Introduction

Numerous automotive components are still produced using castings due to the benefits of design flexibility and weight savings that cast complements can offer. Ferritic SiMo ductile cast iron has a silicon content of 3.5–5% and a molybdenum content of 0.4–1.5% (wt%).1,2 It is frequently used for high-temperature applications due to its favorable thermomechanical properties, including high-temperature tensile strength and resistance to creep rupture because of the formation of a matrix of complex Fe–Mo carbides at the grain boundaries3 as well as beneficial combinations of static strength and ductility for the reason of the high silicon contents.4

More stringent emissions regulations that call for cleaner, more efficient engines must be complied with by the automotive industry. An additional objective is to boost engine performance while preserving low fuel consumption. The specifications for high-temperature corrosion and the mechanical operation of engine parts are getting stricter. This is especially applicable for the turbocharger housing and exhaust manifold of the exhaust system.5

The temperature of the exhaust gases from large diesel engines is frequently not far from 700 °C.6 The manifold temperature cycles that occur during start–stop engine operation cause a thermal gradient. In addition, the components that would lower the exhaust gas temperature in heavy-duty trucks are a liquid-cooled system (using engine coolant) and an air-cooled system.7 Exhaust gases basically consist of nitrogen, water vapor, oxygen, and carbon oxides. Smaller concentrations of HC compounds, nitrogen oxides, and sulfur oxides are also present. These oxides produce nitric and sulfuric acid droplets when they interact with water vapor. Low exhaust gas temperatures cause water vapor to condense in the exhaust manifold pipes with other vapors of sulfuric acid H2SO4 and nitric acid HNO3.8 The entire surface of exhaust channels is severely corroded by such chemically aggressive nitric or sulfuric acid condensates, which also support extensive in-depth corrosion.9 At about 56 °C, all sulfuric acid has almost completely condensed.10

Due to the free graphite content of cast iron (3–4% by weight or 10% by volume), corrosion of cast iron leaves behind an insoluble graphitic layer of corrosion products. These corrosion products are extremely strong, adherent, and dense, and they create a barrier that prevents further corrosion.11 Because cast iron can be protected from corrosive processes by the graphite layer, the presence of microcells does not have a very significant impact on the rate of corrosion for cast iron. The high casting tightness,12 particularly the absence of slag inclusions and gas porosity,13 thermal–mechanical conditions,14 environmental elements, double film inclusions,15 and decarburization process16 all have a positive impact on corrosion resistance. Nofal et al.17 stated that ductile iron exhibited better corrosion resistance than the gray iron in the NaCl solution at its different concentrations. This is because ductile iron has the tendency of forming a uniformly distributed graphite phase as well as forming a barrier film.

Nearly all published works focus on the SiMo ductile iron's oxidation behavior at high temperatures above 700 °C. However, there are no studies on the SiMo corrosion that might happen as a result of exhaust gas condensation on the exhaust manifold channels during cooling cycles. Moreover, the corrosion behavior of SiMo in aqueous solution is poorly understood. Components made of SiMo DI are similarly susceptible to corrosion caused by the outside environment, such as rain and chloride solution. Therefore, understanding how SiMo DI corrodes in aqueous environments as a function of Mo contents is crucial and could help with material loss analysis.

This research looked at how the Mo content affected the corrosion resistance of SiMo cast iron. Electrochemical methods were used to study the corrosion behavior in 0.6 M NaCl and 0.5 M H2SO4 solutions at ambient temperature.

Experimental Procedure

Material

The melting of SiMo alloys with various Mo contents took place in a medium-frequency induction furnace with a 100 kg capacity. High-purity (Sorel) pig iron, steel scrap, ferrosilicon (75%), ferromolybdenum (60%), and carburizing material made up the base molten iron. The molten metal was treated at 1520°C with a 1.4% Mg–Fe–Si spheroidizing alloy containing 9–10% Mg, while the inoculant (70–76% Si, 0.75–1.25% Ca, 0.75–1.25% Al, 1.5–2% Zr, and Fe [Bal.]) with particle size (0.7–3 mm) was added.

The sandwich technique was used for the inoculation and spheroidization treatments. In this technique, the Fe–Si–Mg alloy was put in a pocket that was built into the ladle bottom and covered with high-grade low-carbon steel turnings. As a result of the physical barrier was formed between the Fe–Si–Mg alloy and the liquid iron, slowing the reaction time, increasing alloy use efficiency (magnesium recovery), and reducing flame and flaring. Afterward, the molten alloys were poured into a cylindrical greensand mold of dimensions of 35 mm diameter and a height of 220 mm. The chemical composition of the SiMo ductile iron samples is listed in Table 1.

