Interfacial Microstructure and Corrosion Behaviour of Mild Steel Coated with Alumina Nanoparticles Doped Tin Composite via Direct Tinning Route

Abstract

The improvement of the surface properties of ferrous metallic materials has become a crucial criterion for advanced engineering applications. The interfacial microstructure and corrosion behaviour of mild steel coated with alumina nanoparticles doped in tin composite using the direct tinning technique were investigated. A coating layer of tin composite containing different loads of Al₂O₃ nanoparticles (0.25 wt.%, 0.50 wt.%, 1.00 wt.% and 1.5 wt.%) was prepared and directly deposited on a mild steel substrate. This type of a direct tinning process is considered to be a simple and low-cost route for protecting metallic materials from corrosion. It was found that the thickness of both the composite layer and Fe-Sn intermetallic layer at the coated interfaces was highly affected by the presence of alumina nanoparticles that effectively inhibit the diffusion of Sn atoms into the Fe substrate. For the samples coated with lower content of alumina nanoparticles (0.25 wt.% and 0.50 wt.%), the thickness of the Fe-Sn intermetallic coating (IMC) layer is decreased due to Fe-Sn IMC suppression. Otherwise, for the addition of more alumina nanoparticles (1.00 wt.% and 1.50 wt.%), the thickness of the Fe-Sn IMC layer is slightly increased because of nanoparticle’s agglomeration and flotation. It can be reported that the presence of alumina nanoparticles in the coating layer improves, to a great extent, the corrosion resistance of Sn-composites surface on mild steel substrates. Although the tin composite coating layer with a high quantity of alumina nanoparticles (1.0 wt.%) exhibited better corrosion resistance than the other tested samples, such nanoparticle additions have become increasingly difficult to obtain. It was observed that the Al₂O₃ nanoparticles agglomeration and flotation that were detected in the coating surface may be related to high fraction nanoparticles loading and to the difference in the gravity for Sn and Al₂O₃ nanoparticles. However, based on our investigation, a coating layer that contains 0.50 wt.% alumina nanoparticles is highly recommended for achieving long lasting and high-performance corrosion resistance for coated mild steel with minimal coating layer defects.

1. Introduction

Metallic alloys, such as low carbon steel, are commonly used in advanced engineering applications such as automotive, energy transformation industry parts and petrochemical processes. These metallic alloy materials are used in the manufacturing of engines, pumps, valves and other internal mechanical parts. Generally, these internal mechanical parts are exposed to high temperatures, corrosion, and wear conditions. For example, the mechanical parts of automotive cylinder and valves are greatly affected by the surrounding environment (temperature, humidity, and other operating conditions), which may lead to the corrosion of these elements within a short time period. The corrosion effects minimize the lifetime, functionality and safety of these internal mechanical parts. The present study develops a novel technique for the surface modification of low carbon steel alloys by coating the steel alloy with nano sized Al₂O₃ particles doped with tin paste. Such types of composite paste will enable steel alloys to be high corrosion, wear and oxidation resistance. The coated layer will greatly enhance the surface properties of the steel alloys, consequently, increase their lifetime, and protect them from corrosion. The improvement in the surface properties in turn enhances the mechanical properties of steel alloys that make them more feasible for advanced engineering applications.

However, many investigators with varying perspectives managed the coating process of metallic alloys. As of recent, the main technologies for the surface treatment of metallic alloys are hot-dip aluminizing [1], thermal spraying [2], micro-arc oxidation [3], hot dipping and diffusion aluminizing [4], and pack-aluminizing [5]. Many coated materials were suggested based on the proposed technique. One of the promising techniques is to produce a hard-coated layer using the diffusion technique, which may be more preferable than Physical Vapor Deposition(PVD) and Chemical Vapor Deposition (CVD) processes [6,7]. It was reported that the resistance against corrosion of cast iron could be improved through coating with NbC using diffusion techniques. The results showed that the layers of NbC coatings offer high anticorrosive properties for their protection against corrosion. The corrosion rates were three times lower for the coated samples due to the presence of NbC. The diffusion techniques do not require vacuum equipment, have a low processing cost, and require economical supplies [8,9]. Aluminizing technique is also widely used to protect metallic alloys from corrosion [10,11,12,13,14,15,16,17,18]. It was found that aluminizing the steel improved its oxidation resistance, due to the formation of a protective, dense and stable Al₂O₃ layer on the surface of the aluminide layer on steels during the oxidation process. The surface properties of some steel alloys were modified to improve their corrosion resistance using the boronising technique [19]. A significant increase in the corrosion resistance was a result of the chemical stability and high density of FeB or FeB₂. The analyses revealed that the boride layers do not react with aluminum.

