- Email: [email protected]
Characterization and corrosion behaviour of plasma sprayed Zn-Sn alloy coating on mild steel
SCT-22272; No of Pages 8 Surface & Coatings Technology xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Characterization and corrosion behaviour of plasma sprayed Zn-Sn alloy coating on mild steel O.P. Oladijo a,⁎, M.H. Mathabatha b, A.P.I. Popoola b, T.P. Ntsoane c a b c
Department of Chemical, Materials and Metallurgical Engineering, Botswana International University of Science and Technology, Palapye, Botswana Department of Chemical, Metallurgical and Materials Engineering, Tshwane University of Technology, Pretoria, South Africa Research and Development Division, Necsa Limited, Pretoria, South Africa
a r t i c l e
i n f o
Article history: Received 2 March 2016 Revised 10 April 2017 Accepted in revised form 11 April 2017 Available online xxxx Keywords: Zn-Sn Coating Corrosion Residual stress Plasma spray Mild steel
a b s t r a c t s Extensive effort to enhance the mechanical and electrochemical surface properties of mild steel has been reported by various engineers worldwide. This investigation presents a comparative test of corrosion behaviour, residual stresses and coating characterization for different Zn-Sn alloy composition on mild steel substrate, with aim of enhancing surface properties and extending the application of mild steel in industries. Coating of about 200 μm was successfully deposited by plasma spray on mild steel substrate. The residual stresses were measured by X-ray diffraction sin2ψ technique using Cu-Kα radiation. Characterization and mechanical properties of the surface were investigated using Optical, Scanning electron microscopy, XRD and Emco Dura Scan tester to observe the microstructure, grain size, porosity, phase changes and hardness determination. The corrosion behaviour was examined in NaCl and H2SO4 solution. Direct current (DC) polarization tests were conducted and electrochemical corrosion behaviour of the surfaces was studied using scanning electron microscope (SEM/EDX). The results show an improvement as evidenced by the increased in corrosion resistance in NaCl and H2SO4 solutions, as well as increased in microhardness values on the coating. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Durability and consistency of constructions and parts of machines in operation are closely linked to the steadily continuing processes of corrosion damage and wear which take place on the surface areas of materials [1]. An effective approach of plummeting materials damage is protecting them with layer of a different material with required properties. Therefore desired mechanical properties and combating corrosion can be provided through proper selection of coating materials [1]. Zinc alloy coatings have absorbed major interest owed to their superior anti-corrosion properties compared to pure zinc [2]. Zn-Al, Zn-Ni and Zn-Fe alloys found a variety of engineering applications while ZnCo coatings are less common [3].Electrodeposition of alloys with specific properties is one of the latest trends in plating industry [1–6]. In electroplating the properties of the coated alloys are determined by their structure which in turn depends on the alloy composition, working electrolytes, the nature of plating substrate and deposition conditions [3]. The phase structure of an alloy is the major factor determining its corrosion resistance [3].
⁎ Corresponding author. E-mail address: [email protected] (O.P. Oladijo).
Zinc-Tin alloys are found to be attractive because they combine the barrier property of tin with the cathodic protection offered by zinc [4]. Each metal protects steel by a different mechanism. Tin is nobler than steel and under ordinary atmospheric exposure, protects steel by forming a corrosion resistant envelop around it. However, rusting takes place in pin holes or imperfections in the tin coating and is accelerated galvanically by the difference in potential between steel and tin [4]. At open circuit potential, Zn dissolution is the only corrosion reaction [4].The coatings (Zn-alloy) protect steel better than zinc only under conditions of high humidity and they resist corrosion equally when compared with an equal thickness of cadmium in marine and industrial atmosphere [4]. They also offered better solderability, brightness, ductility and service life with less white corrosion product formation when compared to zinc coatings only. Zn-alloys have better hardness, low hydrogen uptake and are cost effective [4]. Experience has established that electrodeposited tin-zinc alloy coatings of compositions containing 78% Sn and 22% Zn have excellent corrosion resistance. properties [5]. Humidity, salt-spray, hot-water porosity tests have been carried out on steel specimen coated with tin-zinc deposits of various compositions and thicknesses [5]. They reported that optimum tin content has proved to be 75–80%, 0.0003 in. (7.62 × 10−5 m) thickness of the alloy coating gives better protection than the same thickness of
http://dx.doi.org/10.1016/j.surfcoat.2017.04.029 0257-8972/© 2017 Elsevier B.V. All rights reserved.
Please cite this article as: O.P. Oladijo, et al., Surf. Coat. Technol. (2017), http://dx.doi.org/10.1016/j.surfcoat.2017.04.029
2
O.P. Oladijo et al. / Surface & Coatings Technology xxx (2017) xxx–xxx
cadmium or zinc. In addition it has also been found that alloy coatings containing less than 50% Sn have their corrosion resistance marked inferior to that of 78% Sn. Whilst the corrosion tests showed that under saltspray conditions the performance of tin-zinc coatings of 75–80% Sn was very good and superior to the 50 tin: 50 zinc composition [5]. Tin–zinc alloy coatings are recognized as a potential alternative to toxic cadmium (as corrosion resistant deposits) because they combine the barrier protection of tin with the cathodic protection afforded by zinc [5]. The coatings containing 20 wt% zinc, balance tin, offer excellent corrosion protection for steel and do not form gross voluminous white corrosion products like pure zinc or high zinc alloy deposits [5]. On the other hand, the corrosion behaviour of tin–zinc alloy electroplated coatings on steel has been investigated by Dubent et al. [5] using electrochemical methods in a 3 wt% NaCl solution and the salt spray test. The performance of the deposits was compared with cadmium and zinc–nickel electrodeposited coatings. They found that the corrosion resistance of tin–20 wt% zinc alloy coating is superior to that of cadmium and zinc–12 wt% nickel coatings. In recent years, a thermal spraying method that produces anti-corrosion layer on the surface of steel materials by melting Zn or Al, which has a relation to the sacrificial anode, using gas or electricity instead of the conventional corrosion resistance method, is attracting many researchers presently. Thus it is of great importance to select an appropriate thermal spraying method depending on the environment conditions. Sacrificial anode metals, such as tin, magnesium, zinc and aluminium have better corrosion resistance and have been under extensive research studies [6]. However there are few works that quantitatively evaluate the electrochemical performance of thermally sprayed coatings using these metals [6]. S. Schuerz et al. [7] investigated the corrosion behaviour of conventional hot-dip galvanized zinc coated and novel hot-dip galvanized Zn-Al-Mg alloy coated in standardized salt spray test. They found that corrosion proceeds very fast and steel substrate was attacked after 100 h of exposure for conventional hot-dip galvanized zinc coated. However novel hot-dip galvanized showed a different behaviour. This was attributed to the adherent aluminiumrich oxide layer which protects the steel substrate against corrosive attacks. Plasma spraying among the various thermal spraying techniques has attracted considerable attention for its extremely high temperature [8– 11]. A plasma sprayed coating is built up and the microstructure is formed, when individual, fully or partially molten particles, at a particular velocity, flatten, adhere and solidify on impact with the substrate [9]. The spraying parameters, substrate temperature, coating thickness, and surface roughness of substrate during deposition play an important role on the final properties of the coating [10,12]. Spraying distance is an important parameter that influenced on the hardness, porosity and the coating roughness of plasma spraying significantly [11]. Therefore this research work aims to enhancing surface properties of mild steel and extending the application of mild steel in industries. Oladijo et al. [13] reported that the control of residual stresses is a primary issue in coating technology. These are stresses that linger after deformation with all external forces removed. Several authors [13–15] have investigated the origin of residual stresses in coating. They reported that both material and deposition process may produce a residual stress. In the case of thermal spray coatings, quenching stresses due to rapid cooling of the coating, thermal mis-match stresses between the substrate and coating, phase transformation during deposition have been identified as sources. Residual stresses can be compressive or tensile depending on the direction of the forces [13,14]. Residual stresses play a fundamental function in materials as it can either boost or degrade performance. Non destructive stress determination is crucial since it offers the possibility of stress examination as a final quality check before service, or allows stress monitoring development during the components lifetime [15]. Specific to coatings, X-ray diffraction is a valuable technique to measure residual strain.
