Effect of Al₂O₃ and NiO Nanoparticle Additions on the Structure and Corrosion Behavior of Sn—4% Zn Alloy Coating Carbon Steel

Abstract

The application of a higher corrosion resistance coating modified with nano additions can effectively decrease or prevent corrosion from occurring. In the present work, a novel method is successfully developed for the modification of carbon steel surfaces aiming for high corrosion resistance using Sn—4% Zn alloy/nanoparticle composite (NiO+ Al₂O₃) coating. Sn—4% Zn alloy/nanoparticle composite (NiO+ Al₂O₃) coatings were deposed on carbon steel using a direct tinning process that involved a power mixture of Sn—4% Zn alloy along with a flux mixture. Regular coating and interface structures were achieved by individual Al₂O₃ and both NiO and Al₂O₃ nanoparticle combined additions in the Sn-Zn coating. The maximum coating thickness of 70 ± 1.8 µm was achieved for Al₂O₃ nanoparticles in the Sn-Zn coating. Interfacial intermetallic layer thickness decreased with all used nanoparticle additions in individual and hybrid conditions. The minimum intermetallic layer thickness of about 2.29 ± 0.28 µm was achieved for Al₂O₃ nanoparticles in the Sn—Zn coating. Polarization and impedance measurements were used to investigate the influence of the incorporated Al₂O₃, NiO, and hybrid Al₂O₃/NiO nanoparticles on the passivation of the low-carbon steel (LCS) corrosion and the coated Sn—Zn LCS in sodium chloride solution. It was found that the presence of Al₂O₃, NiO, and Al₂O₃/NiO nanoparticles remarkably improved the corrosion resistance. The corrosion measurements confirmed that the corrosion resistance of the coated Sn-Zn carbon steel was increased in the presence of these nanoparticles in the following order: Al₂O₃/NiO > NiO > Al₂O₃.

1. Introduction

Surface treatments are used in different ways to improve the material surface properties of a component. The designers may consider surface-treated components as an alternative to more costly materials. The purpose of these surface-metal treatments is to eliminate harmful oxidation processes and increase their durability. Low resistance to atmospheric corrosion is a major disadvantage of some steels. Chemical heat treatment of steel is required to enhance steel’s corrosion. Such treatment will result in the formation of single- or multi-component coatings [1] that are environmentally resistant without having a negative impact on the other properties of the surface. The electroplating and electrolysis techniques are two major coating techniques in the solution state-coating process [2,3] of carburization [4], nitriding [5], and carbonitriding [6]. However, after nitriding or carburizing, steels with low operating temperatures and low corrosion resistance can be produced. A metallic coating can be applied as a surface treatment in a variety of ways to improve a material’s mechanical [7], electrochemical [8,9], and thermal performance [10]. The method used is largely determined by the material’s application; for example, a high-temperature oxidation resistance [11], the protection of gas turbine blade components, such as cooling channels [12], improvement of corrosion resistance [13,14], and welded can material [15]. For all these applications, the desired coating depth or surface treatment depth can be controlled.

Coatings with nanomaterial additives are very assorted. Usually, nanomaterials are added to a coating material in minor percentages to modify a large variety of properties [16,17]. The aim of improving the protective effect of coatings or creating novel properties in coatings by adding small quantities of nano additives will be the new approach of nanotechnology in the corrosion resistance coating area [16]. Typically, coatings made of metals that are more electrochemically noble than the substrate can only serve as a passive barrier. Examples of this latter kind of metallic coating include pure aluminum layers on copper-rich aluminum alloys [11,12,18] and zinc-based coatings on a steel substrate [19,20].

Nanomaterials are used to achieve greater opacity, enhanced coating-surface interaction, and superior coating durability. Some nanomaterials are suitable for use in transparent coating systems because their particle sizes are less than 100 nm. The material and processing properties of nanomaterial-containing coatings are significantly superior to those of conventional coatings, such as high resistance to indentation, high elasticity, rapid drying, dimensional stability upon contact with water, and high permeability of water vapor [15,16,17].

