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Article

Determination of Corrosion Resistance of High-Silicon Ductile Iron Alloyed with Nb

by
Carlos Rodrigo Muñiz Valdez
1,
Daniel García Navarro
2,
Jesús Salvador Galindo Valdés
1,
Félix Alan Montes González
2,
Efrain Almanza Casas
2 and
Nelly Abigail Rodríguez Rosales
2,*
1
Facultad de Ingeniería, Universidad Autónoma de Coahuila, Fundadores km. 13, Las Glorias, Ciudad Universitaria, Arteaga 25350, Coahuila, Mexico
2
Tecnológico Nacional de México/I.T. Saltillo, Blvd. Venustiano Carranza 2400, Col. Tecnológico, Saltillo 25280, Coahuila, Mexico
*
Author to whom correspondence should be addressed.
Metals 2023, 13(5), 917; https://doi.org/10.3390/met13050917
Submission received: 18 April 2023 / Revised: 3 May 2023 / Accepted: 6 May 2023 / Published: 9 May 2023
(This article belongs to the Topic Alloys and Composites Corrosion and Mechanical Properties)
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Abstract

:
In this study, the effects of Nb on the microstructural characteristics, hardness, and corrosion resistance of high-silicon ductile cast iron (HSDI)-3.6 wt.% Si were investigated. Samples from different castings with 0–0.9 wt.% Nb were obtained and compared to a commercial ductile iron. Microstructures showed that the amount of ferrite in the matrix increased with increasing Nb content, from 34% for unalloyed HSDI to 88% for HSDI-0.9 wt.% Nb. The presence of randomly distributed NbC carbides was identified by EDX for all the samples alloyed with Nb, and the hardness of the HSDI increased with the Nb content. To evaluate the influence of the Nb content on the corrosion resistance of HSDI, potentiodynamic tests were carried out in a solution of H2SO4. The highest corrosion rate on HSDI was obtained for the HSDI-0.3 wt.% Nb sample, with 2802 mills per year, due to the amount of pearlite present and the lowest presence of NbC carbides, compared to the HSDI-0.9 wt.% Nb, with 986 mills per year. This behavior was attributed to the ferrite matrix obtained because of a high Si content in the DI, which delayed the anodic dissolution of the alloy and suppressed the pearlitizing effect of Nb for contents greater than 0.3 wt.%, as well as to the effect of NbC carbides, which acted as inhibitors.