2. Material experiments 2.1. Plasma spraying process Powders mixed in the following ratios: Zn-Sn (25/75), Zn-Sn (50/ 50), and Zn-Sn (75/25) was used for this research work. The powders used were obtained from TLS Technik-Gmbh & Co, Spezialpulver Germany. The equipment used for coating was a 9MC plasma control unit (Sulzer Metco) with argon/nitrogen and helium as inert gases. It consist of a hoop powder feed unit (9MP) and a robot that controls the position and angle of the spray gun at a speed of 400 mm/s. Before deposition, the substrate was grit-blasted using alumina grits with the aim of cleaning and to roughen the specimen surface. The powder was fed into a hopper and Mild steel bar was clamped/placed and tightened on a stand inside the plasma spray booth. The spray gun, with aid of a flame deposits the powder of interest on the surface of the substrate at approximately from 300 to 3000° at different intervals until the required coating thickness of 200 μm is achieved. During the spraying, the gun is stopped frequently to avoid over heating the substrate. Hydrogen (11 L/min) argon (175 L/min) was used as primary and secondary inert gases respectively. The coatings were deposited at a distance of 100 mm and powder feed rate was 32 g/min. 2.2. Coating microstructure The coating cross-sectional part was grind using Struers Tegra Pol21 with MD piano 250 mm diamond grinding disk, followed by 500 and 1000 μm SiC paper. Polishing was done in a 3 step using water based diamond suspensions Allegro largo 9 μm, MD Mol 3 μm and MD Nap 1 μm. Specimens were etched in a Nital to enhance microstructural features such as grain size and phases. Characterization of cross-sections coated samples prior to, and after corrosion tests was carried out using scanning electron microscopy with Energy Dispersive X-Ray Spectroscopy (EDX). Coating hardness was obtained using a Vickers hardness tester at an applied load of 300 g and a 15 s dwell time. Five measurements were made for each cross sectional coating and the average hardness value determined. Porosity measurements were done using dot counting method for SEM images of 5000 ×. The X-ray diffraction (XRD) was conducted for the coatings with Cu-Kα radiation at 40 kV and 20 mA on D8 Discovery. 2.3. Residual stress Residual stress in thermal sprayed coatings may be measured by many techniques, which may be categorized into two methods namely; destructive method and non-destructive method. In the present research, non destructive method was used to measure the residual stress in the thermal spray coated sample, as it gives reliable results of the final residual stress of the material. In addition, no surface preparation is required for this measurement. But the limitation is that X-ray penetration is extremely shallow as it analyse a surface layer whose thickness depends both on the sample material and the incident beam wavelength. The primary side of the equipment consists of a graphite monochromator, a collimator and Cu target (wavelength of 1.544Ǻ). For this particular investigation a 0,8 mm collimator was used. The strain measurements were done on the Zn phase of the Zn-Sn coatings using the Cu-Ka1 radiation with the generator settings of 40 kV and 40 mA. The diffraction data was collected using area detector Vantec500 with no beam optics on the secondary side. Strain measurements were done on the (211) reflections corresponding Bragg peak of ~ 127.492° (2θ). For a complete stress tensor investigation, samples were measured at 6 different azimuth orientation ϕ = 0°, 180°, 90°, 270°, 45°, and 225° with each azimuth measured at 8 different tilt angles at incremental step of 10°. Rotation of the azimuth orientation by 180° allowed measurement in both −ψ and +ψ tilt angles. Measurements were done in side-inclination mode. The data analysis was analysed using Bruker's
Please cite this article as: O.P. Oladijo, et al., Surf. Coat. Technol. (2017), http://dx.doi.org/10.1016/j.surfcoat.2017.04.029
O.P. Oladijo et al. / Surface & Coatings Technology xxx (2017) xxx–xxx
3
Fig. 1. SEM/EDS micrographs of Zn-Sn Powder at higher and lower magnification.
Leptos v6 software. The strain measurements were then converted to stress measurements by using the 211 reflection of Zn with the following elastic constants: S1 = −2.778 × 10−6 MPa, 1/2S2 = 1.204 × 10−5 MPa and Poison's ratio = 0.30. The techniques employed here are similar to the work reported by Oladijo et al. [13].
2.4. Electrochemical tests Comparative electrochemical studies between the Zn-Sn coated and as-received mild steel specimens was carried out using linear polarization technique with the aid of Autolab potential (PGSTAT30). The
Potentiodynamic polarization curve known as a tafel plot, depicts the relationship between potential and corrosion current density. This plot exhibits a linear region, the slope of which is known as the Tafel constants (anodic and cathodic Tafel constants). The intersection of the projection of the linear region of the plot with the open circuit potential (Ecorr) gives the cathodic or anodic corrosion current (Icorr). Once Icorr is determined, the following equation derived from Faraday's law was used to calculate the corrosion rate [16]: corrosion rate
mm 3:27 Icorr EW ¼ yr D
Fig. 2. SEM micrograph of (a) Zn-Sn 25/75, (b) Zn-Sn 50/50 & (c) Zn-Sn 75/25 coating.
Fig. 3. XRD patterns of the as-sprayed Zn-Sn alloy coating.
Please cite this article as: O.P. Oladijo, et al., Surf. Coat. Technol. (2017), http://dx.doi.org/10.1016/j.surfcoat.2017.04.029
ð1Þ
4
O.P. Oladijo et al. / Surface & Coatings Technology xxx (2017) xxx–xxx
Fig. 5. Residual stress results of Zn-Sn coating on mild steel substrate.
4. Discussions
Fig. 4. Hardness of the Zn-Sn coatings and the substrate
The corrosion rate in Eq. (1) is expressed in micrometers per year, μm/y. Icorr is the corrosion current density in μA·cm−2, obtained by dividing Icorr with the exposed surface area of the measured specimen. E.W. is the equivalent weight of steel in g, and d is the density of steel in g/cm3. The polarization resistance Rp (which is the slope of the potential current curve at Ecorr), is related to Icorr through the following Stern-Geary relationship [16]:
Icorr ¼
½βa βc 2:3 ðβa þ βcÞ Rp
ð2Þ
3. Results The morphology of the Zn-Sn powder characterized by SEM depicted in Fig. 1, which were irregular in shape. Fig. 2 shows the microstructure of the Zn-Sn coating, which consist of Zn grain, microcracks as well as some pits. XRD pattern of the as-sprayed coating were shown in Fig. 3. The XRD results revealed that the coating consist of Zn, ZnO, Sn and other complex oxide. Fig. 4 shows the hardness values of the substrate and the coatings, which were all different. The residual stresses determined under the assumption of planar stress conditions are given in Table 1 and Fig. 5. The residual stresses have been determined to be compressive in the as-sprayed Zn-Sn coating, and were relatively low valued. Potentiodynamic polarization curves and electrochemical values of plasma sprayed Zn-Sn coatings under different composition in H2SO4 are shown in Fig. 6 and Table 2 respectively. It is observed that all the Zn-Sn coating exhibit similar polarization curve shapes in the tested solution. Fig. 7 and Table 3 shows the polarization curve shape tested in 3.65% sodium chloride solution. The curve was look similar and different from one another. The representative SEM micrographs taken from the surface of all coating layers after potentiodynamic polarization tests in 1 M H2SO4 and 3.65% NaCl are shown in Figs. 8 and 9 respectively. They were all remarkable and different due to the influence of the test solution.
The typical morphology of the Zn-Sn powder (Fig. 1) was simple and irregular in shape, as they are neither disc nor circle. The powder showed densely structure with Zn cluster cemented by Sn as confirmed by EDX. The SEM/BSE micrographs of the Zn-Sn coatings cross sections are shown in Fig. 2. The coating microstructures were all different despite same substrate and spraying parameters, although Zn-Sn coatings seemed to form an inmate and homogenous mixture. This microstructural difference was attributed to differences in their powder composition, which play an important role in the cooling and solidification of coating after spraying. In addition, the micrographs was characterizing by low porosity, light grain with particle boundaries barely visible. EDX analyses revealed that the coating comprises of Zn, Sn and oxygen (low). Hence partial oxidation likely occurs. Fig. 3 shows the comparison of the as-sprayed Zn-Sn alloy coating on mild steel substrates. Based on the XRD spectra, all coating seem to retain both powder microstructure and secondary phases proving undesirable reaction occurred at high temperature of plasma chamber. The phase identification revealed the presence of Zn, Sn ZnO and ZnSnO3 (larger amount) on all the as-sprayed coating. However, low intensity peak of complex oxide phase (ZnSn(OH)6) was found at 2 theta = 22° and 53° suggesting that phase transformation occur during plasma spraying process. It is thought that the formation of complex oxide (eta phase) suggested that diffusion occur between the substrate and the. Overall, there was also a variation in the XRD intensities of this complex oxide or eta phase (i.e. ZnO, ZnSnO3, ZnSn(OH)6). This could be due to differences in percentage composition of the feedstock powder and residual stress effects since their substrate were the same. Moreover, there was a slight increase in background of the Zn-Sn alloy coating (at a diffraction angle of 2 theta = 15° to 20°). This was due to the following reasons: (i) Micro-strain effect in the coating as a result of the high temperature associated with plasma process. (ii) Grain refinement (iii) Severe plastic deformation of the particle upon impact the as-received mild steel substrate A significant increase in the micro hardness value (Fig. 4) was observed in the Zn-Sn 50/50, Zn-Sn 75/25, & Zn-Sn 25/75 coating respectively. The micro hardness value for mild steel and the Zn-Sn coatings can be arranged in ascending order as follows: Mild steel b Zn-Sn 25/
Table 1 Coating properties in as-coated condition. Coated sample
r11 (MPa)
r22 (MPa)
Residual stress (average) (MPa)
Hardness value (Hv)
Zn-Sn 25/75 Zn-Sn 50/50 Zn-Sn 75/25
−20.3 ± 57.9 −19.5 ± 7.4 −17.7 ± 7.1
−55.3 ± 57.8 −17.4 ± 7.3 −31.1 ± 7.0
−37.8 ± 57.8 −18.45 ± 7.3 −24.4 ± 7.0
474.4 ± 0.01 529.4 ± 0.02 502.7 ± 0.01
Please cite this article as: O.P. Oladijo, et al., Surf. Coat. Technol. (2017), http://dx.doi.org/10.1016/j.surfcoat.2017.04.029
O.P. Oladijo et al. / Surface & Coatings Technology xxx (2017) xxx–xxx
5
Fig. 6. Potentiodynamic polarization curves for as-received and Zn-Sn coatings in 1M H2SO4.