Low-carbon steel (LCS) has a carbon content of between 0.15% and 0.25% and is utilized extensively in engineering and industry applications. Higher ductility and lower tensile strength of low-carbon steel result from its lower carbon content. Surface improvements of low-carbon steel are a relatively more economical fabrication process compared with alloy steels that contain iron, carbon, and other expensive alloying elements. It was reported [18,19] that the environment around low-carbon steel surfaces has a significant impact on the material’s performance, resulting in changing its physical properties and causing damage to the component. As a result, the application of additional elements in the form of diffusion layers via surface coating is an efficient method for safeguarding metal and enhancing its surface properties.

To achieve sustainability goals, the surface of a substrate is coated to decrease or prevent corrosion. This coating process reaped a number of benefits that minimize adverse effects on material resources, environmental quality, and human health and safety. Using a high-performance non-toxic coating with smart additives prevents inefficient and wasteful energy and materials.

Coatings with nano-sized material additives, such as Al₂O₃, SiO₂, SiC, CuO, ZrO₂, and TiO₂, are utilized. The coating typically improves the corrosion resistance of materials through one of three primary mechanisms: inhibition, cathodic protection, or barrier influence. Zinc is considered the most used coating element used to protect steel due to being readily available, providing excellent cathodic protection, being inexpensive, and being friendly to the environment. The majority of researchers used Zn to coat low-carbon steel with Sn—Zn alloys with a high tin content to prevent oxidation [20,21,22,23]. It was reported [24] that the addition of NiO and ZnO nanoparticles to pure tin coatings significantly modified the coating and interface structures as well as the surface corrosion resistance of low-carbon steel. It was reported [16] that the corrosion resistance of Sn—alumina nanoparticle composite surface coating mild steel was improved compared with Sn-coating only. However, the alumina nanoparticle addition to Sn should be kept at a lower percentage (≤0.50 wt.%) in order to achieve both minimal coating defects and higher corrosion resistance for mild steel.

Sn—Zn alloys are promising surface coating alloys due to their combination of the barrier property of Sn as well as the cathodic protection of zinc. Although tin has higher corrosion resistance than carbon steel, the imperfection formations in the surface and/or interface, tin coating/steel accelerate the galvanic cell between Sn and steel [24]. Zn content in Sn—Zn alloys has a significant effect on the crack and pore formation in the resulting corrosion products, whereas, by increasing Zn concentration, the anions transport from the outer layer through the Zn-rich grains. This transportation decreases the corrosion resistance in Sn—Zn coatings. These negative transportation effects can be alleviated by decreasing the Zn content in the alloys Sn—Zn [25].

Accordingly, it is anticipated that the addition of Al₂O₃ and NiO nanoparticles to Sn–Zn alloy will strengthen the Sn—Zn coating/ steel interface bond and enhance resistance to corrosion in an Sn—Zn coating. The Zn addition to Sn is based on its high bonding strength between the coating layer and steel substrate and its high reactivity with Fe. The current study will investigate how the addition of (NiO and Al₂O₃) affects the corrosion behavior and surface properties of low—carbon steel coated with Sn—Zn alloy.

2. Experimental

2.1. Sample Preparation before Coating

Samples of the approximate dimension of 4.7 × 35 × 35 mm³ were cut from a 4.7 mm thickness of low-carbon steel (LCS) sheet (obtained from the local producing company, Riyadh, KSA) and used as the substrate material for this study. The chemical composition (wt.%) of the LCS is shown in

Table 1. Chemical composition of carbon steel substrate, wt.%.

Chemical composition	C	Si	Mn	Cu	Cr	Ni	Al	Fe
Carbon Steel	0.09	0.02	0.32	0.03	0.02	0.05	0.04	Bal.

. All the samples were ground with emery papers of up to 1200 grades.

2.2. Flux Preparation and Coating

After grinding and cleaning, the samples were coated using a direct tinning technique with Sn—4%Zn alloy and Sn—4% Zn reinforced with nanoparticles of Al₂O₃ (0.25 wt.%), NiO (0.25 wt.%), and ZnO/NiO mixture (0.25 wt.%) each.

Table 2. Definitions of the coated carbon steel, wt.%.