1. Introduction

Ductile iron (DI) is a type of cast iron with a metallic matrix of ferrite, pearlite, or a mixture of the two phases with graphite nodules embedded. Due to this morphology, DI has high strength, ductility, and good wear and fatigue resistance; therefore, DI is used in the automotive, energy, and agricultural industries, among others [1,2]. In addition, the microstructure and mechanical properties can be modified by adding different alloying elements or by heat treatment.
Silicon is an important element in DI, as it promotes ferrite formation in as-cast conditions and allows graphite the ability to precipitate during solidification by sulfide formation, surrounded by complex oxides of silicon and magnesium, which serve as nucleation sites for graphite [3]. Furthermore, Si reduces the carbon content of the eutectic composition, increases the amount of precipitated graphite, and prevents the formation of eutectic carbides [4,5,6,7]. In terms of mechanical properties, Si has a significant effect on the toughness and tensile strength of ferritic DI. As the Si content increases, the yield strength and tensile strength also increase [8,9,10,11]. A new DI classification called High-Silicon Ductile Iron (HSDI) has recently been introduced for DI with Si content ranging from 3.2 to 4.3 wt.%. It has been reported that with increasing Si content, hardness, tensile strength, and yield strength properties are promoted, while elongation to failure decreases [8,12,13,14] and machinability and corrosion resistance improve [15,16,17,18]. These results suggest a potential advantage in the production process and performance over DI with lower Si content [10,11].
With the purpose of modifying the structure and properties of HSDI, the addition of different carbide-forming elements, such as V, Cr, Mo, and Mn has been investigated by several authors [18,19,20]. Riebisch et al. [18] have studied the effect of carbide-forming elements on HSDI with 3.8 wt.% Si. They have found that V has a significant negative influence on graphite morphology, as it promotes the formation of chunky graphite due to a long solidification time and its effect of slowing down the diffusion of C into austenite; on the other hand, Cr and Mn have the greatest influence on pearlite area fraction, due to their segregation. Similarly, Górny et al. [19] studied the influence of 0.65 wt.% Mo on HSDI with 4.36 wt.% Si and reported a microstructure of graphite nodules in a ferrite matrix with a small amount of pearlite, as well as the formation of Fe–Mo carbides, which were related to segregation during solidification rather than to cooling rates to the eutectoid temperature. Although some authors have studied HSDI with carbide-forming elements, the role of Si in such castings must be better understood due to the wide range of HSDI applications.
Niobium has a significant impact on DI due to its high affinity for carbon, thus contributing to the formation of NbC carbides. Additionally, Nb has low solubility, especially in ferrous alloys with relatively high carbon content, such as cast iron. The presence of these particles in DI has been reported to improve corrosion resistance and provide good wear resistance with little change in austenite stability and graphite refinement [21,22,23,24,25,26]. However, the effects of a combination of high Si and Nb content on DI have not been reported in the literature.
Sckudlarek et al. [27] reported the formation of dispersed NbC carbides in the microstructure, refinement of graphite nodules with nodularity higher than 90%, and refinement and increase of pearlite volume fraction from 49 to 62% in DI with 2.46 wt.% Si and 0.35 wt.% Nb. On the other hand, Chen et al. [28] found that the addition of Nb up to 0.11 wt.% in the as-cast condition DI had a limited effect on the nodularity and roundness of the graphite nodule. Among the Nb containing DI, the optimum Nb addition in the as-cast condition was about 0.08 wt.% Nb, which gave the highest tensile strength and an elongation of 746 MPa and 8.0%, respectively. This was attributed to the significant microstructural improvement of the pearlite compared to the deterioration of the graphite morphology in DI due to the Nb addition. Furthermore, Ahmed et al. [29] reported that increasing to 0.1 wt.% Nb content in the DI increased the graphite nodule count and eutectic cells and refined the graphite structure, but at the same time, a slight decrement in nodularity was observed. Microstructural analysis showed that by increasing the Nb content, a finer pearlite structure was formed in the matrix.
In contrast, Alias et al. [30] reported an increase in tensile and impact strengths from 26 to 34% and 30 to 80%, respectively, in DI with 1.9 wt.% Si and Nb content in the range of 0.5–2 wt.% Nb compared to unalloyed DI. The formation of NbC carbides and the effect of finer pearlite in the microstructure have been attributed to such increases. In some studies, Nb addition was found to increase cast iron hardness due to the refinement of pearlitic interlamellar spacing, austenitic grain size, and the presence of NbC carbides [31,32,33].
Most studies on Nb addition to DI focus on mechanical properties, leaving aside the study of corrosion resistance, which is an important feature to consider since it can be used in the agriculture, automotive, and aerospace industries, etc. Alloying elements such as Si, Ni, Cr, and Cu, as well as carbide-forming elements (V, Ti, Mo) are commonly used to improve the corrosion resistance of DI [34,35]. Gutiérrez et al. [35] reported that DI alloyed with V and Mo and exposed to three different solutions (H2SO4, NaCl, and NaOH) showed corrosion damage due to the formation of galvanic couples between the graphite nodule and the ferrite. However, a high concentration of V and Mo carbides improves the corrosion resistance in H2SO4, and Si improves the scale resistance of cast iron by forming a light-resistant surface oxide in oxidizing atmospheres, such as H2SO4, HNO3, HCl, and CH3COOH [36]. Recently, Çelik et al. [37] have designed a novel ductile cast iron alloyed with a composition of 3.5 wt.% C, 4 wt.% Si, 1 wt.% Nb and 0–4 wt.% Al. They found, for all alloys, the room temperature phases were graphite, NbC carbides, and ferrite. In addition, they reported that the A1 temperature of the alloy with the 4 wt.% Al addition reached 960 °C, indicating that this alloy can be used at a higher temperature, compared to the commercial SiMo cast iron; and with the increase in Al content, the thermal expansivity of ferrite increased, the graphite content decreased, and the graphite nodularity changed from spheroidal to vermicular. These results are similar to those obtained by Adebayo [38] and Sandikoglu [14] in DI alloyed with Al.
There are some studies about the fabrication of composite coatings on DI deposited by means of the diffusion thermo-reactive treatment with the objective to improve wear resistance [39,40] or corrosion resistance [41], and it has been reported that carbide formation on the substrate surface protects the ductile iron from corrosion in a NaCl environment.
Although the addition of some carbide-forming elements in HSDI has been studied, specific corrosion studies on the effects of Nb are not yet available. Therefore, the main objective of this work is to determine the influence of the high Si content together with the variation in the Nb contents alloyed in HSDI on its microstructure, and the effect on the corrosion rate when exposed to an acid environment.