In other related investigations, the authors of the present work used Sn-based alloys (Babbitt) as an isolated surface lining during the formation of bimetallic materials for bearing applications [20,21,22,23,24]. The optimization of the tinning process of the solid carbon steel substrate after the incorporation of the flux constituents with the tin powder was discussed in the literature [25] to improve the bonding strength and the performance of bimetallic bearings. In Al/Fe bimetallic materials, it was found that the ratio of tin-to-flux 1:10 demonstrated the best performance of the interfacial structure, bonded area and the interfacial shear strength. The improvement of the surface properties can be attributed to a restriction of the formation of Al-Fe and Fe-Al intermetallic phases. The optimizing of the tinning process offers a promising technique for the fabrication of bimetallic materials with a high performance for bearing applications.

The largest category of low-carbon steel is flat-rolled products. The carbon content for these high-formability steels is less than 0.25% C, with up to 0.4% Mn. It is typically used in automobile body panels, tin plates and wire products. For rolled steel structural plates and sections, the carbon content may be increased to approximately 0.30%, with a higher manganese content of up to 1.5%. These materials may be used for forgings, boilerplates, seamless tubes, and many other engineering applications. It is well known that metals are not inherently stable in most environments but tend to revert to more stable compounds. Using metallic coatings with a more noble metal can separate or protect mild steel against the aggressive environment [26,27].

The present work is designed to investigate the surface properties enhancement accompanying the coating of low carbon mild steel through Sn-composites using a direct tinning process that incorporates the flux constituents with tin powder and alumina nanoparticles. The interfacial microstructure for the produced coated materials together with the corrosion behavior were investigated under different Sn- nanocomposites conditions. A direct tinning process is considered as a promising technique for protecting metallic materials from corrosion as a simple and low-cost method.

2. Experimental

2.1. Coating Process

Low carbon steel specimens of approximately 42 × 18 × 7 mm³ that were used as substrates in the present study were grinded with emery papers of up to 400 grades (namely S0 to S5). These specimens were coated by Sn-metal containing different amounts of alumina nanoparticles (50 nm) and S2–S5 substrates. The tinning process of the steel substrate which involves powder tin and the flux mixture deposition has been explained in detail elsewhere [20,21,22,25]. In the present work, the tinning process of mild steel solid substrate surfaces was performed using powder tin (with and without alumina nanoparticles) and a flux mixture. For the tinning process of mild steel solid substrates without alumina nanoparticles, flux constituents of 24 g ZnCl₂, 6 g NaCl, 3 g NH₄Cl, 1 mL HCl and 1 mL H₂O were mixed. Then, 1 g of powder pure Sn (with grain size ≈ 50 μm) was mixed with 10 g of the flux. A layer of Sn- flux mixture amounting to 0.2 g/cm² was distributed on the steel surface area (42 mm × 18 mm). The steel substrates with an Sn-flux mixture were heated using a hotplate for 2.5 min at 350 °C. After the tinning process, the steel substrates were cooled and washed using worm water to remove the remaining flux on the surface of tin layer. Two reference steel specimens without/with coating tin composite were also prepared (S0 without any coating and S1 substrate coating with tin composite without any addition of alumina nanoparticles).

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

Chemical Composition	C	Si	Mn	Cu	Cr	Ni	Fe
Steel substrate	0.14	0.30	0.41	0.20	0.14	0.09	Bal.

displays the chemical compositions of the steel substrate as used for the coating process in the current study. The produced specimens (S0–S5) were cross-sectional cut, grinded with emery papers of up to 1200 mesh, polished and etched with nital of 4% HNO₃ and ethyl alcohol for microstructural investigations.