75 b Zn-Sn 75/25 b Zn-Sn 50/50. The hardness of the coatings was higher than the as-received sample as expected. This confirmed the significant or important that coating render the surface of engineering material. In addition the coated Zn-Sn 50/50 has the highest hardness value while Zn-Sn 25/75 has the lowest hardness value. The difference in their hardness of the coating might be due to minor diffusion between coating and substrate. The increase in hardness of the Zn-Sn 50/50 coated specimen was due to the presence of the higher concentration of tin as the reinforcement additive. Perhaps could be due to the low values of porosity and compressive residual stress respectively (Fig. 5). It is thought that the high peak intensities of ZnSO3 and ZnSn(OH)6 at 2 theta = 22° could have added as advantage (although the exact amount on the coating surface does not quantify). However the reduction in the hardness of Zn-Sn 75/25 coating (about 53% reductions) and Zn-Sn 25/ 75 (about 11% reductions) could be due to the composition of their feedstock powder. Overall the reduction in the hardness can be ascribed to the increased porosity level, micro crack present with the coating as well as weak bonding. Residual stress effect maybe either beneficial or detrimental depending on upon the sign and stresses distribution with respect to some external factors. The experimental results of the residual stresses found on the coated samples (Fig. 5) were compressive stress, but different from each other due to powder composition used (i.e. Zn and Sn). It is thought that a compressive stress has a beneficial effect on the coating adhesion and mechanical properties of a coating. The compressive stresses increases with higher content of Zn and Sn alloy powder, but decreased when the ratio of two powders were equal. With higher Sn content (75%) the compressive stress is higher than the other coatings. The variation in the residual stresses could be due to the following mechanism: (1) Melting behaviour of the spray particles in the hot gas jet since the powder compositions were varies [15]. (2) Difference in the surface roughness of the coating which was slightly difference due to their differences in powder composition which might play important roles in the melting of charge
particles inside the vacuum. It is thought that surface roughness also contributed to the residual stresses, especially in the attenuation of beam into the top bond coat of the coating. (3) The rate of cooling after spraying can significantly influence the residual stress profile in the coating, which was not considered here, because the cooling rates were assumed to be the same [15]. (4) The exact measuring position used for the measurement since their surface roughnesses were assumes not the same. (5) The differences in the compressive stress was deduced to be due to different in coefficient of thermal expansion, interaction between the coating and substrate which had been discussed by the current author elsewhere [13–15]. Also, moderate compressive stresses found on the as-sprayed coated samples reflect the dominance of peening mechanism of stress (caused by grit-blasting) over thermal quenching. Furthermore, the compressive residual stress observed on the as-sprayed Zn-Sn 25/75 was higher than other as-sprayed coating. This could be due to the homogeneous microstructure of the coating, as well as the surface hardening caused by a grit-blasted surface. In addition, a relatively numerical fitting (i.e. error bar) was found on the coated sample (Table 1 and Fig. 5). These error bars symbolize the minimum and maximum rages in which residual stress condition can be attained. However relatively large fitting (error bar) found on the as-sprayed Zn-Sn 25/75 in comparison to the others coated sample suggested that significant accuracy improvement can be achieved by increasing the intensity through the measurement time. The calculated penetrating depth of Cu–Kα radiation in Zn coating is 26.5 μm, suggested that the residual stress result cover several micrometers deep into the coating. This value was calculated based on [13,17]. However the results obtained by X-ray diffraction analysis were unable to highlight the differences of stress variation across the as-sprayed coating/ substrate due to the very low penetration depth. Therefore further studies will be conducted with the neutron diffraction measurement to compliments the X-ray diffraction results.
Table 2 Summary of potentiodynamic polarization results in 1M H2SO4 solution at ambient temperature. Sample
Ecorr (v)
Icorr (A/cm2)
Bc (v/dec)
Ba (v/dec)
CR (mm/yr)
Rp (Ω)
Substrate Zn-Sn 25/75 Zn-Sn 50/50 Zn-Sn 75/25
−0.29247 −0.24722 −0.26659 −0.19539
5.2411 × 10−3 8.166 × 10−5 1.1626 × 10−3 4.3581 × 10−4
2.261 0.031384 0.31409 0.032842
2.419 0.20822 0.31508 0.075225
60.9156 0.9491 13.5132 5.0653
96.947 145.2 58.82 22.807
Please cite this article as: O.P. Oladijo, et al., Surf. Coat. Technol. (2017), http://dx.doi.org/10.1016/j.surfcoat.2017.04.029
6
O.P. Oladijo et al. / Surface & Coatings Technology xxx (2017) xxx–xxx
Fig. 7. Potentiodynamic polarization curves for as-received and Zn-Sn coatings in 3.65% NaCl solution.
Furthermore, it is of no doubt that the compressive residual stress measured tend to counteract the Vickers hardness indenter from deeper penetration into Zn-Sn coating resulting in high hardness [18]. Therefore, compressive residual stress plays a crucial role in determine the mechanical properties of a coated sample. The propensity of mild steel to corrosion dilapidation in 3.65% NaCl and 1 M H2SO4 was examined using the linear polarization technique. Fig. 6 shows the linear polarization curve of the substrate and coated samples in 1 M H2SO4. The Potentiodynamic polarization data from the Tafel plot are presented in Table 2. A vast corrosion degradation of 60.91 mm/year was discovered as a result of deficient surface protection from the experimental analysis. The substrate presents a high current density of 5.241 × 10−3 A/cm2 signifying hastening behaviour for the metal dissolution. The corrosion potential of the sample was −0.29247 V. Through observation and research, the disastrous damage of the substrate by aggressive sulphate ion (SO2− 4 ) took place consistently. Also the substrate had less passive film created on its surface ensuing severe corrosion attack from the sulphuric acid solution. Furthermore, since the substrate lacked surface protection, a high rate of corrosion was expected, as lack of surface protection has been substantiated to stimulate sever deterioration [19]. When mild steel corrodes, there is usually a loss of metal to a solution in some form, by a product –favored oxidation-reduction reaction. The following mechanism for the corrosion of iron and steel in acid solution has been proposed [20–22]. The anodic iron dissolution: Fe þ An− → FeAn− ads
ð1Þ
FeAn− ads→ FeAn− ads þ ne−
ð2Þ
FeAn− ads→ FeAnþ ads þ ne−
ð3Þ
FeAnþ ads → Fe2þ ads þ An−
ð4Þ
The cathodic hydrogen evolution: Fe þ H þ → FeH þ ads
ð5Þ
FeH þ ads þ ne− →ðFeH Þads FeH
þ
ð6Þ
ads þ H þ þ ne− →Fe þ H 2
ð7Þ
In the absence of a protective coating in acid solutions, the mild steel surface becomes extremely corroded with areas of pitting corrosion. This phenomenon is often intensified in the presence of chlorides by unleashing severe attack in coatings [23]. Pitting corrosion takes advantage of the different metallurgical phases present on the surface of most common alloys such as mild steel. Pitting occurs as a result of localized anodic dissolution where the anodic portion of corrosion cell is dominated by the larger cathodic portion. Under common operating conditions, oxide films are formed on the iron surface. These oxide films mostly present brown rust (hematite, (γ -Fe2O3)), a black rust (Fe3O4) and yellow rust (FeO(OH)H2O). Table 2 also shows the corrosion current density (Icorr), corrosion rate (Cr) and polarization resistance (Rp) values of each coated sample. Zn-Sn 25/75 exhibited the highest polarization resistance, lowest corrosion rate and corrosion current density compared to the other Zn-Sn coatings and the substrate in 1 M sulphuric acid solution. The Zn-Sn 50/50 and Zn-Sn 75/25 coating exhibited higher polarization resistance, lower corrosion rates and corrosion currently densities than the substrate respectively. Metal exposed to sulphuric acid, in this case 1 M H2SO4 without coating shows the presence of carbon, oxygen and iron suggesting the formation of a compound including iron oxide/hydroxide and carbon [24]. Iron oxide (rust) developed on the surface which is not protective because it doesn't form a continuous, adherent film. Instead, it spalls, exposing fresh iron to the atmosphere which in turn allows more corrosion to occur [24]. It can be seen that the substrate exhibited a corrosion rate (mm/yr) which is 63 times that of the ZnSn 25/75. Lower corrosion rate of the coated samples is attributed to the formation of zinc sulphate (ZnSO4) which dissolves the protective zinc hydroxide layers (ZnOH2). With increased concentration of Sn in
Table 3 Summary of potentiodynamic polarization results in 3.65% NaCl solution at ambient temperature. Sample
Ecorr (V)
Icorr (A/cm2)
Bc (V/dec)
Ba (V/dec)
CR(mm/yr)
Rp (Ω)
Substrate Zn-Sn 25/75 Zn-Sn 50/50 Zn-Sn 75/25
−0.3383 −1.0608 −1.1017 −0.68001
2.39 × 10–2 5.24 × 10–4 4.44 × 10–5 9.29 × 10–4
0.218 0.13901 0.14656 0.33085
0.216 0.084413 0.045705 0.24792
278.514 6.0554 0.51604 10.797466
1.968 43.49 354.71 66.288
Please cite this article as: O.P. Oladijo, et al., Surf. Coat. Technol. (2017), http://dx.doi.org/10.1016/j.surfcoat.2017.04.029
O.P. Oladijo et al. / Surface & Coatings Technology xxx (2017) xxx–xxx
7
Fig. 8. SEM micrograph of (a) Zn-Sn 25/75, (b) Zn-Sn 50/50& (c) Zn-Sn 75/25 in 1 M H2SO4 solution.