Table 1 The Chemical Composition of SiMo Iron Samples
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Metallographic Investigation

Samples (10 mm thick) were cut at a height of 80 cm from the rod bottom. Figure 1 depicts the placement of the specimens used for microstructure and corrosion examinations. The samples were machined as disks of 25 mm in diameter and 10 mm in thickness. To perform a metallographic investigation to look at the microstructure, the samples were polished and etched in 2% Nital for 5 minutes at room temperature then well washed with water and dried. The nodule count and nodularity were calculated in unetched samples at a 100x magnification using the ZEISS image analyzer software according to ISO 945-1 and ISO 945-4. Grenier et al.18 provided a full description and analysis of the nodule count and nodularity measurements method utilizing image analysis. Field emission scanning electron microscopy (FE-SEM; Quanta EG) outfitted with energy-dispersive X-ray spectroscopy (EDX) was used to analyze the microstructure, eutectic carbides, and fine precipitate phases forming close to the cell boundaries of SiMo cast iron samples.

Figure 1
figure 1

Position of samples for microstructure investigation and corrosion tests.

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Corrosion Test

To assess the corrosion performance of the various samples, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements were made with 0.5 M H2SO4 acidic solution and 0.6 M NaCl solution at ambient temperature. Before the corrosion test, samples were mechanically polished using SiC sheets of varying grits, washed in water, and dried.

Utilizing the IviumStat instrument, electrochemical measurements were made (supplied by Ivium Technologies, Eindhoven, the Netherlands). IviumStat instrument can be managed by a computer using the IviumSoft software (the Netherlands) (PC), as illustrated in Figure 2. The personal computer (PC) was used to specify the measurement's parameters, display the measured curves, and compute the measurement's outcomes. The software enables automatic or manual calculation of the Icorr and Tafel slopes depending on the Tafel line's shape. The Tafel lines were recorded using the potentiodynamic polarization technique, which automatically changed the electrode potential from −500 mV (cathodic direction) to +500 mV (anodic direction) at a scan rate of 0.5 mVs−1. To achieve this, a three-electrode system was used, with a platinum sheet serving as the counter electrode, a working electrode with a surface area of 0.785 cm2, and a silver/silver chloride electrode (Ag/AgCl) serving as the reference electrode, which has an E° of 199 mV vs. normal hydrogen electrode (NHE). The open-circuit potential (OCP) was maintained steady for 30 min while the three electrodes were submerged in 0.6 M NaCl solution or 0.5 M H2SO4 acid solution. The intersection of the anodic and cathodic lines at zero polarization was named the corrosion current (Icorr). Using alternating current (AC) signals with an amplitude value of 10 mV and a frequency range of 35 kHz–100 mHz, the impedance of the coated samples was tested. The equivalent circuit based on the OCP was used to analyze the results of the impedance test.

Figure 2
figure 2

A schematic drawing of IviumStat instrument displays the main parts.

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Results and Discussion

Microstructure Investigation

The changes in the high Si DI microstructure owing to the variation in Mo content are shown in the OM microstructures (Figure 3). The 0% Mo sample has a fully ferritic matrix and a significant amount of nodular graphite (Figure 3a). On the other hand, the microstructures of SiMo ductile iron alloys with different Mo contents are similar. The three SiMo alloys are made up of nodular graphite and different amounts of fine precipitates that are dispersed throughout the ferrite matrix (Figure 3b–d). The nodule count and the percentage of ferrite, pearlite, and graphite, as well as cell-boundary precipitated carbides in each alloy, were calculated using Zeiss microscopy software for metallographic analysis, as listed in Table 2. The highest nodule count was at 0% Mo, while the nodular count gradually decreased with increasing Mo contents. The volume fracture of cell-boundary precipitates increased significantly with the increased Mo contents while ferrite decreased (Table 2). This is due to the high tendency of molybdenum to segregate at cell boundaries.19,20

Figure 3
figure 3

Microstructure of SiMo ductile iron: (a) 0% Mo, (b) 0.5% Mo, (c) 1% Mo, and (d) 1.5% Mo. Nital (2%) etched samples (100x magnification).

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Table 2 Microstructure Calculations of Examined SiMo Samples
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For further investigation by SEM, these precipitates were in the matrix of SiMo ductile iron. Fishbone-similar precipitates are present at grain boundaries, and both lamellar pearlite and finely spherical carbides surround these fishbone-similar precipitates (Figure 4). From EDS analysis (see Figure 5), these fishbone-similar precipitates are proposed to be M6C carbides (M = Mo, Si, and Fe), containing almost 32% of Mo. The amount of M6C carbides and the surrounding fine precipitates of lamellar pearlite increases with increasing Mo contents; consequently, the percentage of ferrite decreases. These obtained results agree well with earlier microstructural investigations mentioned in the previous works.19,20,21,22

Figure 4
figure 4

SEM photomicrographs of SiMo ductile iron exhibit: (a) grain-boundary precipitates and (b) fishbone-similar carbides at 1% Mo sample.