2.2. Microstructure Characterization

An optical microscope and scanning electron microscope (FEI Quanta 250 SEM, Eindhoven, Netherlands) were used to investigate the surface morphology and the structure of the coated layer. The chemical compositions of the interfaces of the coated layer were measured using energy-dispersive X-ray spectroscopy (EDAX-AMETEK, Mahwah, NJ, USA). The thickness of the surface layers was measured using a micrograph image analysis from software (Image Analyzer Software, Olympus GX51, Tokyo, Japan). The thickness of the formed layer was measured at the different areas of the micrograph to confirm our results. The average thickness layer was obtained for each sample.

2.3. Corrosion Resistance Test

2.3.1. Electrochemical Setup

For electrochemical measurements as shown in Figure 1, a typical jacketed three-electrode cell was employed. A Pt electrode and a saturated calomel electrode (SCE) were used as the auxiliary electrode and a reference electrode, respectively. All potentials were referred to on the SCE potential scale. A potentiostat/galvanostat AUTOLAB (PGSTAT30, Metrohm, Herisau, Switzerland) was used in conjunction with an Autolab frequency response analyzer (FRA) with FRA2 module linked to a PC for electrochemical studies. For each run, there were 200 mL of the test solution inside the inner compartment of the cell. The volume of the test solution was great enough to withstand any considerable driftage in the composition during the run.

Prior to the electrochemical runs, the test solutions were deoxygenated for at least 30 min by bubbling Ar through them. With the use of a temperature-controlled bath and water flowing through the outer cell jacket, the temperature was fixed at (25 °C ± 0.2 °C). At least three different experiments were conducted for each electrochemical run to guarantee that the results were repeatable. Although the corrosion resistance of the coatings usually evaluated by electrochemical techniques without and with lubricated conditions [28,29,30,31], the present work focused on corrosion tests without lubricating conditions. The information gathered was judged to be statistically significant. The arithmetic mean and standard deviation were calculated and reported appropriately.

2.3.2. Uniform Corrosion Studies

A combination of two d.c. electrochemical techniques, namely linear polarization resistance (LPR) and Tafel polarization were utilized to analyze the uniform corrosion behavior of the studied alloys. After stabilizing the working electrode in the test solution for 2 h, LPR was conducted, which was then followed by Tafel polarization measurements. With an exceedingly slow potential scan rate of 0.167 mV s⁻¹, LPR measurements were conducted by changing the potential of the working electrode linearly from −20 to +20 versus the corrosion potential (E_corr). Once the LPR was completed, Tafel polarization measurements were conducted by polarizing the working electrode within the Tafel potential region (E = E_corr ± 250 mV) and employing a potential sweep rate of 1.0 mV s⁻¹.

3. Results and Discussion

3.1. Microstructural Changes Accompanying Coating Process

The microstructure of mild steel (S0 substrate) used in the current study is shown in Figure 2, in which the common phases of ferrite and pearlite can be observed. The percentage of the two phases primarily verifies the carbon % in the mild steel. Among a huge variety of metallic materials, mild steel is the most extensively used engineering material for its outstanding mechanical properties, its reasonable conductivity and its cost efficiency [32]. However, mild steel and other types of steel alloys are considered reactive metals that easier to corrode in different environmental systems. Barrier coatings [33] have been used to protect the carbon from corrosion.

Cross-section microstructures of mild steel with tin coating and tin-nanocomposites coating, containing 0.25 wt.%, 0.50 wt.%, 1.00 wt.% and 1.50 wt.% alumina nanoparticles, are presented in Figure 3. It is evident that the morphology and thickness of Sn-composites coating layers are highly affected by the presence of alumina nanoparticles. The mild steel coated only with tin (without alumina nanoparticles addition) shows irregular and relatively lower coated layer thickness (Figure 3a). However, the mild steel coating with tin doped nanoparticles containing 0.25 wt.% and 0.50 wt.% alumina shows regular and higher coated layer thicknesses (see Figure 3b,c). By increasing the alumina nanoparticles addition to 1.00 wt.% and 1.50 wt.% in tin paste, a remarkable change in the coated layer is observed, as they become irregular in shape and lessen in their thicknesses (see Figure 3d,e). It was reported [34] that the addition of Al₂O₃ nanoparticles in a lower percentage resulted in an increase in the wetting area and an increase in the microhardness of the Sn- based SAC0307 solder alloy that contains a lower percentage of Ag and Cu. The regularity of Sn-composites coating layers is improved due to the improvement in wettability between the coating layer and the steel for lower addition conditions. Otherwise, for a higher addition of alumina nanoparticles (1.00 and 1.5 wt.%), the wettability decreases due to alumina particle agglomerations, resulting in irregular tin-composite coating [34].