2. Materials and Methods

The HSDI was produced in a medium-frequency induction furnace. Steel scrap was used as raw material, e.g., crankshafts and foundry waste. Graphite and copper C11000 were used to adjust the composition of the DI. Nodularization and inoculation processes were carried out with FeSi (75 wt.% Si) and FeSiMg (7 wt.% Mg) alloys by sandwich and ladle methods, respectively. In order to obtain different Nb contents in the HSDI, an FeNb alloy (65 wt.% Nb) in powder form was added to the ladle to achieve better dissolution during casting. After maintaining a temperature of 1400 ± 20 °C, the molten iron was poured into Y-shaped molds (175 × 150 × 25 mm3) according to ASTM A-536-84 [42]. Molds were allowed to cool for 24 h before the samples were cut out of the useful area of the Y-block to obtain smaller samples of 25 × 25 × 15 mm3, as shown in Figure 1.
Chemical composition was determined using a Thermo ARL 3460 Optical Emission Spectrometer (Thermo Fisher Scientific, Waltham, MA, USA), taking the average of three measurements of each casting alloy. Samples were polished to a mirror finish and examined using an Olympus BX-51 optical microscope (OM, Olympus, Tokyo, Japan). Phase identification, nodule count, and nodularity were measured and quantified using Image Pro-Plus software (version 6.0, Rockville, MD, USA).
The surface of each sample was etched with Nital 2% for 20 s to reveal the phases and evaluate the microstructural evolution. In addition, the samples were analyzed in a JSM-6610LV scanning electron microscope (SEM, JEOL USA, Inc., Pleasanton, CA, USA), along with Energy-dispersive X-ray Spectroscopy (EDS, JEOL USA, Inc., Pleasanton, CA, USA) to determine the presence of Nb in the samples and the corroded surfaces.
Three corrosion tests were performed for each HSDI composition. Each sample was exposed to a 1N H2SO4 solution at room temperature in a flat cell using a three-electrode Gamry 1000 A potentiostat/galvanostat with an Ag/AgCl reference electrode and a scan rate of 1.0 mV/s ranging from −1000 to 1000 mV vs. Ecorr. Corrosion data (Ecorr and icorr) were graphically extrapolated using the Tafel method from potentiodynamic curves.
In addition, Rockwell C hardness measurements were performed in an MACROMET® 5100 Rockwell Type Hardness Tester, (Rockwell, Lake Bluff, IL, USA) with a test load of 150 kN to determine the influence of Nb content on HSDI.