the coatings, the corrosion potential was significantly low. Passivation of Sn in alloys containing over 70% of Zn occurred at more negative potentials. This could be due to the formation of a non-equilibrium surface layer enriched in Sn and structural defects as a result of selective dissolution of Zn as reported by Korobov et al. [24]. In addition, Zn-Sn coatings did not suffer horrible corrosion attack from sulphate ions when compared with the great degradation done on the substrate. According to electrochemical principles, the corrosion rate is proportional to the corrosion current density, and reduced rate in corrosion current density will enhance passivity of the sample as reported by [19]. Perhaps excellent improvement in the corrosion resistance of the alloyed samples could be due to homogeneous oxide films and stable coating formed in the coating as showed by the XRD (Fig. 3). The passivation behaviour is also due to the formation of adherent corrosion products, certainly consisting of very stable oxides and hydroxides of zinc. A good decrease in corrosion current density may also be due to the formation of ZnO. The phases, vismirnovite (ZnSn(OH)6), zinc oxide (ZnO) and zinc stannate in the Zn-Sn coatings led to the formation of a more densely packed corrosion product layer on the coating surface, which enhanced the corrosion resistance [19]. Table 3 and Fig. 7 show the corrosion behaviour of the substrate and coatings in 3.65% sodium chloride solution. The substrate exhibited a considerably high corrosion rate (mm/yr) which is enormously higher than the coating. This is attributed to the absence of a passive layer/ film on its surface. However the substrate also exhibited the lowest polarization resistance and higher corrosion current density compared to the coatings, thus more prone to corrosion attack/susceptible to corrosion. In a study conducted by Fayomi and popoola et al. [25], mild steel corrosion behaviour was studied in 3.65% sodium chloride using open circuit potential scans. The potential was observed to shift towards the negative region indicating strong dissolution of the mild steel due to the absence of passivation. Among the coatings, Zn-Sn 50/50 had the highest polarization resistance, lower corrosion rate and current density. Zn-Sn 75/25 exhibited a higher polarization resistance and corrosion current density than Zn-Sn 25/75 whereas its corrosion rate was found to be high which was not expected. It is thought that a higher polarization resistance will also result in low corrosion rate but it was not the case. From the Tafel values it
can be seen that the coated values exhibited polarization resistances significantly higher than that of the substrate and corrosion rates which are considerably lower than the substrate. This is because the presence of Zn and Sn both combine the barrier property of tin with the cathodic protection offered by zinc [4]. Each metal protects steel by a different mechanism. Tin is nobler than steel and under ordinary atmospheric exposure, protects steel by forming corrosion resistant envelop around it [4]. The microstructures of the corroded samples were highly corroded after immersion inside different medium (Figs. 8 and 9). The microstructures revealed some pitting/pores on the samples after immersion in corrosion media which was due to the aggressive sulphate ion (SO42−) and chloride ion (Cl−). The Zn-Sn coating in general did not suffer horrible corrosion attack from the chloride and sulphate ion as compared to the substrate. 5. Conclusions Deposition Zn-Sn alloys was successfully carried out on mild steel surface by plasma spraying technique. The coated Zn-Sn samples in different ratio were carried out on the surface of mild steel substrate. The substrate exhibited corrosion rate considerably higher that the coatings. A substantial advancement in corrosion resistance was attained in the coated specimens. In 1M H2SO4 solution, the best corrosion resistance was exhibited by Zn-Sn 25/75, Zn-Sn 75/25 and Zn-Sn 50/50 respectively. In 3.65% NaCl solution the highest corrosion resistance was exhibited by Zn-Sn 50/50, followed by Zn-Sn 25/75 and Zn-Sn 75/25 coating respectively. The micro hardness values show that Zn-Sn coatings had improved hardness values as compared to the substrate. Zn-Sn 50/50 had the highest hardness value compared to the other coatings. The residual stresses of the Zn-Sn coating were all compressive in nature and of relatively lower valued. Acknowledgements The authors wish to acknowledge the financial and technical support received from National Research Foundation, South Africa (91027) and Tshwane University of Technology. The authors are also thankful to Mr.
Fig. 9. SEM micrograph of (a) Zn-Sn 25/75, (b) Zn-Sn 50/50 & (c) Zn-Sn 75/25 in 3.65% NaCl solution.
Please cite this article as: O.P. Oladijo, et al., Surf. Coat. Technol. (2017), http://dx.doi.org/10.1016/j.surfcoat.2017.04.029
8
O.P. Oladijo et al. / Surface & Coatings Technology xxx (2017) xxx–xxx
Tshepo Ntsoane from Necsa (Residual stress analyser), Surface Engineering Research Center TUT (Mr. Fatoba and Joy Matsilele) and Dr. Oluwasayo (UJ) for their great assistance and contributions. References [1] H. Kazimierczak, P. Ozga, K. Berent, M. Kot, Microstructure and micromechanical properties electrodeposited of Zn-Mo coatings on steel, J. Alloys Compd. 636 (2015) 156–164. [2] F. Fontenay, Electrodeposited zinc and zinc alloy coatings and their corrosion resistance. Part 1, Galvanotechnik 93 (2002) 2534–2541. [3] V.S. Vasantha, M. Pushpavanam, V.S. Muralidharan, Tin-zinc alloy electrodepositphysicochemical characterisation, Ind. J. Chem. Technol. 7 (2000) 216–222. [4] S. Dubent, M. Mertens, M. Saurat, Electrodeposition, characterization and corrosion behaviour of tin-20wt% zinc coatings electroplated from a non-cyanide alkaline bath, Mater. Chem. Phys. 120 (2010) 371–380. [5] S.K. Ray, Electro-Deposition of Tin-Zinc Alloy as a Protective Coating of Ferrous Metals. Ing: Non-Ferrous Metal Industry in India, 1957 266–271. [6] Hong-Bok Choe, Jun-Ho Shin Han-Scung, Experimental study on the electrochemical anti-corrosion properties of steel structures applying the arc thermal metal spraying method, Materials 7 (2014) 7722–7736. [7] S. Schuerz, M. Fleischanderl, G.H. Luckeneder, K. Preis, T. Haunschmied, G. Mori, A.C. Kneissl, Corrosion behaviour of Zn-Al-Mg coated steel sheet in sodium chloridecontainning environment, Corros. Sci. 51 (2009) 2355–2363. [8] A. Satapathy, H. Sutar, S.C. Mishra, S.K. Sahoo, Characterization of plasma sprayed pure red mud coatings: an analysis, Am. Chem. Sci. J. 3 (2) (2013) 151–163. [9] Z. Yina, S. Tao, X. Zhou, Effect of the thickness on properties of Al2O3 coatings deposited by plasma spraying, Mater. Charact. 62 (2011) 90–93. [10] O. Sarikaya, Effect of some parameters on microstructure and hardness of alumina coatings prepared by the air plasma spraying process, Surf. Coat. Technol. 190 (2005) 388–393. [11] T.S. Sidhu, S. Prakash, R.D. Agrwal, Studies on the properties of high-velocity oxyfuel thermal spray coatings for higher temperature applications, Mater. Sci. 41 (2005) 6. [12] Y. Gao, X. Xu, Z. Yan, G. Xin, High hardness alumina coatings prepared by low power plasma spraying, Surf. Coat. Technol. 154 (2002) 189–193.
[13] O.P. Oladijo, A.M. Venter, L.A. Cornish, N. Sacks, X-ray diffraction measurement of residual stress in WC-co thermally sprayed on to metal substrates, Surface & Coatings Technology 206 (2012) 4725–4729. [14] A.M. Venter, O.P. Oladijo, V. Luzin, L.A. Cornish, N. Sacks, Performance characterization of metallic substrates coated by HVOF WC-Co, Thin Solid Films 549 (2013) 330–339. [15] O.P. Oladijo, A.M. Venter, L.A. Cornish, Correlation between residual stress and abrasive wear of WC-17Co coatings, Int. J. Refract. Met. Hard Mater. 44 (2014) 68–76. [16] M. Geary, A.L. Geary, Electrochemical polarization, No 1 theoretical analysis of the shape of polarization curves, J. Electrochem. Soc. 104 (1957) 56–57. [17] I.C. Noyan, J.B. Cohen, Residual Stress Measurement by Diffraction and Interpretation, Springer, New York, 1987. [18] A. Milanti, V. Matikainen, H. Koivuluoto, G. Bolelli, L. Lusvarghi, P. Vuoristo, Effect of spraying parameters on the Microstructurural and corrosion properties of HVAFsprayed Fe-Cr-Ni-B-C coating, Surf. Coat. Technol. 277 (2015) 81–90. [19] O.S. Fatoba, A.P.I. Popoola, T. Fedotova, Characterization and corrosion behaviour of Zn-Sn binary alloy coatings in 0.5 M H2SO4 solution, J. Electrochem. Sci. Technol. 6 (2) (2015) 65–74. [20] M.S. Morad, A.M. Kamal El-dean, 22′-Dithiobis (3-cyano-4-6dimethylpyridine) a new class of acid corrosion inhibitors for mild steel, Corros. Sci. 48 (2006) 3398–3412. [21] K. Tebbji, H. Oudda, B. Hammouti, M. Benkaddour, M. El Kodadi, F. Malek, A. Ramdani, Inhibitive action of two bipyrazolic isomers towards corrosion of steel in 1 M HCl, Appl. Surf. Sci. 241 (2005) 326–334. [22] A. Yurt, A. Balaban, S.U. Kandemir, G. Bereket, B. Erk, Investigation on some Schiff bases as HCl corrosion inhibitors for carbon steel, Mater. Chem. Phys. 854 (2004) 20–426. [23] R.T. Loto, Pitting corrosion evaluation of austenitic stainless steel type 304 in acid and chloride media, J. Mater. Environ. Sci. 4 (4) (2013) 448–459. [24] V.I. Korobov, L.V. Petrenko, V.V. Poltavelts, Effect of phase composition on the anodic dissolution and passivation of zinc-based alloys, Univ. J. Chem. 2 (5) (2014) 76–85. [25] A.P.I. Popoola, O.S. Fayomi, A.P.I. Popoola, Electrochemical and Mechanical Properties of Mild Steel Electro-plated with Zn-Al, Int. J. Electrochem. Sci. 7 (2012) 4898–4917.