SEM images of a cross section of mild steel substrate coating with tin composite without any addition of alumina nanoparticles (S1 substrate) alongside the presentation of an EDS microanalysis are shown in Figure 4. The typical microstructures of a tin coating layer can be detected at the Fe-Sn IMC interface and a ferrite–pearlite matrix is observed (Figure 4a). The EDS patterns in the tin coating surface layer near the coating outer surface, adjacent to the interface layer in the tin-coating surface layer/ mild steel are shown in Figure 4b,c. The EDS pattern of tin coating surface layer adjacent to interface layer displays Fe element due to a diffusion from higher concentrations of iron and mild steel to Sn-coating (Figure 4c). The interfacial SEM image (Figure 4a) shows the presence of a clear and relatively thick interface between the tin coating surface layer/mild steel indicating the presence of intermetallic phases [35] (Fe-Sn intermetallic phases) due to the reaction of tin with iron, as revealed in Figure 4d. In the jointing process, the molten tin metal can react with the steel substrates to form Fe-Sn IMCs at the interface. The evolution and growth of IMCs play an important role in the mechanical properties and reliability of the joints, due to their inherent and brittle properties [36].

Figure 5 shows the effect of alumina nanoparticles additive on the thickness of tin surface layer and an iron-tin IMC interface. It can be observed that the thickness of the tin surface layer increases with an increase in the alumina nanoparticles additives. Such a trend can be explained in terms of morphology changes of the surface layer during the tinning process. At lower additions of alumina nanoparticles (from 0.0 wt.% and up to 0.50 wt.%), the thickness of the tin surface layer increases due to IMC repression. For higher additions of alumina nanoparticles (1.25–1.50 wt.%), an increase of the tin surface layer could be attributed to higher porosity formation. Previous studies [37,38] reported that the porosity percentage of the pore morphology of the Sn–0.7Cu alloy-based nanocomposite solders increased with the weight of the percentage of Al₂O₃ reinforcement. On the other hand, the effect of the addition of alumina nanoparticles on the thickness of Fe-Sn IMC interface seems to be minimal, particularly for a higher content of alumina nanoparticles (1.25–1.50 wt.%). At lower amounts of nanoparticles (0.0–0.25–0.50 wt.%), a remarkable decrease in the thickness of the IMC interface is observed. In conclusion, a higher concentration of alumina nanoparticles facilitated the Fe and Sn reaction, due to particles agglomeration phenomena.

Figure 6a presents the SEM image for a cross-section of mild steel, coated with 0.25 wt.% alumina nanoparticles doped tin-composite (S2). The EDS analysis results across the interface of the S2 sample are displayed in Figure 6b,c, respectively. The analysis confirms the formation of the intermetallic Fe-Sn at the interface layer. The EDS analysis shows the presence of composition of the tin-composite coating surface layer and minor percentages of Al and O with almost constant values through the line proofing representing a good distribution. Figure 6b indicates a comparative Al and O at the tin-composite coating surface layer, which was verified in the graph of the EDS line analysis detecting for the presence of Al₂O₃ reinforcement.

The SEM image of a mild steel that is coated with a tin-composite doped with 0.5 wt.% alumina nanoparticles alongside a line scan analysis is shown in Figure 7. The same observation as that of the previous figure is detected for Sn-composite coating. It contains 0.5 wt.% Al₂O₃, a minor percentage of Al and O with almost constant values across the line, again confirming their good distribution.

Figure 8 shows the SEM image and graphical representation of the EDS microanalysis of the tin coating surface layer and interface layer in the mild steel for sample S4 (1.0 wt.% alumina nanoparticles). A relatively higher percentage of micropores are observed (Figure 8a). It has been reported [37] that a higher percentage of Al₂O₃ reinforcement particulates in the lead-free Sn–0.7Cu composite, a higher aspect ratio and shape factor that results in a higher irregularity of the pores. The EDS point analyses (Figure 8b–d) of the sub-surface coating (point 1), the near interface (point 2) and an interface (point 3) confirm the inhomogeneity and uneven distribution of Al and O (Al₂O₃ nanoparticles) in the coating surface layer.