3. Results and Discussion

3.1. Chemical Composition

The chemical composition of commercial DI and the different HSDIs produced in this study are shown in Table 1. It can be seen that the Si content in all alloys was higher than the conventional DI (>2.6 wt.% Si); according to [43], they correspond to HSDI. As Nb content varied from 0.3–0.9 wt.%, the samples were designated as U-HSDI for unalloyed HSDI, and depending on the Nb content, as HSDI-0.3% Nb, HSDI-0.6% Nb, and HSDI-0.9% Nb.

3.2. Microstructural Analysis

Figure 2 shows the different microstructures identified by SEM for all samples. Graphite nodules embedded in a full pearlitic matrix can be seen for commercial DI in Figure 2a, whereas a pearlitic-ferritic matrix for U-HSDI is observed in Figure 2b. For all HSDI samples with different Nb content, a change in the phase fraction in the metallic matrix was observed, as well as the presence of small irregular particles randomly distributed and corresponding to polygonal primary NbC carbides. These particles form before the casting due to the high affinity between Nb and C atoms, so they are called primary carbides [18,21,22,24,25,26,27,28,29,30,31]. Unlike other carbide-forming elements (Mo, Cr, and V), precipitation of NbC carbides occurs randomly throughout the material rather than by positive segregation. Regarding the changes in the matrix, it is observed that for the HSDI-0.3% Nb sample, there is a refinement in the pearlite in comparison with the U-HSDI and the commercial DI samples, as reported by [27,28,29]; however, for the HSDI-0.6% Nb and HSDI-0.9% Nb samples, this refinement is not observed. On the contrary, a mostly ferritic matrix was obtained, which is attributable to the high silicon content in the alloy.
To quantify the size and amount of NbC carbides in each sample and the Nb distribution in the matrix using EDX mapping, five different fields were taken from each HSDI-Nb sample at 1000×. It can be seen that the distribution of NbC carbides is random both within the matrix and the graphite nodules, and the carbide size varies with Nb content, whereas the distribution of Nb is shown homogeneously in the matrix. In the HSDI-0.9% Nb sample, the size and amount of NbC carbides varies more than in the other two HSDI-Nb samples, as shown in Figure 3; i.e., the presence of NbC carbides increases with Nb content in HSDI. The average number of NbC carbides over a 1200 µm2 area for each HSDI with 0.3, 0.6, and 0.9 wt.% Nb was 7, 12, and 16 particles, respectively. The NbC carbides exhibit a polygonal shape, as shown in Figure 2c–e, with average sizes of 0.5 and 4.5 µm for all samples, and a 4.49 µm size for the HSDI-0.3% Nb sample, showing a slight decrease in size as the Nb content increases to 3.48 µm for the HSDI-0.9% Nb sample. These variations occurred because FeNb was added in powder form during melting to achieve better dissolution efficiency into the liquid, resulting in fine carbides, similar to those reported in [21,24,28].
Summarizing, as the Nb content of HSDI increases, smaller precipitates are obtained, but in greater proportion. Figure 4 shows the average size (µm) and the number of NbC carbides in the HSDI-Nb.
Figure 5a shows the EDX analysis performed on the polygonal particles and the graphite nodule center of the HSDI-0.9% Nb to determine the presence of Nb, as shown in Figure 5b,c, respectively. This indicates that for this composition, the precipitation of primary carbides produces heterogeneous nucleation for graphite. Thus, in addition to the inoculant, some of the NbC carbides act as nucleation sites for graphite nodule growth.
Similarly, mapping was obtained by EDX at 1000× to determine the general distribution of the alloying elements HSDI-Nb, for C, Si, and Cu, which are shown to be homogeneously distributed, as shown Figure 6, with no segregation visible.
Nodularity showed a linear trend between 86 and 90% for the four different compositions. This can be explained by the effects of the corresponding amount of cerium oxide and the residual amount of Mg in the alloy [44,45,46,47]. In contrast, nodule count decreased with the presence of Nb in the HSDI, from 182 n/mm2 for U-HSDI to a range of 147–154 n/mm2 for HSDI-Nb, as shown in Figure 7. The solidification of conventional DI starts with the nucleation of the graphite nodules, whereas for HSDI-Nb, the primary NbC carbides were simultaneously formed in addition to the nucleation of the graphite nodules. These results indicate that HSDI-Nb has a lower amount of free carbon due to the formation of NbC carbides, reducing the nodule count. Some authors [25,27,48,49] report a similar pattern in which the nodule count is reduced by the addition of various carbide-forming elements such as V, Cr, and Nb. Moreover, the addition of more Nb had little effect on the nodule count. This can be confirmed with microstructures shown in Figure 2, where the HSDI-0.9% Nb sample has smaller NbC carbides in the center of the graphite nodules compared to the HSDI-0.3% Nb sample. This is due to the heterogeneous nucleation that occurs with a higher Nb content and keeps the nodules in the microstructure.
The changes in the HSDI matrix due to the variation in Nb content are shown in OM microstructures of Figure 8. Whereas an increase in pearlite (dark phase) is observed in the HSDI-0.3% Nb sample compared to the U-HSDI sample, ferrite (light phase) increased in the HSDI-0.6% Nb and HSDI-0.9% Nb samples, until the ferritic matrix is almost complete. Although Nb is an element that refines pearlite [21,33,50,51], the results show that when the Nb content exceeds 0.3 wt.% in HSDI, this effect is attenuated by the formation of larger amounts of ferrite, which is mainly due to the fact that Si is a ferrite-stabilizing primary element, and its content in HSDI helps to suppress the pearlitizing effect of Nb in the matrix [52,53]. In addition, the strong affinity between Nb and C to form carbides reduces the amount of carbon available to form pearlite when the Nb content in HSDI increases.
The eutectoid transformation was limited by the reduction of free carbon in the liquid, and as a result of the precipitation of graphite and the NbC primary carbides, the ferrite content in the microstructure varies from 34% for U-HSDI to 88% for HSDI-0.9% Nb. Nevertheless, when only the metallic matrix is considered, the ferrite content increased from 39 to 98% with the addition of Nb, as shown in Figure 9. According to some studies [21,24,26,50], the addition of Nb as a microalloying element (0.09–0.11 wt.%) increases hardenability and the equilibrium carbon content in austenite, and causes further pearlitic transformation to a cast structure of DI. In the present study, the resulting microstructures of different HSDI-Nbs show that the pearlite is refined only to HSDI-0.3% Nb sample in comparison to U-HSDI; nevertheless, an increase of more than 0.3 wt.% Nb ended up decreasing the pearlite volume fraction.