Please cite this article as: O.P. Oladijo, et al., Surf. Coat. Technol. (2017), http://dx.doi.org/10.1016/j.surfcoat.2017.04.029
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Characterization and corrosion behaviour of plasma sprayed Zn-Sn alloy coating on mild steel O.P. Oladijo a,⁎, M.H. Mathabatha b, A.P.I. Popoola b, T.P. Ntsoane c a b c
Department of Chemical, Materials and Metallurgical Engineering, Botswana International University of Science and Technology, Palapye, Botswana Department of Chemical, Metallurgical and Materials Engineering, Tshwane University of Technology, Pretoria, South Africa Research and Development Division, Necsa Limited, Pretoria, South Africa
a r t i c l e
i n f o
Article history: Received 2 March 2016 Revised 10 April 2017 Accepted in revised form 11 April 2017 Available online xxxx Keywords: Zn-Sn Coating Corrosion Residual stress Plasma spray Mild steel
a b s t r a c t s Extensive effort to enhance the mechanical and electrochemical surface properties of mild steel has been reported by various engineers worldwide. This investigation presents a comparative test of corrosion behaviour, residual stresses and coating characterization for different Zn-Sn alloy composition on mild steel substrate, with aim of enhancing surface properties and extending the application of mild steel in industries. Coating of about 200 μm was successfully deposited by plasma spray on mild steel substrate. The residual stresses were measured by X-ray diffraction sin2ψ technique using Cu-Kα radiation. Characterization and mechanical properties of the surface were investigated using Optical, Scanning electron microscopy, XRD and Emco Dura Scan tester to observe the microstructure, grain size, porosity, phase changes and hardness determination. The corrosion behaviour was examined in NaCl and H2SO4 solution. Direct current (DC) polarization tests were conducted and electrochemical corrosion behaviour of the surfaces was studied using scanning electron microscope (SEM/EDX). The results show an improvement as evidenced by the increased in corrosion resistance in NaCl and H2SO4 solutions, as well as increased in microhardness values on the coating. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Durability and consistency of constructions and parts of machines in operation are closely linked to the steadily continuing processes of corrosion damage and wear which take place on the surface areas of materials [1]. An effective approach of plummeting materials damage is protecting them with layer of a different material with required properties. Therefore desired mechanical properties and combating corrosion can be provided through proper selection of coating materials [1]. Zinc alloy coatings have absorbed major interest owed to their superior anti-corrosion properties compared to pure zinc [2]. Zn-Al, Zn-Ni and Zn-Fe alloys found a variety of engineering applications while ZnCo coatings are less common [3].Electrodeposition of alloys with specific properties is one of the latest trends in plating industry [1–6]. In electroplating the properties of the coated alloys are determined by their structure which in turn depends on the alloy composition, working electrolytes, the nature of plating substrate and deposition conditions [3]. The phase structure of an alloy is the major factor determining its corrosion resistance [3].
⁎ Corresponding author. E-mail address: [email protected] (O.P. Oladijo).
Zinc-Tin alloys are found to be attractive because they combine the barrier property of tin with the cathodic protection offered by zinc [4]. Each metal protects steel by a different mechanism. Tin is nobler than steel and under ordinary atmospheric exposure, protects steel by forming a corrosion resistant envelop around it. However, rusting takes place in pin holes or imperfections in the tin coating and is accelerated galvanically by the difference in potential between steel and tin [4]. At open circuit potential, Zn dissolution is the only corrosion reaction [4].The coatings (Zn-alloy) protect steel better than zinc only under conditions of high humidity and they resist corrosion equally when compared with an equal thickness of cadmium in marine and industrial atmosphere [4]. They also offered better solderability, brightness, ductility and service life with less white corrosion product formation when compared to zinc coatings only. Zn-alloys have better hardness, low hydrogen uptake and are cost effective [4]. Experience has established that electrodeposited tin-zinc alloy coatings of compositions containing 78% Sn and 22% Zn have excellent corrosion resistance. properties [5]. Humidity, salt-spray, hot-water porosity tests have been carried out on steel specimen coated with tin-zinc deposits of various compositions and thicknesses [5]. They reported that optimum tin content has proved to be 75–80%, 0.0003 in. (7.62 × 10−5 m) thickness of the alloy coating gives better protection than the same thickness of
http://dx.doi.org/10.1016/j.surfcoat.2017.04.029 0257-8972/© 2017 Elsevier B.V. All rights reserved.
Please cite this article as: O.P. Oladijo, et al., Surf. Coat. Technol. (2017), http://dx.doi.org/10.1016/j.surfcoat.2017.04.029
2
O.P. Oladijo et al. / Surface & Coatings Technology xxx (2017) xxx–xxx
cadmium or zinc. In addition it has also been found that alloy coatings containing less than 50% Sn have their corrosion resistance marked inferior to that of 78% Sn. Whilst the corrosion tests showed that under saltspray conditions the performance of tin-zinc coatings of 75–80% Sn was very good and superior to the 50 tin: 50 zinc composition [5]. Tin–zinc alloy coatings are recognized as a potential alternative to toxic cadmium (as corrosion resistant deposits) because they combine the barrier protection of tin with the cathodic protection afforded by zinc [5]. The coatings containing 20 wt% zinc, balance tin, offer excellent corrosion protection for steel and do not form gross voluminous white corrosion products like pure zinc or high zinc alloy deposits [5]. On the other hand, the corrosion behaviour of tin–zinc alloy electroplated coatings on steel has been investigated by Dubent et al. [5] using electrochemical methods in a 3 wt% NaCl solution and the salt spray test. The performance of the deposits was compared with cadmium and zinc–nickel electrodeposited coatings. They found that the corrosion resistance of tin–20 wt% zinc alloy coating is superior to that of cadmium and zinc–12 wt% nickel coatings. In recent years, a thermal spraying method that produces anti-corrosion layer on the surface of steel materials by melting Zn or Al, which has a relation to the sacrificial anode, using gas or electricity instead of the conventional corrosion resistance method, is attracting many researchers presently. Thus it is of great importance to select an appropriate thermal spraying method depending on the environment conditions. Sacrificial anode metals, such as tin, magnesium, zinc and aluminium have better corrosion resistance and have been under extensive research studies [6]. However there are few works that quantitatively evaluate the electrochemical performance of thermally sprayed coatings using these metals [6]. S. Schuerz et al. [7] investigated the corrosion behaviour of conventional hot-dip galvanized zinc coated and novel hot-dip galvanized Zn-Al-Mg alloy coated in standardized salt spray test. They found that corrosion proceeds very fast and steel substrate was attacked after 100 h of exposure for conventional hot-dip galvanized zinc coated. However novel hot-dip galvanized showed a different behaviour. This was attributed to the adherent aluminiumrich oxide layer which protects the steel substrate against corrosive attacks. Plasma spraying among the various thermal spraying techniques has attracted considerable attention for its extremely high temperature [8– 11]. A plasma sprayed coating is built up and the microstructure is formed, when individual, fully or partially molten particles, at a particular velocity, flatten, adhere and solidify on impact with the substrate [9]. The spraying parameters, substrate temperature, coating thickness, and surface roughness of substrate during deposition play an important role on the final properties of the coating [10,12]. Spraying distance is an important parameter that influenced on the hardness, porosity and the coating roughness of plasma spraying significantly [11]. Therefore this research work aims to enhancing surface properties of mild steel and extending the application of mild steel in industries. Oladijo et al. [13] reported that the control of residual stresses is a primary issue in coating technology. These are stresses that linger after deformation with all external forces removed. Several authors [13–15] have investigated the origin of residual stresses in coating. They reported that both material and deposition process may produce a residual stress. In the case of thermal spray coatings, quenching stresses due to rapid cooling of the coating, thermal mis-match stresses between the substrate and coating, phase transformation during deposition have been identified as sources. Residual stresses can be compressive or tensile depending on the direction of the forces [13,14]. Residual stresses play a fundamental function in materials as it can either boost or degrade performance. Non destructive stress determination is crucial since it offers the possibility of stress examination as a final quality check before service, or allows stress monitoring development during the components lifetime [15]. Specific to coatings, X-ray diffraction is a valuable technique to measure residual strain.