Similarly to our current tinning process, solders prepared through paste mixing contain a percentage of nanoparticles, and the rest of the nanoparticles remain in the flux residue after the reflow, showing a smaller percentage compared with the nominal percentage added [39]. Many processing factors may affect the difference between the nominal and the actual percentages of the amounts of nanoparticles remaining in the coating layer as well as their distribution in the matrix. Although forming a composite by combining different kinds of materials is a promising technique to obtain performance monolithic materials, the optimum benefits of different kinds of materials are difficult to obtain due to the difficulty of dispersing and distributing the nanometer scale reinforcing materials uniformly within the matrix [40,41].

Considering the strengthening mechanisms in the metal matrix nanocomposite, a significant enhancement of the nanocomposite properties can be achieved through a combination of the high-strength and the thermal stability of the ceramic nanoparticles with the metal matrix [41]. The expected improvements were rarely achieved to attain the high percentage of nanoparticles additions, due to the difficulty in achieving dispersion and a uniform distribution of the nanoparticles in the metal matrix [42]. The SEM image of the mild steel coated with 1.5 wt.% alumina nanoparticles doped tin-composite and the graphical presentation of the EDS microanalysis of alumina nanoparticles agglomeration and tin oxide in a thin coating layer are shown in Figure 9. The large subsurface voids and surface coating nanoparticles agglomeration can be easily detected (Figure 9a). The EDS microanalysis of agglomerated particles confirms the presence of Al₂O₃ nanoparticles along with a small amount of tin oxides (Figure 9b).

Based on the above analysis and results, obtaining a high fraction (1 wt.% and 1.5 wt.%) of alumina nanoparticles is becoming difficult to achieve using the current tinning process of mild steel using alumina nanoparticles-doped Sn-coating composite. The Al₂O₃ nanoparticles agglomeration and flotation observed in the Sn-coating surface could be related to high fraction nanoparticles loading and the difference in specific gravity between the Sn and Al₂O₃ nanoparticles.

3.2. Corrosion Resistance Enhancement

Figure 10 depicts the cathodic and anodic polarization curves recorded for the five-coated mild steel samples (S1–S5) through a comparison with the uncoated mild steel substrate sample (S0). Measurements were performed in a 3.5 wt.% NaCl solution at a potential scan rate of 1.0 mV s⁻¹ at 25 °C.

It is inferable from Figure 10 that between S0 and S4 the current associated with both the cathodic and anodic polarization plots decreases without a definite trend for the corrosion potential (E_corr). In addition, the current density accompanying both the anodic and cathodic polarization curves is markedly reduced. Such results suggest that the coatings employed inhibit both the anodic and cathodic processes on the substrate surface.

A further examination of the cathodic and anodic polarization curves revealed that, in all cases, a typical Tafel response (linear E-log j relationship) is displayed by the cathodic curves, whereas the anodic curves contradict Tafel behavior. The absence of the linear E-log j relationship on the anodic polarization curves can be attributed to a corrosion product deposition and/or passivation. The cathodic polarization curves’ Tafel region makes it possible to undertake a precise evaluation of the cathodic Tafel slope (βc), and therefore, the rate of the uniform corrosion in terms of corrosion current density (j_corr), via the Tafel extrapolation method. However, the anodic Tafel slope (βa) values, estimated from the software, are thought to be inaccurate due to the absence of the Tafel response on the anodic domains [43,44,45,46,47,48].

The various electrochemical kinetic parameters, derived from fitting the cathodic and anodic polarization curves with the Tafel equation, are displayed in

Table 2. Mean value (standard deviation) of the various electrochemical kinetic parameters, recorded for the investigated coated steel samples in comparison with the uncoated steel substrate. These parameters were derived from polarization measurements employing the Tafel extrapolation and LPR methods.