3.3. Hardness

Hardness measurements were made for each composition to determine the effects of high Si and NbC carbide precipitation, as shown in Figure 10. The commercial DI presented with 29.9 HRC due to the pearlitic matrix, whereas in the Nb-HSDI samples, hardness incremented from 30.44 HRC for U-HSDI to 32.87, 35.93, and 38.38 HRC as Nb content increased. These values were attributed to the higher NbC precipitation with the Nb addition. These carbides increase hardness in HSDI; however, it is important to highlight that this property is a function of the overall microstructure present [24,25,29,30].
Compared to U-HSDI, HSDI-0.3% Nb increased hardness by about 8%, and as would be expected with increasing Nb content in HSDI, HSDI-0.9% Nb increased hardness by about 26%. Si is one of the most common elements always present in DI chemical composition and exerts a strong effect on the mechanical properties of ferritic DI, as mentioned before [54,55]. Due to Si promoting ferrite formation in as-cast conditions, the presence of this element in the present HSDI-Nb conditions resulted in a fully ferritic microstructure, especially in the HSDI-0.9% Nb sample, as shown in Figure 8. Because NbC carbides could act as metallic matrix reinforcement, it would be expected that with higher Nb content, hardness and mechanical resistance would increase due to carbide precipitation. However, as the solubility limit of Nb is exceeded, the morphology, distribution, and segregation of carbides reduce the toughness of DI [44,45]. The results obtained indicate that the increase in hardness is mainly due to the greater presence of NbC carbides, but that the high Si content also favors the formation of ferrite, resulting in a moderate increase in hardness in HSDI alloyed up to 0.3 wt.% Nb.