Sample	Definitions
Sample 1 (S1)	Low-carbon steel (LCS)
Sample 2 (S2)	LCS + (Sn—4% Zn) alloy coating
Sample 3 (S3)	LCS + (Sn—4% Zn) with 0. 25 wt.% Al2O3 nanoparticle addition alloy coating
Sample 4 (S4)	LCS + (Sn—4% Zn) with 0.25 wt.% NiO nanoparticle addition alloy coating
Sample 5 (S5)	LCS + (Sn—4% Zn) with (0.25 wt.% Al2O3 + 0.25 wt% NiO) nanoparticle addition alloy coating

provides a summary of the sample definitions, which range from the bare LCS to all coated steels. The direct tinning process [16,26] was performed on LCS substrates using Sn—4%Zn alloy powder (with and without adding nanoparticles) mixed with flux. The constituents of the flux included 24 g ZnCl₂, 6 g NaCl, 3 g NH₄Cl, 1 mL HCl, and 1 mL H₂O. Flux was added in order to improve the wetting as well as the dissolution of the oxide layer from the steel surface, in addition to protecting the cleanliness of the surfaces and promoting the flow of the coating into the interface. The evaluated proportion of nanoparticles (0.25 wt.%) was mixed with 10 g of flux and 1 g of (Sn—4%Zn alloy) powder. The mixture was spread out on the surface of the LCS substrate in a layer of 0.15 g/cm².

Afterward, the coated steel substrates were subsequently heated on a hotplate for 3 min at 350 °C. After the coating process, the coated LCS samples were moved away carefully, and the remaining flux on the surface was removed by rinsing in hot water and then cooled in normal ambient air. After cutting the samples, the cross-sectional surfaces of the coated samples (S1–S5) were ground with emery papers of up to 1200 grit size and etched with nital (4% HNO₃ in ethyl alcohol) for microstructural study.

Previously [15,23,27,28], the microstructures of the coated layers and the intermetallic layer thicknesses between mild steel and coated layer were changed with the addition of some particles, such as Si, Zn, Cr, Al₂O₃, Fe₂O₃, and others. These particle additions to the coated layer improve the mechanical properties and durability of the coated steel, which can be attributed to the change of morphology of the undesirable intermetallic compound layer. A previous study [16] reported that a tin coating layer that contains 0.50 wt.% alumina nanoparticles shows a long-lasting and high-corrosion resistance for coated mild steel with minimal coating layer defects. It was reported [26] that the presence of Sn coating provides protection to LCS, and this protection effect increased after the addition of 0.25% NiO nanoparticles. Based on the previously mentioned information, the percentage of nanoparticle additions was kept below 0.5%. Moreover, the combination of Al₂O₃ and NiO nanoparticles is designed to achieve the maximum benefit of improved interlayer intermetallic morphology (Al₂O₃ addition) and higher corrosion resistance (NiO addition).

2.3. Tools for Characterization

An optical microscope (Olympus GX51, Tokyo, Japan) was used to examine the surface morphology as well as the microstructures of the coatings and interfaces between the uncoated and coated samples. A scanning electron microscope (FEI Quanta 250 SEM, Eindhoven, Netherlands) was used to examine the samples’ morphology and elemental composition. Energy Dispersive X-ray Spectroscopy (EDS) provided a composition profile of the elements’ distribution at the interface. A software-based micrograph image analysis (Image Analyzer Software, Olympus GX51, Tokyo, Japan) was used to measure the coating layer thickness. To verify our findings, we measured the coating layer thickness in various micrograph locations. For each sample, the average thickness of the layer was calculated.

The coating phase composition and the size of the crystallites produced were determined through X-ray diffraction (XRD) tests. The measurements were carried out on a Shimadzu X-ray Diffractometer using radiation from a Cu Kα source at a wavelength of 1.5404 nm, an accelerating voltage of 40 kV, a current of 30 mA, and a scan speed of two degrees per minute at room temperature. For optical microstructures, SEM and XRD, at least three specimens were evaluated.