Tested Sample	Tafel Extrapolation Method						LPR Method
	Ecorr/ mV (SCE)	jcorr/ mA cm−2	βc/ mV dec−1	βa/ mV dec−1	υ/ mm/yr	I (%)	jcorr/ mA cm−2	Rp/ Ω cm2	υ/ mm/yr	I (%)
S0	−1004 (11)	1.51 (0.03)	−330 (7)	282 (5.2)	18.1 (0.22)	-	1.44 (0.02)	40.97 (1.1)	17.3 (0.2)	-
S1	−1008 (9)	1.072 (0.02)	−366 (8)	727 (10)	12.9 (0.15)	29.0 (0.35)	0.97 (0.018)	109.1 (2.1)	11.6 (0.2)	32.6 (0.4)
S2	−1050 (13)	0.29 (0.005)	−207 (4)	173 (3.2)	3.5 (0.06)	80.8 (1.15)	0.25 (0.004)	162.8 (3)	3.0 (0.05)	82.6 (1.2)
S3	−1060 (12)	0.16 (0.003)	−190 (3.6)	260 (5)	1.9 (0.03)	89.4 (1.3)	0.14 (0.002)	340.7 (6.4)	1.7 (0.04)	90.3 (1.5)
S4	−1020 (11)	0.076 (0.002)	−243 (4.2)	284 (4.7)	0.9 (0.02)	95.0 (1.5)	0.068 (0.0022)	831.5 (11)	0.8 (0.01)	95.3 (1.3)
S5	−960 (8)	0.45 (0.01)	−250 (4.8)	281 (5.3)	5.4 (0.08)	70.2 (1.1)	0.40 (0.009)	143.4 (2.8)	4.8 (0.07)	72.2 (0.9)

. The estimated βc values are higher than anticipated. The coated steel samples recorded high βc values ranging from −190 to 366 mV dec⁻¹ for the cathodic processes, (i.e., the reduction of both water molecules and the dissolved O₂ occurring on its surface) in a 3.5 wt.% NaCl solution. The estimated βa values were also considerable, covering the range of 173–727 mV dec⁻¹. There is no apparent change or behavior of the Tafel slope values of the studied coated steel samples. Analogous data were previously reported by M. Metikos-Hukovic et al. [49] who incorporated organic additives to effectively control the perchloric acid corrosion of Al. High βc values (235–245 mV dec⁻¹) were estimated during the described study [49]. High Tafel slope values are commonly considered irregular as such great Tafel slopes cannot be predicted for any given mechanism [49]. The higher values of βc were attributed by Metikos-Hukovic and his coworkers [49] to the Al passivity that occurs instantaneously, forming a virtually stable ineffectual passive layer, which restricts the reduction capacity of Al [50]. This, in turn, slows any potential reduction process occurring at the surface by influencing the energy of the double-layer reaction and/or through the introduction of a barrier to the transfer of the charge through the film [51]. An accurate way of describing the high Tafel slopes is through the barrier-film model for the observed HER [51].

The corrosion-current density (j_corr) values, and the uniform corrosion rate, decrease in the sequence: bare steel substrate (1.51 mA cm⁻²) > tin-coated steel (1.07 mA cm⁻²) > Al₂O₃ (0.25%)-tin-steel (0.29 mA cm⁻²) > Al₂O₃ (0.5%)-tin-steel (0.16 mA cm⁻²) > Al₂O₃ (1.0%)-tin-steel (0.076 mA cm⁻²). A further increase in the Al₂O₃ content within the coating, namely the Al₂O₃ (1.5%)-tin-steel sample, significantly enhanced the corrosion current density by up to 0.45 mA cm⁻², a value that is between those of the tin-coated steel and Al₂O₃ (0.25%)-tin-steel, presenting an enhanced uniform corrosion rate than compared with Al₂O₃ (0.25%)-tin-steel, Al₂O₃ (0.5%)-tin-steel, and Al₂O₃ (1.0%)-tin-steel samples.

To clarify and further assess the high uniform corrosion resistance of the investigated Al₂O₃ (x%)-tin-steel samples (x = 0.25, 0.5, and 1.0%), samples S2–S4, versus the uncoated steel substrate (sample S0), linear polarization resistance (LPR) plots were constructed (Figure 11).

The slopes of such linear plots define the polarization resistance R_p (the polarization curve’s tangent at E_corr), Equation (1) [52].