3.4. Corrosion

The corrosion resistance of DI and HSDI samples was evaluated by potentiodynamic tests in an acidic medium (H2SO4). Figure 11 shows the polarization curves obtained for the different samples with similar behavior between them. The DI curve presents a lower potential than those of Nb-HSDI, so there are no significant differences in corrosion potentials. However, as the Nb content of HSDI increased, the current density shifted to lower values, indicating a decrease in the corrosion rate in HSDI-Nb.
Nevertheless, the HSDI-0.3 wt.% Nb sample has a higher current density value than the other samples, resulting in lower corrosion resistance in sulfuric acid. This behavior can be explained by a higher percentage of pearlite in the matrix of HSDI, as presented in Figure 9. Since the pearlite matrix is a laminar mixture of ferrite and cementite, there is a larger reaction surface between these phases, which exhibits electrochemical properties similar to those of grain boundaries and produces more high-energy and chemically active sites [55].
Figure 12 shows the corrosion rate of commercial DI and HSDI with different Nb contents. For DI with a conventional Si content, a corrosion rate of 4174 mpy (mills per year) was obtained, which is 3.5 times higher than that of the U-HSDI (1187 mpy). Thus, it was determined that the addition of Si to the HSDI significantly reduced the corrosion rate. In addition, it was found that the corrosion rate decreases as ferrite content increases, which is explained by the distribution of carbon between graphite and NbC carbides during the solidification of HSDI. Since the corrosion resistance of the phases present in Fe–C alloys depends on the amount of interstitial carbon present, ferrite is highly susceptible to pitting corrosion in acidic media. However, cast iron contains Si in addition to being an Fe–C alloy, which leads to the formation of a layer that acts as a protective barrier [56,57]. In an acidic medium, the corrosion of iron is controlled by the cathodic reaction, and the reaction rate increases when the pH decreases, which means that the corrosion rate is determined by the rate of hydrogen evolution [35]. As a result of the higher proportion of Si-saturated ferrite in HSDI, the surface develops a more compact and complete passive layer when interacting with the environment, which reduces the corrosion rate. In addition, Si reduces the potential difference between ferrite and graphite, increases the ferrite potential, and protects the substrate [57].
It is obvious that the addition of Nb at concentrations higher than 0.3 wt.% to HSDI decreases the corrosion rate when comparing the U-HSDI sample with the HSDI-% Nb samples. The amount of fine NbC carbides precipitated in the metal matrix increased the corrosion resistance; as a result, the corrosion rate was reduced by 59% (1156 mpy) and 65% (986 mpy) for the HSDI-0.6 wt.% Nb and HSDI-0.9 wt.% Nb samples, respectively, compared to the DI-0.3 wt.% Nb sample (2802 mpy).
The corrosion resistance of cast iron alloys is primarily determined by their chemical composition, which causes changes in the microstructure. Figure 13 shows the SEM microstructures of all corroded surfaces where a preferential attack of pearlite in the matrix is evident upon exposure to acid solution; it can be assumed that a higher pearlite volume fraction reduced the corrosion resistance of HSDI in this environment.
Figure 14 shows the scheme of the corrosion mechanism of Nb-HSDI exposed to H2SO4, as a result of potentiodynamic curves and microstructures analysis. In Figure 14a the HSDI is presented with a mixture of ferritic–pearlitic matrix, with embedded graphite nodules and NbC dispersed within the microstructure. At this stage, it was not yet in contact with the electrolyte; that is, it did not start the corrosion process. At the beginning of the corrosion process, as can be seen in Figure 14b, upon contact with the electrolyte, galvanic pairs were formed between the graphite and matrix (Figure 14c), resulting in a corroded surface; however, the graphite nodules were unaffected, as shown in Figure 14d.
Si atoms play an important role in current foundries because adding Si in cast iron produces cathodic protection [58,59]; as they are present in large numbers, it is possible to find them throughout the matrix. However, in the ferritic matrix, these atoms are more readily available to associate with oxygen, unlike pearlite. Figure 14c is an illustration of an arrangement of Si atoms, while Figure 14d shows that the residual Si atoms are oxidized to form a protective layer of silicon oxide (SiO2). Iron with a high content of Si can easily form a SiO2 passivation layer to protect the material [58,59] because the enthalpy of Si–O chemical bond formation is significantly more negative than that of Fe–O.
In the present study it was found that the electrochemical corrosion occurs due to the formation of a galvanic couple between the graphite and the metallic matrix. Therefore, it can be concluded that the corrosion resistance of HSDI in an acidic environment is significantly influenced by the high Si content and Nb content, which contribute to the reduction of the corrosion rate. Nb promotes carbide formation, while Si promotes a ferritic matrix, which in combination results in HSDI being passive. Therefore, Nb content higher than 0.3 wt.% and high Si content in DI can be used as protective systems against cathodic corrosion.