2.4. Corrosion Test

For the electrochemical impedance spectroscopy (EIS) and potentiodynamic cyclic polarization (PCP) measurements, a three-electrode electrochemical cell that can hold 250 mL of 3.5% sodium chloride solution was used. Merck provided 99 percent pure NaCl salt, which was utilized as received. EIS and PCP data were collected using an Autolab Potentiostat-Galvanostat model PGSTAT-302N provided by Metrohm (Amsterdam, The Netherlands).

By way of the working electrodes, the various coated and uncoated samples were assembled. Silver–silver chloride, Ag/AgCl in a saturated KCl solution served as the reference electrode, while a Pt sheet served as the counter electrode. The EIS data were measured from the corrosion potential (OCP) values after 40 min immersion in the test solution. An AC wave of 5 mV peak-to-peak was overlayed over the EIS data, which were collected in a frequency range that began at 100,000 Hz and ended at 0.1 Hz. Sweeping the potential in the forward direction between −1.2 V and 0.0 V produced the PCP curves. To complete the circle, the potential was contentiously swept again in the backward direction once more at the same scan rate of 0.00167 V/s.

3. Results and Discussion

3.1. Microstructure and Interface

Figure 1 shows an optical micrograph of the base material (low-carbon steel) where a homogeneous distribution of fine pearlitic structure is clear. The common phase of ferrite can be observed. Mild steel low carbon is the engineering material that is used the most because of its excellent mechanical properties, reasonable conductivity, and cost-effectiveness. Mild steel and other steel alloys, on the other hand, are regarded as reactive metals because they are easier to corrode in various environmental systems. Carbon has been shielded from rust by barrier coatings [29,30].

Figure 2 represents macro images of carbon steel with different surface coatings. Surfaces with a fine texture and complete surface coverage are obtained. They lack any visible defects, such as spallation, cracks, or delamination. Sample 2 (S2) and sample 5 (S5), steel coated with Sn—4% Zn alloy and Sn—4% Zn with (0.25 wt% Al₂O₃ + 0.25 wt% NiO) nanoparticle addition, show smoother and finer coating surfaces compared with other samples.

Figure 3 depicts the cross-sectional microstructural features of the LCS substrate coated with an Sn—Zn alloy with various nano additions. The findings demonstrated that the presence of nanoparticle additions has an effect on the morphology and thickness of Sn—Zn composite coating layers [16,31]. The LCS coated only with Sn-Zn (without any nanoparticle addition) and with Sn—Zn coating, including nano additives, shows different coated layer thicknesses and morphology.

The microstructure images of coated steels show two layers of the coating: a thicker outer layer and a thin intermediate layer separating the outer layer and the steel substrate. This intermediate layer is an intermetallic layer that is mostly made up of the diffusion between the LCS substrate and the Sn—Zn alloy coating.

The adhesion, stability, and other bonding properties of coatings are significantly influenced by their interfaces with substrates. The effects of adding Al₂O₃, NiO, or hybrid Al₂O₃ + NiO nanoparticles to an Sn—Zn coating on the coating thicknesses, interfacial microstructures, and IMC thicknesses of coated LCS are depicted, respectively, in Figure 4 and Figure 5.

In general, the Sn-Zn coating thicknesses are increased with the addition of nanoparticles. The maximum average coating thickness of about 70 µm is achieved by Al₂O₃ nanoparticles. This can be ascribed to the uniform coating thickness achieved by the tinning process on the surface layer.

In accordance with the data that have already been published, the addition of Al₂O₃ + NiO nanoparticles (Al₂O₃ + NiO nanoparticles) to the Sn—Zn alloy significantly reduces the thickness of the intermetallic layer during solid/liquid interdiffusion [32,33] due to the fact that the presence of both nano additives prevents Fe diffusion, resulting in a thinner diffusion layer as well as a uniform thickness of the intermediate layer in Sample 5 [16].