R_p = (dE/dj)E = E_corr

(1)

The value of R_p significantly increased, corresponding to an enhanced corrosion resistance, from 40.97 Ω cm² for the uncoated steel substrate (sample S0) to 109.05, 162.8, 340.7, and 831.47 Ω cm² for samples S1–S4, respectively. These findings present further evidence with regard to the outstanding uniform corrosion resistance of the Al₂O₃-tin-coated steel samples in NaCl solutions, which enhances with the Al₂O₃ content by up to 1.0%. Once again, a further increase in the Al₂O₃ content (up to 1.5%) within the coating, such as in sample S5, which is an an Al₂O₃ (1.5%)-tin-steel, decreases the polarization resistance value to 143.4 Ω cm², corresponding to an accelerated rate of corrosion. These findings confirm the results derived from the Tafel extrapolation method.

The rates of the uniform corrosion, ν (expressed in mm y⁻¹) and the coatings’ inhibition efficiency (I %) values are also depicted in Table 2. Equation (2) is employed to convert the values of j_corr (measured in μA cm⁻²), which are estimated using the Tafel extrapolation method via the extrapolation of the cathodic Tafel line to E_corr, to the corrosion rate, υ (given in mm/yr) [52]:

υ = K × (j_corr/ρ) × EW

(2)

where K is a constant (3.27 × 10⁻³ mm/μA cm yr), EW is the equivalent weight of Fe (27.9225 g), and ρ is its density (7.86 g/cm³).

A pronounced correlation exists between the LPR method’s corrosion rate values and those derived from the Tafel extrapolation method. These findings affirm the validity of the Tafel extrapolation method in evaluating uniform corrosion rates [46].

The value of I (%) is calculated using the corrosion current densities for the uninhibited (jo_corr) and inhibited (j_corr) NaCl solutions using Equation (3):

I (%) = {(jo_corr − j_corr)/jo_corr} × 100

(3)

In order to obtain the precise values of j_corr and accurately to measure the corrosion rate, the values of βa and βc, which are calculated from the analysis of the experimental anodic and calculated cathodic Tafel segments, have been included in the Stern—Geary equation, Equation (4) [53] along with the Rp values derived from the LPR method.

j_corr = B/R_p = {(βa × βc)/2:303 (βa + βc)}/R_p

(4)

However, unlike other methods that use different techniques to protect the surfaces of metallic materials such as welding [54], the use of expensive alloys containing Cr, Ni, Mo and Ti [55], or even using heat treatment [56], the direct tinning method used in this research is a simple and low-cost technique that can be applied to flat horizontal surfaces. The percentage of different nanoparticles that are added can also be controlled easily and optimized as desired.

Through this research, it has become clear that an increase in the percentage of alumina nanoparticles by up to 1% to the tin coating layer improves the general corrosion resistance. However, to maximize the benefits of adding nanoparticles, to obtain the ideal properties for the coating layer, and for ensuring that it is free of voids along with a significant resistance level to corrosion, we recommend the addition of 0.5% of alumina nanoparticles to the tin coating layer.

4. Conclusions

Tin-based alumina nanocomposite coatings were successfully fabricated via a direct, simple and low-cost tinning process route to protect mild steel from corrosion. Tin-composite surface layers that contained a range of 0.25 wt.% to 1.5 wt.% alumina nanoparticles coated on mild steel were investigated in detail. It was observed that the morphology and the thickness of the Fe-Sn intermetallic layer at the coated Sn/Fe interface were highly affected by alumina nanoparticles additions that effectively inhibit the diffusion of Sn atoms into the Fe substrate. The thickness of the Fe-Sn IMC layer is decreased at lower additions of alumina nanoparticles (0.25 wt.% and 0.50 wt.%) because of an Fe-Sn IMC suppression, while it is slightly increased for higher additions of alumina nanoparticles (1.00 wt.% and 1.50 wt.%) due to nanoparticle agglomeration and flotation. The nanocomposite tin coating containing 1.00 wt.% alumina nanoparticles promoted a superior corrosion resistance in comparison with other amounts used in the current study. However, for a long lasting and high-performance corrosion resistant tin-nanocomposite coating and for a coating layer with minimum defects (voids and particles agglomerations), a coating layer containing 0.50 wt % alumina nanoparticles is highly recommended.