou et al. [
Vickers microhardness (HV) values of Zn-Ni alloy deposits are shown in Figure
Evolution of microhardness of Zn-Ni alloy coatings (a) and Zn-Ni alloy composites (b) electroplated onto steel substrate at 25 mA/cm2 for 15 min.
The evolution of coatings hardness showed an increase from about 140 HV for pure Zn-Ni alloy to about 396 HV for samples prepared with SiO2 particles. We can conclude that the microhardness was affected by the incorporation of this type of nanoparticles into deposits. In turn, crystallite size is an important variable affecting the hardness value [
The thermal stability of Zn-Ni alloy electrodeposits is important to their general evaluation for future applications, particularly in applications where the coatings are expected to perform at elevated temperature such as on some automotive parts. In this study, in order to evaluate the stability of the deposits, we followed the microhardness evolution of deposits treated at 200°C continuously for 30 min, 2 hours, and 24 hours (Figure
Variation of microhardness of Zn-Ni alloy deposit and coating composite electroplated onto steel substrate at 25 mA/cm2 for 15 min and heat treated at 200°C for 30 min, 2 h, and 24 h.
After annealing at 200°C, deposits revealed reduced hardness values after 30 min of thermal treatment, especially the hardness of composites alloy. The reduced hardness could be attributed to surface oxidation phenomena or a rapid grain growth and a decrease of internal stresses as reported by Apachitei et al. [
Figure
Anodic polarization curves of steel substrates coated with Zn-Ni alloy coating (a) and Zn-Ni alloy composite (b) elaborated under DC (25 mA/cm2, 15 min); immersion in 3% NaCl.
Based on previous results [
Samples | Peak 1 | Peak 2 | Peak 3 | ||||
Zn-Ni alloy coating | −1020 | −800 | 15 | −615 | 8 | −445 | 5 |
Zn-Ni alloy composite | −941 | −815 | 5 | −544 | 13 | −404 | 10.5 |
As mentioned in Table
We followed the loss of metal from Zn-Ni alloy deposit by stripping with X-ray fluorescence. In comparison with Zn-Ni alloy coatings, we noted that the dezincification rate of Zn-Ni alloy composites decreased from 24% to 2% until the last remaining phase (
EIS was used to evaluate the barrier properties of the coatings after 24 hours of immersion in 3% NaCl and to determine their polarization resistance. Figure
Nyquist plots obtained for Zn-Ni alloy coating (a) and Zn-Ni alloy composite (b), after 24 h immersion in 3% NaCl solution. Symbols are the experimental data, and lines were modeled using fitting model.
The Nyquist-like diagram of the samples is characteristic of electrode process under kinetic and charge transfer control. The polarization resistance values could be approximately determined by fitting the Nyquist response to an equivalent circuit consisting of three RC components: the first time constant (
Data obtained by electrochemical impedance spectroscopy of coatings immersed in 3% NaCl solution for 24 h.
Coating systems | Composition analyses (±0.2 wt.%) | |||||||
---|---|---|---|---|---|---|---|---|
Zn-Ni alloy coating | 0.4 | 320 | Zn | Ni | Cl | Fe | Si | |
71 | 6 | 10.8 | 1.30 | — | ||||
Zn-Ni alloy composite | 6.5 | 3 × 10 | 560 | 85.5 | 9.73 | 4 | 0.47 | 0.8 |
Composition analyses of deposits immersed in 3% NaCl for 24 h (Table
MEB micrographs indicated that Zn-Ni alloy coating immersed in 3% NaCl for 24 hours suffered from defects with grains of salt deposited in clusters on the surface (Figure
Comparison of SEM micrographs of the surfaces of deposits elaborated at 25 mA/cm2 for 15 min onto steel substrate and immerged for 24 hours in 3% NaCl solution: (a) Zn-Ni alloy coating, (b) Zn-Ni alloy composite.
This work represented the electrodeposition and the corrosion behavior of Zn-Ni alloy deposits elaborated in absence or in presence of 5 g/L of nano-SiO2 in the plating bath. This study was made to evaluate the influence of nanoparticles addition on some properties such as hardness, morphologic, structure characteristics, and corrosion resistance. In comparison with Zn-Ni alloy coatings, the results revealed that Zn-Ni alloy composites: revealed a higher percentage of Ni and an incorporation of 1.54% of silica in the deposit, formed a mixture of two phases, zinc and exhibited higher values of microhardness and were thermally stable up to 24 hours at 200°C, revealed better corrosion resistance in corrosive media of 3% NaCl. This behavior indicates that Ni content, morphology, and the content of nanoparticles in the composite are responsible for the observed corrosion behavior.
The authors would like to acknowledge the financial support provided by “Action Intégrée Franco-Tunisienne du Ministère des Affaires Etrangères et Européennes Français et du Ministère de l’Enseignement Supérieur, de la Recherche Scientifique et de la Technologie Tunisien.”