2. Material experiments 2.1. Plasma spraying process Powders mixed in the following ratios: Zn-Sn (25/75), Zn-Sn (50/ 50), and Zn-Sn (75/25) was used for this research work. The powders used were obtained from TLS Technik-Gmbh & Co, Spezialpulver Germany. The equipment used for coating was a 9MC plasma control unit (Sulzer Metco) with argon/nitrogen and helium as inert gases. It consist of a hoop powder feed unit (9MP) and a robot that controls the position and angle of the spray gun at a speed of 400 mm/s. Before deposition, the substrate was grit-blasted using alumina grits with the aim of cleaning and to roughen the specimen surface. The powder was fed into a hopper and Mild steel bar was clamped/placed and tightened on a stand inside the plasma spray booth. The spray gun, with aid of a flame deposits the powder of interest on the surface of the substrate at approximately from 300 to 3000° at different intervals until the required coating thickness of 200 μm is achieved. During the spraying, the gun is stopped frequently to avoid over heating the substrate. Hydrogen (11 L/min) argon (175 L/min) was used as primary and secondary inert gases respectively. The coatings were deposited at a distance of 100 mm and powder feed rate was 32 g/min. 2.2. Coating microstructure The coating cross-sectional part was grind using Struers Tegra Pol21 with MD piano 250 mm diamond grinding disk, followed by 500 and 1000 μm SiC paper. Polishing was done in a 3 step using water based diamond suspensions Allegro largo 9 μm, MD Mol 3 μm and MD Nap 1 μm. Specimens were etched in a Nital to enhance microstructural features such as grain size and phases. Characterization of cross-sections coated samples prior to, and after corrosion tests was carried out using scanning electron microscopy with Energy Dispersive X-Ray Spectroscopy (EDX). Coating hardness was obtained using a Vickers hardness tester at an applied load of 300 g and a 15 s dwell time. Five measurements were made for each cross sectional coating and the average hardness value determined. Porosity measurements were done using dot counting method for SEM images of 5000 ×. The X-ray diffraction (XRD) was conducted for the coatings with Cu-Kα radiation at 40 kV and 20 mA on D8 Discovery. 2.3. Residual stress Residual stress in thermal sprayed coatings may be measured by many techniques, which may be categorized into two methods namely; destructive method and non-destructive method. In the present research, non destructive method was used to measure the residual stress in the thermal spray coated sample, as it gives reliable results of the final residual stress of the material. In addition, no surface preparation is required for this measurement. But the limitation is that X-ray penetration is extremely shallow as it analyse a surface layer whose thickness depends both on the sample material and the incident beam wavelength. The primary side of the equipment consists of a graphite monochromator, a collimator and Cu target (wavelength of 1.544Ǻ). For this particular investigation a 0,8 mm collimator was used. The strain measurements were done on the Zn phase of the Zn-Sn coatings using the Cu-Ka1 radiation with the generator settings of 40 kV and 40 mA. The diffraction data was collected using area detector Vantec500 with no beam optics on the secondary side. Strain measurements were done on the (211) reflections corresponding Bragg peak of ~ 127.492° (2θ). For a complete stress tensor investigation, samples were measured at 6 different azimuth orientation ϕ = 0°, 180°, 90°, 270°, 45°, and 225° with each azimuth measured at 8 different tilt angles at incremental step of 10°. Rotation of the azimuth orientation by 180° allowed measurement in both −ψ and +ψ tilt angles. Measurements were done in side-inclination mode. The data analysis was analysed using Bruker's
Please cite this article as: O.P. Oladijo, et al., Surf. Coat. Technol. (2017), http://dx.doi.org/10.1016/j.surfcoat.2017.04.029
O.P. Oladijo et al. / Surface & Coatings Technology xxx (2017) xxx–xxx
3
Fig. 1. SEM/EDS micrographs of Zn-Sn Powder at higher and lower magnification.
Leptos v6 software. The strain measurements were then converted to stress measurements by using the 211 reflection of Zn with the following elastic constants: S1 = −2.778 × 10−6 MPa, 1/2S2 = 1.204 × 10−5 MPa and Poison's ratio = 0.30. The techniques employed here are similar to the work reported by Oladijo et al. [13].
2.4. Electrochemical tests Comparative electrochemical studies between the Zn-Sn coated and as-received mild steel specimens was carried out using linear polarization technique with the aid of Autolab potential (PGSTAT30). The
Potentiodynamic polarization curve known as a tafel plot, depicts the relationship between potential and corrosion current density. This plot exhibits a linear region, the slope of which is known as the Tafel constants (anodic and cathodic Tafel constants). The intersection of the projection of the linear region of the plot with the open circuit potential (Ecorr) gives the cathodic or anodic corrosion current (Icorr). Once Icorr is determined, the following equation derived from Faraday's law was used to calculate the corrosion rate [16]: corrosion rate
mm 3:27 Icorr EW ¼ yr D
Fig. 2. SEM micrograph of (a) Zn-Sn 25/75, (b) Zn-Sn 50/50 & (c) Zn-Sn 75/25 coating.
Fig. 3. XRD patterns of the as-sprayed Zn-Sn alloy coating.
Please cite this article as: O.P. Oladijo, et al., Surf. Coat. Technol. (2017), http://dx.doi.org/10.1016/j.surfcoat.2017.04.029
ð1Þ
4
O.P. Oladijo et al. / Surface & Coatings Technology xxx (2017) xxx–xxx
Fig. 5. Residual stress results of Zn-Sn coating on mild steel substrate.
4. Discussions
Fig. 4. Hardness of the Zn-Sn coatings and the substrate
The corrosion rate in Eq. (1) is expressed in micrometers per year, μm/y. Icorr is the corrosion current density in μA·cm−2, obtained by dividing Icorr with the exposed surface area of the measured specimen. E.W. is the equivalent weight of steel in g, and d is the density of steel in g/cm3. The polarization resistance Rp (which is the slope of the potential current curve at Ecorr), is related to Icorr through the following Stern-Geary relationship [16]:
Icorr ¼
½βa βc 2:3 ðβa þ βcÞ Rp
ð2Þ
3. Results The morphology of the Zn-Sn powder characterized by SEM depicted in Fig. 1, which were irregular in shape. Fig. 2 shows the microstructure of the Zn-Sn coating, which consist of Zn grain, microcracks as well as some pits. XRD pattern of the as-sprayed coating were shown in Fig. 3. The XRD results revealed that the coating consist of Zn, ZnO, Sn and other complex oxide. Fig. 4 shows the hardness values of the substrate and the coatings, which were all different. The residual stresses determined under the assumption of planar stress conditions are given in Table 1 and Fig. 5. The residual stresses have been determined to be compressive in the as-sprayed Zn-Sn coating, and were relatively low valued. Potentiodynamic polarization curves and electrochemical values of plasma sprayed Zn-Sn coatings under different composition in H2SO4 are shown in Fig. 6 and Table 2 respectively. It is observed that all the Zn-Sn coating exhibit similar polarization curve shapes in the tested solution. Fig. 7 and Table 3 shows the polarization curve shape tested in 3.65% sodium chloride solution. The curve was look similar and different from one another. The representative SEM micrographs taken from the surface of all coating layers after potentiodynamic polarization tests in 1 M H2SO4 and 3.65% NaCl are shown in Figs. 8 and 9 respectively. They were all remarkable and different due to the influence of the test solution.
The typical morphology of the Zn-Sn powder (Fig. 1) was simple and irregular in shape, as they are neither disc nor circle. The powder showed densely structure with Zn cluster cemented by Sn as confirmed by EDX. The SEM/BSE micrographs of the Zn-Sn coatings cross sections are shown in Fig. 2. The coating microstructures were all different despite same substrate and spraying parameters, although Zn-Sn coatings seemed to form an inmate and homogenous mixture. This microstructural difference was attributed to differences in their powder composition, which play an important role in the cooling and solidification of coating after spraying. In addition, the micrographs was characterizing by low porosity, light grain with particle boundaries barely visible. EDX analyses revealed that the coating comprises of Zn, Sn and oxygen (low). Hence partial oxidation likely occurs. Fig. 3 shows the comparison of the as-sprayed Zn-Sn alloy coating on mild steel substrates. Based on the XRD spectra, all coating seem to retain both powder microstructure and secondary phases proving undesirable reaction occurred at high temperature of plasma chamber. The phase identification revealed the presence of Zn, Sn ZnO and ZnSnO3 (larger amount) on all the as-sprayed coating. However, low intensity peak of complex oxide phase (ZnSn(OH)6) was found at 2 theta = 22° and 53° suggesting that phase transformation occur during plasma spraying process. It is thought that the formation of complex oxide (eta phase) suggested that diffusion occur between the substrate and the. Overall, there was also a variation in the XRD intensities of this complex oxide or eta phase (i.e. ZnO, ZnSnO3, ZnSn(OH)6). This could be due to differences in percentage composition of the feedstock powder and residual stress effects since their substrate were the same. Moreover, there was a slight increase in background of the Zn-Sn alloy coating (at a diffraction angle of 2 theta = 15° to 20°). This was due to the following reasons: (i) Micro-strain effect in the coating as a result of the high temperature associated with plasma process. (ii) Grain refinement (iii) Severe plastic deformation of the particle upon impact the as-received mild steel substrate A significant increase in the micro hardness value (Fig. 4) was observed in the Zn-Sn 50/50, Zn-Sn 75/25, & Zn-Sn 25/75 coating respectively. The micro hardness value for mild steel and the Zn-Sn coatings can be arranged in ascending order as follows: Mild steel b Zn-Sn 25/
Table 1 Coating properties in as-coated condition. Coated sample
r11 (MPa)
r22 (MPa)
Residual stress (average) (MPa)
Hardness value (Hv)
Zn-Sn 25/75 Zn-Sn 50/50 Zn-Sn 75/25
−20.3 ± 57.9 −19.5 ± 7.4 −17.7 ± 7.1
−55.3 ± 57.8 −17.4 ± 7.3 −31.1 ± 7.0
−37.8 ± 57.8 −18.45 ± 7.3 −24.4 ± 7.0
474.4 ± 0.01 529.4 ± 0.02 502.7 ± 0.01
Please cite this article as: O.P. Oladijo, et al., Surf. Coat. Technol. (2017), http://dx.doi.org/10.1016/j.surfcoat.2017.04.029
O.P. Oladijo et al. / Surface & Coatings Technology xxx (2017) xxx–xxx
5
Fig. 6. Potentiodynamic polarization curves for as-received and Zn-Sn coatings in 1M H2SO4.