3.2. Cross-Section Morphology

Scanning electron micrographs of the polished cross-section coated steel and EDS elemental line scan results of the coating layer, interface, and steel substrate are presented in Figure 6 and Figure 7, respectively. The coating presents a rather dense microstructure with the typical lamellar characteristic. A thin, continuous, and uniform IMC layer provides good bonding for the coating layers for all the samples. The resulting coating layers had a structure consisting of two layers: the outer layer was an Sn—Zn alloy layer, and the inner layer was a diffusion layer formed by the inter-diffusion of the Fe, Zn, and Sn elements. It is clear that all fabricated coating layers of Sn—Zn with and without nanoparticles had a dense and smooth interface without microscopic defects, such as cracks or holes, except Sn—Zn coating with Al₂O₃ nanoparticles. Sn—Zn + Al₂O₃ nanoparticle coating exhibited micro-voids distributed in the coating layer. These micro-voids (Figure 6b) are due to particle agglomeration phenomena accompanied by the presence of Al₂O₃ nanoparticles that leads to vacancy diffusion in the opposite direction and the accumulation of vacancies.

EDS line scanning was used to investigate the distribution profile of Sn, Zn, Ni, Al, and Fe from the coating surface to steel substrate across the coating cross-section of the Sn—4% Zn coating with and without different nanoparticle additives. The results are shown in Figure 7, including the intermetallic compound layer (IMC layer) next to the LCS substrate, indicating the ongoing transfer and interaction between the atoms of Sn, Zn, and Fe through the coating depth. Otherwise, the outer surface layer of the coating contains Sn and Zn with a negligible amount of Fe.

EDS line scan collects a spectrum across a live line scan through the IMC layer as the region of interest (ROI). According to research, a layer known as an intermetallic compound (IMC) forms close to the steel substrate. This signifies the transport of Sn and Zn as the dominating diffusing species to the substrate, as well as the diffusion of Fe in the opposite direction. The abscissa in Figure 7 expresses the distance corresponding to the image, while the ordinate shows the mole fractions of the elements, respectively.

In the center part of the coating, where the Sn profile reaches a plateau, It has been observed that the level of Fe rises very slowly. For an Sn—Zn coating alloy (S2), the level of Sn decreases at a distance of approximately 40 µm from the steel substrate, about 27 μm for Sn—Zn + Al₂O₃ nanoparticle coating (S3), about 32 μm for Sn—Zn + NiO nanoparticle coating (S4), and about 19 μm for Sn—Zn + Al₂O₃ + NiO nanoparticle coating (S5). The formation of the intermetallic Fe-Sn at the interface layer is confirmed by the analysis (Fe-Sn intermetallic phases) due to the iron-tin reaction in all of the samples. There was an inter-diffusion reaction between the molten Sn—Zn alloy and the solid iron. Compared to the diffusion rate of Sn atoms in the solid Fe, the rate of diffusion of Fe atoms in the molten Sn was significantly faster [34]. The thickness of the IMC interface is noticeably decreased when nanoparticles are added.

3.3. X-ray Diffractograms

Figure 8 shows the XRD pattern obtained for Sn—4%Zn coating (S2) and the influence of adding Alumina NPs (S3), Nickle Oxide NPs (S4), and Alumina/Nickle Oxide NPs (S5) additives on the crystalline structures. The observed “d” spacings and the respective prominent peaks correspond to reflections of the (200), (101), (220), (211), (301), (112), (400), and (321) planes and are in good agreement with the standard data (ICDD Card No. 04-021-7599). Thus, the XRD pattern reveals the polycrystalline nature of the Sn-4% Zn coating, either by incorporating nanoparticles or not. According to the XRD patterns, the different nano additives did not change the characteristics of the diffraction pattern, as there are no detectable dopant-related peaks. This is due to the low concentration of nano additives (0.25 wt.%); consequently, there are not enough nanoparticles for the potential influences on the crystalline structures. The most intense peak for all of the samples corresponds to (200) and (101), while sample 4 shows the most intense peak at (211). This indicates that the orientation of the grain growth for Sn—4%Zn coating with different nano additives is along different directions.

Figure 9 reveals a significant difference in the (200) peak shifts to higher values of θ in the case of Sn—Zn + Al₂O₃ NPs coating (S3) as well as Sn—Zn + NiO NPs coating (S4). This significant displacement confirmed the successful incorporation of this small ratio (0.25%) of Al₂O₃ and NiO nanoparticles in the host Sn—4%Zn matrix, resulting in an increase in the crystallite size of S3 and S4 while adding both Al₂O₃ and NiO nanoparticles (hybrid doping (S5)) results in higher nucleation probability that can induce small crystallite size (as will be shown in Figure 10), and also results in peak broadening [35,36,37].