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Figure 5
figure 5

SEM-EDX analysis for Mo-rich carbides of SiMo ductile iron (0.5% Mo).

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The total number of graphite nodules reduces as the Mo content increases, but its size and distribution are only slightly affected, according to Figures 6 and 7. Table 2 also shows that an increase in Mo contents led to a decrease in nodularity almost from 75 to 60%. This might be the case because factors such as section thickness, inoculation effectiveness, and pouring temperature have an impact on the graphite's size and distribution. As shown in Figure 6, the inoculant with Zr+O+ S in the current study encourages the formation of graphite nodules with a variety of sizes and distributions. Due to Zr's affinity for oxygen and sulfur, which results in heterogeneous oxides and sulfides, graphite can form at these locations.23

Figure 6
figure 6

Size distribution of the nodular graphite in all SiMo alloys: (a) 0% Mo, (b) 0.5% Mo, (c) 1% Mo, and (d) 1.5% Mo, attached by result images at 100x magnifications.

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Figure 7
figure 7

Nodule count vs. Mo contents.

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Corrosion Measurement

To evaluate the corrosion resistance of SiMo cast iron with various Mo contents in different environments, electrochemical techniques (electrochemical polarization and electrochemical impedance) were used.

Electrochemical polarization measurement

Electrochemical polarization measurement was used to gauge each sample's corrosion resistance. The measurement was carried out in 0.6 M NaCl and 0.5 M H2SO4 solutions at ambient temperature as shown in Figure 8a–b. The figure shows the cathodic and anodic polarization curves for SiMo cast iron containing various Mo contents in 0.6 M NaCl and 0.5 M H2SO4 solutions. Using polarization curves as a starting point, the Tafel extrapolation method is used to calculate the corrosion potential (Ecorr), corrosion current density (Icorr), and polarization resistance (Rp) using polarization curves. Table 3 provides a summary of the findings.

Figure 8
figure 8

Anodic and cathodic potentiodynamic polarization curves at different corrosive mediums: (a)0.6 M NaCl and (b) 0.5 M HSO4.

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Table 3 The Polarization Parameters of Different SiMo Cast Irons in 0.5 M H2SO4 and 0.6 M NaCl Solution
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For the corrosion evaluation in NaCl solution, first, the open-circuit potential of the SiMo samples was measured as a function of time at 25°C in 0.6 M NaCl aerated solution until the steady-state value is reached, and then, the polarization measurements using Tafel lines were carried out and presented in Figure 8a and Table 3. The results revealed that a lower corrosion current density value (2.895 \(\times {10}^{-5}\) A/cm2) was recorded for the sample containing 1 wt% Mo than the other cast iron investigated samples, and consequently, a lower corrosion rate (0.2186 mm/y) was obtained compared with the corrosion rate of cast iron 0 wt% Mo (1.415 mm/y) (Table 3). This may be attributed to reduce the formation of nodular graphite which led to reduce the galvanic effects.

A ferrite metallic matrix, graphite nodules, and the amount of segregation in the form of carbides (i.e., Fe2MoC and M6C) surrounded by a pearlite island make up the majority of the SiMo ductile iron's microstructure (Figure 4) and each phase having its own electrochemical potential. These disparities in potential could result in the accumulation of leftover cementite and graphite at the cathodic sites and the dissolution of the less noble ferrite matrix phase at the anodic site. These multiphases in the microstructure have a significant impact on the corrosion resistance. Since graphite has a higher electrochemical potential than the ferrite matrix, micro-galvanic cells are created at the graphite–matrix interface. Galvanic coupling causes the matrix surrounding the graphite to dissolve preferentially, leaving cavities and a porous mass of corrosion products in the area of the graphite nodule. Graphitic corrosion is a term used to describe this phenomenon.24

The pH of a solution has an important factor in determining the corrosion resistance of materials because hydrogen ions can interact and change the surface and therefore alter the corrosion resistance of a material. Figure 8b and Table 3 illustrate the effect of 0.5 M H2SO4 solutions on the corrosion behavior of SiMo containing different Mo contents. For the corrosion evaluation in H2SO4, the results indicate that all the sample alloys have suffered from corrosion to varying extents during the polarization test in this solution, with a much higher corrosion rate than in a 0.6 M NaCl solution.