75 b Zn-Sn 75/25 b Zn-Sn 50/50. The hardness of the coatings was higher than the as-received sample as expected. This confirmed the significant or important that coating render the surface of engineering material. In addition the coated Zn-Sn 50/50 has the highest hardness value while Zn-Sn 25/75 has the lowest hardness value. The difference in their hardness of the coating might be due to minor diffusion between coating and substrate. The increase in hardness of the Zn-Sn 50/50 coated specimen was due to the presence of the higher concentration of tin as the reinforcement additive. Perhaps could be due to the low values of porosity and compressive residual stress respectively (Fig. 5). It is thought that the high peak intensities of ZnSO3 and ZnSn(OH)6 at 2 theta = 22° could have added as advantage (although the exact amount on the coating surface does not quantify). However the reduction in the hardness of Zn-Sn 75/25 coating (about 53% reductions) and Zn-Sn 25/ 75 (about 11% reductions) could be due to the composition of their feedstock powder. Overall the reduction in the hardness can be ascribed to the increased porosity level, micro crack present with the coating as well as weak bonding. Residual stress effect maybe either beneficial or detrimental depending on upon the sign and stresses distribution with respect to some external factors. The experimental results of the residual stresses found on the coated samples (Fig. 5) were compressive stress, but different from each other due to powder composition used (i.e. Zn and Sn). It is thought that a compressive stress has a beneficial effect on the coating adhesion and mechanical properties of a coating. The compressive stresses increases with higher content of Zn and Sn alloy powder, but decreased when the ratio of two powders were equal. With higher Sn content (75%) the compressive stress is higher than the other coatings. The variation in the residual stresses could be due to the following mechanism: (1) Melting behaviour of the spray particles in the hot gas jet since the powder compositions were varies [15]. (2) Difference in the surface roughness of the coating which was slightly difference due to their differences in powder composition which might play important roles in the melting of charge
particles inside the vacuum. It is thought that surface roughness also contributed to the residual stresses, especially in the attenuation of beam into the top bond coat of the coating. (3) The rate of cooling after spraying can significantly influence the residual stress profile in the coating, which was not considered here, because the cooling rates were assumed to be the same [15]. (4) The exact measuring position used for the measurement since their surface roughnesses were assumes not the same. (5) The differences in the compressive stress was deduced to be due to different in coefficient of thermal expansion, interaction between the coating and substrate which had been discussed by the current author elsewhere [13–15]. Also, moderate compressive stresses found on the as-sprayed coated samples reflect the dominance of peening mechanism of stress (caused by grit-blasting) over thermal quenching. Furthermore, the compressive residual stress observed on the as-sprayed Zn-Sn 25/75 was higher than other as-sprayed coating. This could be due to the homogeneous microstructure of the coating, as well as the surface hardening caused by a grit-blasted surface. In addition, a relatively numerical fitting (i.e. error bar) was found on the coated sample (Table 1 and Fig. 5). These error bars symbolize the minimum and maximum rages in which residual stress condition can be attained. However relatively large fitting (error bar) found on the as-sprayed Zn-Sn 25/75 in comparison to the others coated sample suggested that significant accuracy improvement can be achieved by increasing the intensity through the measurement time. The calculated penetrating depth of Cu–Kα radiation in Zn coating is 26.5 μm, suggested that the residual stress result cover several micrometers deep into the coating. This value was calculated based on [13,17]. However the results obtained by X-ray diffraction analysis were unable to highlight the differences of stress variation across the as-sprayed coating/ substrate due to the very low penetration depth. Therefore further studies will be conducted with the neutron diffraction measurement to compliments the X-ray diffraction results.
Table 2 Summary of potentiodynamic polarization results in 1M H2SO4 solution at ambient temperature. Sample
Ecorr (v)
Icorr (A/cm2)
Bc (v/dec)
Ba (v/dec)
CR (mm/yr)
Rp (Ω)
Substrate Zn-Sn 25/75 Zn-Sn 50/50 Zn-Sn 75/25
−0.29247 −0.24722 −0.26659 −0.19539
5.2411 × 10−3 8.166 × 10−5 1.1626 × 10−3 4.3581 × 10−4
2.261 0.031384 0.31409 0.032842
2.419 0.20822 0.31508 0.075225
60.9156 0.9491 13.5132 5.0653
96.947 145.2 58.82 22.807
Please cite this article as: O.P. Oladijo, et al., Surf. Coat. Technol. (2017), http://dx.doi.org/10.1016/j.surfcoat.2017.04.029
6
O.P. Oladijo et al. / Surface & Coatings Technology xxx (2017) xxx–xxx
Fig. 7. Potentiodynamic polarization curves for as-received and Zn-Sn coatings in 3.65% NaCl solution.
Furthermore, it is of no doubt that the compressive residual stress measured tend to counteract the Vickers hardness indenter from deeper penetration into Zn-Sn coating resulting in high hardness [18]. Therefore, compressive residual stress plays a crucial role in determine the mechanical properties of a coated sample. The propensity of mild steel to corrosion dilapidation in 3.65% NaCl and 1 M H2SO4 was examined using the linear polarization technique. Fig. 6 shows the linear polarization curve of the substrate and coated samples in 1 M H2SO4. The Potentiodynamic polarization data from the Tafel plot are presented in Table 2. A vast corrosion degradation of 60.91 mm/year was discovered as a result of deficient surface protection from the experimental analysis. The substrate presents a high current density of 5.241 × 10−3 A/cm2 signifying hastening behaviour for the metal dissolution. The corrosion potential of the sample was −0.29247 V. Through observation and research, the disastrous damage of the substrate by aggressive sulphate ion (SO2− 4 ) took place consistently. Also the substrate had less passive film created on its surface ensuing severe corrosion attack from the sulphuric acid solution. Furthermore, since the substrate lacked surface protection, a high rate of corrosion was expected, as lack of surface protection has been substantiated to stimulate sever deterioration [19]. When mild steel corrodes, there is usually a loss of metal to a solution in some form, by a product –favored oxidation-reduction reaction. The following mechanism for the corrosion of iron and steel in acid solution has been proposed [20–22]. The anodic iron dissolution: Fe þ An− → FeAn− ads
ð1Þ
FeAn− ads→ FeAn− ads þ ne−
ð2Þ
FeAn− ads→ FeAnþ ads þ ne−
ð3Þ
FeAnþ ads → Fe2þ ads þ An−
ð4Þ
The cathodic hydrogen evolution: Fe þ H þ → FeH þ ads
ð5Þ
FeH þ ads þ ne− →ðFeH Þads FeH
þ
ð6Þ
ads þ H þ þ ne− →Fe þ H 2
ð7Þ
In the absence of a protective coating in acid solutions, the mild steel surface becomes extremely corroded with areas of pitting corrosion. This phenomenon is often intensified in the presence of chlorides by unleashing severe attack in coatings [23]. Pitting corrosion takes advantage of the different metallurgical phases present on the surface of most common alloys such as mild steel. Pitting occurs as a result of localized anodic dissolution where the anodic portion of corrosion cell is dominated by the larger cathodic portion. Under common operating conditions, oxide films are formed on the iron surface. These oxide films mostly present brown rust (hematite, (γ -Fe2O3)), a black rust (Fe3O4) and yellow rust (FeO(OH)H2O). Table 2 also shows the corrosion current density (Icorr), corrosion rate (Cr) and polarization resistance (Rp) values of each coated sample. Zn-Sn 25/75 exhibited the highest polarization resistance, lowest corrosion rate and corrosion current density compared to the other Zn-Sn coatings and the substrate in 1 M sulphuric acid solution. The Zn-Sn 50/50 and Zn-Sn 75/25 coating exhibited higher polarization resistance, lower corrosion rates and corrosion currently densities than the substrate respectively. Metal exposed to sulphuric acid, in this case 1 M H2SO4 without coating shows the presence of carbon, oxygen and iron suggesting the formation of a compound including iron oxide/hydroxide and carbon [24]. Iron oxide (rust) developed on the surface which is not protective because it doesn't form a continuous, adherent film. Instead, it spalls, exposing fresh iron to the atmosphere which in turn allows more corrosion to occur [24]. It can be seen that the substrate exhibited a corrosion rate (mm/yr) which is 63 times that of the ZnSn 25/75. Lower corrosion rate of the coated samples is attributed to the formation of zinc sulphate (ZnSO4) which dissolves the protective zinc hydroxide layers (ZnOH2). With increased concentration of Sn in
Table 3 Summary of potentiodynamic polarization results in 3.65% NaCl solution at ambient temperature. Sample
Ecorr (V)
Icorr (A/cm2)
Bc (V/dec)
Ba (V/dec)
CR(mm/yr)
Rp (Ω)
Substrate Zn-Sn 25/75 Zn-Sn 50/50 Zn-Sn 75/25
−0.3383 −1.0608 −1.1017 −0.68001
2.39 × 10–2 5.24 × 10–4 4.44 × 10–5 9.29 × 10–4
0.218 0.13901 0.14656 0.33085
0.216 0.084413 0.045705 0.24792
278.514 6.0554 0.51604 10.797466
1.968 43.49 354.71 66.288
Please cite this article as: O.P. Oladijo, et al., Surf. Coat. Technol. (2017), http://dx.doi.org/10.1016/j.surfcoat.2017.04.029
O.P. Oladijo et al. / Surface & Coatings Technology xxx (2017) xxx–xxx
7
Fig. 8. SEM micrograph of (a) Zn-Sn 25/75, (b) Zn-Sn 50/50& (c) Zn-Sn 75/25 in 1 M H2SO4 solution.