3.4. Crystallite Size, Dislocation Density, and Microstrain

In order to acquire additional structural data, the Scherrer formula listed below [38] was utilized to calculate the average crystallite size (D) of each sample. Figure 10 depicts the findings.

$$D=\frac{K\lambda }{\beta cos \theta }$$

(1)

where λ is the X-ray wavelength (λ = 1.5406 Å), β the observed angular width at half maximum intensity (FWHM) of the peaks and θ the Bragg angle. K is the shape factor, which takes a value between 0.8–1.2; it was considered 0.9, assuming spherical crystals with cubic symmetry [39,40].

The formula provided below by Williamson and Smallman was used to calculate the dislocation density (δ)

$$\delta =\frac{n}{D^2}$$

(2)

where “n” is a factor, which when equal to unity gives the minimum dislocation density, and “D” is the average crystallite size.

The average microstrain (ε) developed in the coating layer was calculated by using the relation [41]

$$\epsilon =\frac{\beta }{4tan\theta }$$

(3)

where “β” is the full width at half maximum and θ is the Bragg angle.

As can be seen in Figure 11, both the dislocation density and the microstrain decrease as the crystallite size increases. This is because the coating crystallite size has increased, resulting in a smaller number of grain boundaries. Hybrid doping increases the probability of nucleation, which can result in smaller crystallite sizes and higher values of crystal microstrain. Both the small crystallite size and crystal microstrain are the two factors that induce a higher dislocation density.

3.5. Corrosion Behavior

3.5.1. EIS Measurements

EIS measurements have been widely exhibited to report the corrosion and corrosion protection of metals and alloys in various corrosive solutions [42,43,44]. Here, the Nyquist plots were obtained for (1) low-carbon steel (LCS), (2) LCS coated with Sn—4% Zn, (3) LCS coated with Sn—Zn + 0.25 Al₂O₃, (4) LCS coated with Sn—Zn + 0.25% NiO, and (5) LCS coated with Sn—4%Zn + 0.25% NiO + 0.25% Al₂O₃. The samples, after being immersed for 40 min in 3.5% NaCl solution before measurements, are shown in Figure 12. It is seen from the plots that LCS (sample 1) surface shows the lowest semicircle diameter. Coating the LCS surface with Sn and Zn (plot 2) increases the impedance, as confirmed by the increase in the diameter of the corresponding semicircle. Further coating the surface of LCS with Sn—Zn + Al₂O₃ (spectrum 3) highly increases the impedance, and this effect was further increased when substituting Al₂O₃ with NiO (sample 4), which remarkably increases the resistance of the surface, as confirmed by spectrum 4 of Figure 12. The highest resistance is clearly seen for the LCS coated with Sn—4%Zn + 0.25% NiO + 0.25% Al₂O₃ (plot 5), as indicated by recording the widest semicircle diameter. The Nyquist plots thus reveal that coating the LCS surface with the mentioned coating nanoparticles highly improves the corrosion resistance of LCS.

The measured impedance data were best fitted to an equivalent circuit model, which is schematically drawn in Figure 13. The symbols shown within the circuit model can be defined as the solution resistance (R_S), constant phase elements (Q, CPEs), and first polarization resistance (R_P1), and it is mostly expressed as the corrosion resistance at the interface between the surface of the samples and the formed layer on that surface, a second polarization resistance (R_P2), which represents the corrosion resistance for the interface between the outer surface of a layer formed on the test samples and the test solution, and a double layer capacitance (C_dl). The values obtained for these parameters are listed in

Table 3. Parameters obtained from fitting the EIS data.