When SiMo alloy reacts with an aggressive solution (0.5 M H2SO4), which causes a quick interaction between acid and air, a soluble passive layer (oxide film) forms on the surface of cast iron. It results in no protection being provided on the surface and prevents a solution from emerging. Iron corrosion in H2SO4 solution generally demonstrated that hydrogen depolarization occurs. The anodic dissolution reaction (Eqn. 1) and the analogous cathodic reaction (Eqn. 2) can be used to describe the spontaneous dissolving of iron. According to Mathur and Vasudevan,25 the hydrogen evolution reaction, which takes place in two phases (Eqns. 3 and 4), is cathodically responsible for controlling the corrosion of metals in acidic solutions.

$$ {\text{Fe}} \to {\text{Fe}}^{2 + } + 2{\text{e}}^{ - } $$
(1)
$$ 2{\text{H}}^{ + } + \, 2{\text{e}}^{ - } \to {\text{H}}_{2} \uparrow $$
(2)
$$ {\text{H}}^{ + } + {\text{ e}}^{ - } \to {\text{H}}_{{{\text{ads}}}} $$
(3)
$$ {\text{H}}_{{{\text{ads}}}} + {\text{ H}}_{{{\text{ads}}}} \to {\text{H}}_{2} \uparrow $$
(4)

The Ecorr of the SiMo containing various Mo contents was approximately unaltered in the H2SO4 solutions. In contrast, the Icorr markedly dropped as Mo was raised up to 1.5 wt% Mo. The results showed that SiMo cast iron alloy with 1.5 wt% Mo had a lower corrosion current (Icorr) (79.0x10−5 A/cm2) in the H2SO4 solution, which resulted in the lowest corrosion rate (5.958 mm/y) compared with the free Mo cast iron (Table 3).

According to this study, as the molybdenum level rose, the amount of nodular graphite decreased, its size somewhat increased, the amount of ferrite dropped, and the graphite/matrix contact weakened. These elements reduce corrosion rate, improving corrosion resistance. Contrarily, as the number of Mo-rich precipitates (such as carbides and lamellar pearlite) increases, the rate of corrosion increases and the corrosion resistance decreases in the samples. As a result, the improvement in corrosion resistance was mitigated by this aspect. In order to prevent the development of unintended carbides that affect corrosion behavior, the Mo content must be kept within a certain range.

A transition layer of corrosion reaction products is created on the surface of the exhaust piping as a result of the reaction between the exhaust gas condensates and the material. The intensity of oxygen penetration into the wall depth is influenced by the diffusion of oxygen into the metal and, conversely, the diffusion of reaction products toward the surface, as well as by the elemental potential of cast iron and the compactness of the corroded layer. After the corrosion test, the corrosion on the sample's surface is comparatively uniform. To better understand the corrosion behavior of SiMo alloys in such a corrosive condition, optical microscopy was used to examine the corroded surface of sample materials. Figure 9 depicts the corroded surfaces along with their microstructure. From the inspection of the surfaces after the corrosion test, it can be seen that the nodular graphite sites are what the corrosive medium targets the most. As shown in Figure 9, the ferrite matrix for the 0% Mo sample erodes more slowly than the nodular graphite, which is also partially degraded. Figure 9 shows that in SiMo samples, the ferritic matrix is less degraded than the nodular graphite that is followed by Mo precipitates. In comparison with the NaCl medium, the corroded surfaces in the sulfuric acid medium appeared to be significantly more corroded. This is due to the quicker corrosion rate as was previously mentioned.

Figure 9
figure 9

The corroded surfaces of some investigated samples.

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According to earlier research, corrosion resistance is enhanced by reducing the amount of nodular graphite present.26,27 At the ferrite grain boundary, the high impurity concentration and potential may lead to boundary instability. Additionally, the creation of an electrochemical circuit is facilitated by the potential difference between graphite and ferrite.28 Moreover, corrosion resistance is related to the graphite/matrix interface. Although the current study found that higher Mo levels (more than 1.5 wt%) decrease nodular graphite and ferrite, their positive influence on corrosion resistance was not surprising. This means that the amount of nodular graphite and the number of Mo precipitates present in the microstructure both affect the corrosion behavior of SiMo ductile iron.