the coatings, the corrosion potential was significantly low. Passivation of Sn in alloys containing over 70% of Zn occurred at more negative potentials. This could be due to the formation of a non-equilibrium surface layer enriched in Sn and structural defects as a result of selective dissolution of Zn as reported by Korobov et al. [24]. In addition, Zn-Sn coatings did not suffer horrible corrosion attack from sulphate ions when compared with the great degradation done on the substrate. According to electrochemical principles, the corrosion rate is proportional to the corrosion current density, and reduced rate in corrosion current density will enhance passivity of the sample as reported by [19]. Perhaps excellent improvement in the corrosion resistance of the alloyed samples could be due to homogeneous oxide films and stable coating formed in the coating as showed by the XRD (Fig. 3). The passivation behaviour is also due to the formation of adherent corrosion products, certainly consisting of very stable oxides and hydroxides of zinc. A good decrease in corrosion current density may also be due to the formation of ZnO. The phases, vismirnovite (ZnSn(OH)6), zinc oxide (ZnO) and zinc stannate in the Zn-Sn coatings led to the formation of a more densely packed corrosion product layer on the coating surface, which enhanced the corrosion resistance [19]. Table 3 and Fig. 7 show the corrosion behaviour of the substrate and coatings in 3.65% sodium chloride solution. The substrate exhibited a considerably high corrosion rate (mm/yr) which is enormously higher than the coating. This is attributed to the absence of a passive layer/ film on its surface. However the substrate also exhibited the lowest polarization resistance and higher corrosion current density compared to the coatings, thus more prone to corrosion attack/susceptible to corrosion. In a study conducted by Fayomi and popoola et al. [25], mild steel corrosion behaviour was studied in 3.65% sodium chloride using open circuit potential scans. The potential was observed to shift towards the negative region indicating strong dissolution of the mild steel due to the absence of passivation. Among the coatings, Zn-Sn 50/50 had the highest polarization resistance, lower corrosion rate and current density. Zn-Sn 75/25 exhibited a higher polarization resistance and corrosion current density than Zn-Sn 25/75 whereas its corrosion rate was found to be high which was not expected. It is thought that a higher polarization resistance will also result in low corrosion rate but it was not the case. From the Tafel values it
can be seen that the coated values exhibited polarization resistances significantly higher than that of the substrate and corrosion rates which are considerably lower than the substrate. This is because the presence of Zn and Sn both combine the barrier property of tin with the cathodic protection offered by zinc [4]. Each metal protects steel by a different mechanism. Tin is nobler than steel and under ordinary atmospheric exposure, protects steel by forming corrosion resistant envelop around it [4]. The microstructures of the corroded samples were highly corroded after immersion inside different medium (Figs. 8 and 9). The microstructures revealed some pitting/pores on the samples after immersion in corrosion media which was due to the aggressive sulphate ion (SO42−) and chloride ion (Cl−). The Zn-Sn coating in general did not suffer horrible corrosion attack from the chloride and sulphate ion as compared to the substrate. 5. Conclusions Deposition Zn-Sn alloys was successfully carried out on mild steel surface by plasma spraying technique. The coated Zn-Sn samples in different ratio were carried out on the surface of mild steel substrate. The substrate exhibited corrosion rate considerably higher that the coatings. A substantial advancement in corrosion resistance was attained in the coated specimens. In 1M H2SO4 solution, the best corrosion resistance was exhibited by Zn-Sn 25/75, Zn-Sn 75/25 and Zn-Sn 50/50 respectively. In 3.65% NaCl solution the highest corrosion resistance was exhibited by Zn-Sn 50/50, followed by Zn-Sn 25/75 and Zn-Sn 75/25 coating respectively. The micro hardness values show that Zn-Sn coatings had improved hardness values as compared to the substrate. Zn-Sn 50/50 had the highest hardness value compared to the other coatings. The residual stresses of the Zn-Sn coating were all compressive in nature and of relatively lower valued. Acknowledgements The authors wish to acknowledge the financial and technical support received from National Research Foundation, South Africa (91027) and Tshwane University of Technology. The authors are also thankful to Mr.
Fig. 9. SEM micrograph of (a) Zn-Sn 25/75, (b) Zn-Sn 50/50 & (c) Zn-Sn 75/25 in 3.65% NaCl solution.
Please cite this article as: O.P. Oladijo, et al., Surf. Coat. Technol. (2017), http://dx.doi.org/10.1016/j.surfcoat.2017.04.029
8
O.P. Oladijo et al. / Surface & Coatings Technology xxx (2017) xxx–xxx
Tshepo Ntsoane from Necsa (Residual stress analyser), Surface Engineering Research Center TUT (Mr. Fatoba and Joy Matsilele) and Dr. Oluwasayo (UJ) for their great assistance and contributions. References [1] H. Kazimierczak, P. Ozga, K. Berent, M. Kot, Microstructure and micromechanical properties electrodeposited of Zn-Mo coatings on steel, J. Alloys Compd. 636 (2015) 156–164. [2] F. Fontenay, Electrodeposited zinc and zinc alloy coatings and their corrosion resistance. Part 1, Galvanotechnik 93 (2002) 2534–2541. [3] V.S. Vasantha, M. Pushpavanam, V.S. Muralidharan, Tin-zinc alloy electrodepositphysicochemical characterisation, Ind. J. Chem. Technol. 7 (2000) 216–222. [4] S. Dubent, M. Mertens, M. Saurat, Electrodeposition, characterization and corrosion behaviour of tin-20wt% zinc coatings electroplated from a non-cyanide alkaline bath, Mater. Chem. Phys. 120 (2010) 371–380. [5] S.K. Ray, Electro-Deposition of Tin-Zinc Alloy as a Protective Coating of Ferrous Metals. Ing: Non-Ferrous Metal Industry in India, 1957 266–271. [6] Hong-Bok Choe, Jun-Ho Shin Han-Scung, Experimental study on the electrochemical anti-corrosion properties of steel structures applying the arc thermal metal spraying method, Materials 7 (2014) 7722–7736. [7] S. Schuerz, M. Fleischanderl, G.H. Luckeneder, K. Preis, T. Haunschmied, G. Mori, A.C. Kneissl, Corrosion behaviour of Zn-Al-Mg coated steel sheet in sodium chloridecontainning environment, Corros. Sci. 51 (2009) 2355–2363. [8] A. Satapathy, H. Sutar, S.C. Mishra, S.K. Sahoo, Characterization of plasma sprayed pure red mud coatings: an analysis, Am. Chem. Sci. J. 3 (2) (2013) 151–163. [9] Z. Yina, S. Tao, X. Zhou, Effect of the thickness on properties of Al2O3 coatings deposited by plasma spraying, Mater. Charact. 62 (2011) 90–93. [10] O. Sarikaya, Effect of some parameters on microstructure and hardness of alumina coatings prepared by the air plasma spraying process, Surf. Coat. Technol. 190 (2005) 388–393. [11] T.S. Sidhu, S. Prakash, R.D. Agrwal, Studies on the properties of high-velocity oxyfuel thermal spray coatings for higher temperature applications, Mater. Sci. 41 (2005) 6. [12] Y. Gao, X. Xu, Z. Yan, G. Xin, High hardness alumina coatings prepared by low power plasma spraying, Surf. Coat. Technol. 154 (2002) 189–193.
[13] O.P. Oladijo, A.M. Venter, L.A. Cornish, N. Sacks, X-ray diffraction measurement of residual stress in WC-co thermally sprayed on to metal substrates, Surface & Coatings Technology 206 (2012) 4725–4729. [14] A.M. Venter, O.P. Oladijo, V. Luzin, L.A. Cornish, N. Sacks, Performance characterization of metallic substrates coated by HVOF WC-Co, Thin Solid Films 549 (2013) 330–339. [15] O.P. Oladijo, A.M. Venter, L.A. Cornish, Correlation between residual stress and abrasive wear of WC-17Co coatings, Int. J. Refract. Met. Hard Mater. 44 (2014) 68–76. [16] M. Geary, A.L. Geary, Electrochemical polarization, No 1 theoretical analysis of the shape of polarization curves, J. Electrochem. Soc. 104 (1957) 56–57. [17] I.C. Noyan, J.B. Cohen, Residual Stress Measurement by Diffraction and Interpretation, Springer, New York, 1987. [18] A. Milanti, V. Matikainen, H. Koivuluoto, G. Bolelli, L. Lusvarghi, P. Vuoristo, Effect of spraying parameters on the Microstructurural and corrosion properties of HVAFsprayed Fe-Cr-Ni-B-C coating, Surf. Coat. Technol. 277 (2015) 81–90. [19] O.S. Fatoba, A.P.I. Popoola, T. Fedotova, Characterization and corrosion behaviour of Zn-Sn binary alloy coatings in 0.5 M H2SO4 solution, J. Electrochem. Sci. Technol. 6 (2) (2015) 65–74. [20] M.S. Morad, A.M. Kamal El-dean, 22′-Dithiobis (3-cyano-4-6dimethylpyridine) a new class of acid corrosion inhibitors for mild steel, Corros. Sci. 48 (2006) 3398–3412. [21] K. Tebbji, H. Oudda, B. Hammouti, M. Benkaddour, M. El Kodadi, F. Malek, A. Ramdani, Inhibitive action of two bipyrazolic isomers towards corrosion of steel in 1 M HCl, Appl. Surf. Sci. 241 (2005) 326–334. [22] A. Yurt, A. Balaban, S.U. Kandemir, G. Bereket, B. Erk, Investigation on some Schiff bases as HCl corrosion inhibitors for carbon steel, Mater. Chem. Phys. 854 (2004) 20–426. [23] R.T. Loto, Pitting corrosion evaluation of austenitic stainless steel type 304 in acid and chloride media, J. Mater. Environ. Sci. 4 (4) (2013) 448–459. [24] V.I. Korobov, L.V. Petrenko, V.V. Poltavelts, Effect of phase composition on the anodic dissolution and passivation of zinc-based alloys, Univ. J. Chem. 2 (5) (2014) 76–85. [25] A.P.I. Popoola, O.S. Fayomi, A.P.I. Popoola, Electrochemical and Mechanical Properties of Mild Steel Electro-plated with Zn-Al, Int. J. Electrochem. Sci. 7 (2012) 4898–4917.
Please cite this article as: O.P. Oladijo, et al., Surf. Coat. Technol. (2017), http://dx.doi.org/10.1016/j.surfcoat.2017.04.029