Sample	RS/ Ω cm2	Q		RP1/ Ω cm2	Cdl/ µF cm−2	RP2/ Ω cm2
		YQ/F cm−2	n
Low-carbon steel (LCS)	4.542	0.005957	0.98	14.63	0.0337	119
LCS + Sn + 4% Zn	5.489	0.001936	0.84	59.97	0.0093	537
LCS + Sn + 4% Zn + 0.25 Al2O3	6.795	0.000997	0.73	116.78	0.0073	1518
LCS + Sn + 4% Zn + 0.25% NiO	7.487	0.000928	0.62	142.28	0.0058	4039
LCS + Sn + 4% Zn + 0.25 Al2O3+ 0.25% NiO	8.291	0.000596	0.60	189.50	0.0038	5658

, and it can be seen that it confirms the data presented in Figure 12. Here, coating the LCS sample only with Sn and Zn is seen to increase the value of the obtained R_P1 and R_P2 while decreasing the values of Y_Q and C_dl. Adding Al₂O₃ to the coating (plot 3) is also seen to highly increase the values of the polarization resistances and inversely decrease both Y_Q and C_dl values. It is also seen that the addition of NiO highly increases this effect, but the maximum values of the polarization resistance, R_P1 and R_P2, as well as the lowest values of Y_Q and C_dl, were recorded for the LCS sample that was coated with Sn—4%Zn + 0.25% NiO + 0.25% Al₂O₃.

3.5.2. Polarization Data

Figure 14 displays the potentiodynamic cyclic polarization, PCP, and curves obtained for (a) low-carbon steel (LCS, 1), (2) LCS coated with Sn —4% Zn, (b) LCS (1), (3) LCS coated with Sn—4% Zn + 0.25 Al₂O₃, (c) LCS (1), (4) LCS coated with Sn—4% Zn + 0.25% NiO, and (d) LCS (1), and (5) LCS coated with Sn—4%Zn + 0.25% NiO + 0.25% Al₂O₃; samples after 40 min immersion in 3.5% NaCl solution differ from samples after 40 min immersion in 3.5% NaCl solutions. The PCP technique has been successfully performed to report the corrosion and corrosion protection of metals and alloys in corrosive environments. Some corrosion parameters can be reported from the polarization measurement, namely, the cathodic Tafel slope (βc) and anodic Tafel slope (βa), the corrosion potential (E_Corr), the corrosion current density (j_Corr), the corrosion (polarization) resistance (R_P), and the corrosion rate (R_Corr) [26,43]. The values of all these parameters are listed in

Table 4. Corrosion parameters obtained from polarization curves.

Sample	βc (mV/dec)	ECorr (mV)	βa (mV/dec)	jCorr (μA/cm2)	RP (kΩ cm2)	RCorr (mmy−1)
Low-carbon steel (LCS)	160	−857	195	36.5	1.05	0.4303
LCS + Sn—4% Zn	172	−838	205	18.6	2.19	0.2193
LCS + Sn —4% Zn + 0.25 Al2O3	154	−860	203	13.4	2.84	0.1580
LCS + Sn—4% Zn + 0.25% NiO	146	−858	185	2.25	15.8	0.0265
LCS + Sn—4% Zn + 0.25 Al2O3+ 0.25% NiO	140	−870	166	0.76	43.4	0.0090

and were obtained as reported in the previous studies [26,39,40].

The PCP curves collected for (a) low-carbon steel (LCS, curve 1) and LCS coated with Sn—4% Zn (curve 2), (b) LCS (curve 1) and LCS coated with Sn—4% Zn + 0.25 Al₂O₃ (curve 3), (c) LCS (curve 1) and LCS coated with Sn —4% Zn + 0.25% NiO (curve 4), and (d) LCS (curve 1) and LCS coated with Sn—4%Zn + 0.25% NiO + 0.25% Al₂O₃ (curve 5), samples after 40 min immersion in 3.5% NaCl solution are shown in Figure 14. It is well known that the reduction of currents in the cathodic branch with scanning the potential in an anodic direction for all samples is due to the reduction of oxygen [44]:

2H₂O + O₂ + 4e⁻ = 4OH⁻

(4)

Coating LCS with different layers is also seen to reduce the obtained cathodic currents due to the ability of the coating layer to suppress the cathodic reactions via its passivation. On the other hand, the anodic reaction that takes place on the steel surface is known to be the dissolution of iron as per the following reaction [43,44]:

Fe⁰ = Fe²⁺ + 2e⁻

(5)