Electrochemical impedance spectroscopy (EIS)

Electrochemical impedance spectroscopy was used to examine the corrosion behavior of SiMo cast iron containing different Mo contents in 0.6 M NaCl and 0.5 M H2SO4 solutions. The electrochemical impedance diagrams are depicted in Figure 10a–b and Table 4. The impedance spectra were given in the Nyquist representation. All the impedance data points were analyzed using the equivalent circuit shown in Figure 10 (inset figure). This equivalent circuit fitted well with the experimental results. The main parameters deduced from the analysis of the Nyquist diagram are the solution resistance Rs, the charge transfer Rct (diameter of the high-frequency loop), Warburg diffusion (W), and the capacitance of double-layer Cdl.

Figure 10
figure 10

The Nyquist plots of impedance spectra for SiMo cast iron in 0.6 M NaCl and H2SO4 solutions.

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Table 4 EIS Parameters of the Investigated Samples in NaCl and H2SO4 Solutions
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In Figure 9a, for the evaluation of corrosion behavior in 0.6 M NaCl solution, it is clear that the Nyquist plots of the different samples of cast iron show a depressed semicircular shape. The inset image in Figure 10a depicts the circuit model of the examined samples that were used to further match the EIS test parameters and assess the corrosion behavior. This model matched the outcomes of the experiment. Voltage is applied across the circuit as a result of the Warburg diffusion one-time constant. The impedance spectra for the corrosion product generated by the corrosion process on the surface after exposure to a corrosive electrolyte (0.6 M NaCl) are explained using this model. This observation indicates that the corrosion of cast iron in 0.6 M NaCl solution is mainly controlled by a charge transfer process and that the corrosion process is characterized by a single time constant with Warburg diffusion Rs (RctCdlW), implying the presence of a double layer at the sample surface/electrolyte interface.29 The high charge transfer resistance value (Rct=197.7 Ω) for the currently examined sample S3 compared to free Mo cast iron sample S1 (164 Ω) implies that Mo has a good effect on corrosion resistance up to 1.0 wt%.

The Nyquist plots are not perfect semicircles and are generally attributed to the frequency dispersion and mass transport resistance. Cdl stands for double-layer capacitance, which is basically the capacitance between the electrolyte and the working electrode. In order to account for the deflection of the frequency dispersion semicircle caused by an experimental system due to surface inhomogeneity, surface roughness, electrode porosity, surface disorder, geometric irregularities, and others, a constant phase element (CPE) must be used in place of a pure capacitor, as is done in the case of pure capacitors.30

The main parameters deduced from the Nyquist diagram are reported in Table 4. These parameters illustrated that the value of Rct increases with increasing the Mo contents and the maximum value with 1.0 wt% Mo, while the value of double-layer capacitance Cdl (CPE) decreases. This attributed to the increase in thickness of the electric double layer, suggested that Mo gives better protection against corrosion. These results are in good agreement with the results obtained from the potentiodynamic polarization electrochemical technique.

The inset image in Figure 10b represents the circuit model of the samples under scrutiny that was used to further match the EIS test parameters and assess the corrosion behavior. This model was used to describe the impedance spectra for the corrosion product formed by the corrosion process on the surface exposed to a corrosive electrolyte (0.5 M H2SO4) and matched the experimental data. The circuit is, therefore, a one-time constant. The main parameters deduced from the analysis of the Nyquist diagram which is reported in Table 3 illustrated that the value of Rct increases with increasing the Mo contents and the maximum value (54.3 Ω) with 1.5 wt% Mo, while the value of double-layer capacitance Cdl (CPE) decreases. These findings are well in line with those of the electrochemical potentiodynamic polarization technique.

Conclusions

The variation in Mo content affects the microstructure of SiMo cast iron and hence the corrosion behavior as well. The corrosion behavior of SiMo ductile cast iron has been investigated through potentiodynamic polarization measurements and EIS in 0.6 M NaCl and 0.5 M H2SO4 solutions. The results of this investigation suggest some conclusions, which can be summarized as follows:

  1. 1.

    The microstructure of Mo-free DI contains a significant amount of nodular graphite in a fully ferritic matrix, while SiMo alloys consist of nodular graphite, M6C carbides, and lamellar pearlite, embedded in a ferritic matrix due to Mo segregation at grain boundaries according to SEM-EDX investigations.

  2. 2.

    The results revealed that the microstructure plays a chief role in the corrosion behavior as the extent of corrosion was related to the Mo content.

  3. 3.

    The SiMo containing 1 wt% Mo has lower corrosion rates in 0.6 M NaCl solution, while the SiMo containing 1.5 wt% Mo has lower corrosion rates in 0.5 M H2SO4 solution.

  4. 4.

    The analysis of potentiodynamic polarization and microstructure results showed that the corrosion of SiMo ductile cast iron may be due to the galvanic action resulting from the difference in electrode potential between the nodular graphite and the matrix in all studied samples.