The current continuously increases with applying more negative anodic potential, which leads to the further dissolution of the steel surface. Further increasing the anodic potential in the more negative direction leads to the appearance of a little passive region. The appearance of this region mainly results from the reaction of the iron surface with oxygen from the solution to form Fe (OH)₂ and Fe₃O₄ [43,44]:

Fe⁰ + ½ O₂ + H₂O = Fe (OH)₂

(6)

3Fe (OH)₂ + ½ O₂ = Fe₃O₄ + 3H₂O

(7)

Application of the most negative anodic potential, E > 0.5 V (Ag/AgCl), as seen from Figure 14, leads to an abrupt increase in the recorded currents for LCS (curve 1). This is perhaps due to the dissolution of any formed layer (an oxide film and/or a corrosion product layer) due to the breakdown of that formed film. Moreover, this behavior also leads to the occurrence of pitting corrosion to the surface of the sample and was confirmed through the appearance of a hysteresis loop. Such a loop results when the obtained current in the backward direction increases more than the obtained current from the forward direction. At this condition, the dissolution of the LCS surface via pitting corrosion may happen as follows [43,44]:

Fe_(s) + 2Cl⁻_(aq) = FeCl_2(aq) + 2e⁻

(8)

The application of a coated layer onto the surface of the LCS sample is seen to greatly suppress both the cathodic and anodic currents, and this effect increases in the following order: Sn—4%Zn + 0.25% NiO + 0.25% Al₂O₃ > Sn—4%Zn + 0.25% NiO > Sn—4%Zn + 0.25% Al₂O₃ > Sn—4%Zn > LCS bare surface. This decrease in the cathodic and the anodic currents is directly proportional to the increased passivation of the surface with the applied coating layer. More important is the appearance of a long passive region in the presence of the different coatings, which enhances the suggestion that the presence of the coating layer minimizes the chloride ions attack as well as the increase in the less negative potential to a certain limit. After these certain potential values, the current increases because of the reduced resistance, and pitting corrosion may occur. The effectiveness of the applied coating layer on the passivation of the surface of LCS against corrosion in NaCl solution was also confirmed by the data listed in Table 4, whereas the coating layer Sn—4%Zn with 0.25% NiO + 0.25% Al₂O₃ additions has the best performance via providing the lowest corrosion current and corrosion rate as well as the highest corrosion resistance. PCP measurements thus confirm the data obtained by EIS; both confirmed that the current applied coatings increase the corrosion resistance of LCS surface and the ability of these coatings increase in the following direction: Sn + 4%Zn + 0.25% NiO + 0.25% Al₂O₃ > Sn—4%Zn + 0.25% NiO > Sn—4%Zn + 0.25% Al₂O₃ > Sn—4%Zn. The combinations of Al₂O₃ and NiO nanoparticles with a percentage of 0.5% improve both interlayer morphology and Sn—Zn coating corrosion resistance.

4. Conclusions

Effects of Al₂O₃ and NiO nanoparticle addition on the structure and corrosion behavior of Sn—4% Zn alloy coating carbon steel were studied. Regular coating and interface structures were achieved by individual Al₂O₃, NiO, and both NiO and Al₂O₃ nanoparticle-combined addition in the Sn—Zn coating. The Sn-Zn coating thickness increased with all used nanoparticle additions in both individual and hybrid conditions. The maximum coating thickness of 70 ± 1.8 µm was achieved for Al₂O₃ nanoparticle additions in the Sn—Zn coating. The interfacial intermetallic layer thickness decreased with all used nanoparticles in individual and hybrid conditions, and the minimum intermetallic layer thickness of about 2.29 ± 0.28 µm was achieved for Al₂O₃ nanoparticle additions in the Sn-Zn coating. For all Sn—Zn coatings samples, adding Al₂O₃, NiO, and hybrid Al₂O₃/NiO nanoparticles greatly improved corrosion resistance compared with the Sn—Zn coating without additions. The addition of hybrid Al₂O₃/NiO nanoparticles to Sn-Zn coating shows the best improvement in corrosion resistance. The technological findings of this study are thought to be useful as corrosion protection enables less frequent maintenance performing and industrial applications where less expensive surface treatment methods are needed.