4. Conclusions

The current study shows the effects of Nb on the microstructural characteristics, hardness, and corrosion resistance of high-silicon ductile cast iron (HSDI)-3.6 wt.% Si. It was found that:
  • The formation of polygonal NbC carbides was observed in HSDI upon the Nb addition in a range of 0.3–0.9 wt.%. These carbides have a size between 0.5 and 4.5 µm, which decreases with increasing Nb content in HSDI.
  • The presence of NbC carbides increases with Nb content, resulting in heterogeneous graphite nodule nucleation.
  • Nodularity maintained a linear trend between 86 and 90%, and the nodule count decreased with the presence of Nb in the HSDI, from 182 n/mm2 for U-HSDI to 147–154 n/mm2 for HSDI-% Nb samples.
  • An increment in hardness is due to the greater presence of NbC carbides in combination with high Si, resulting in an increase from 30.44 to 38.38 HRC for U-HSDI and HSDI-0.9 wt.% Nb, respectively.
  • Electrochemical corrosion is achieved by the formation of a galvanic couple between the graphite and the metal matrix which preferentially attacks the pearlite, with the greatest corrosion occurring at HSDI-0.3 wt.% Nb.
  • The combination of Nb with Si in HSDI helps to reduce the corrosion rate, since Nb promotes the development of NbC carbides and Si promotes the ferritic matrix. If the Nb content exceeds 0.3 wt.% and the Si content in the DI is more than 2.8 wt.%, protective systems against cathodic corrosion can be applied to the HSDI and it can be used in agriculture applications.
For future work, HSDI austempering is one of the main interests of the authors as a highly developed alternative for applications in the agricultural industry, since with this research it has been shown that Si and Nb in ductile iron play an important role in the resulting microstructures as well as in the distribution of NbC, affecting the mechanical properties and the corrosion resistance of such castings.

Author Contributions

Conceptualization, N.A.R.R., C.R.M.V. and D.G.N.; Methodology, N.A.R.R. and C.R.M.V.; Validation, J.S.G.V. and E.A.C.; Formal Analysis, N.A.R.R., D.G.N. and F.A.M.G.; Investigation, D.G.N. and N.A.R.R.; Resources, C.R.M.V.; Data Curation, E.A.C. and D.G.N.; Writing—Original Draft Preparation, N.A.R.R. and D.G.N., Software, D.G.N., E.A.C. and F.A.M.G.; Writing—Review and Editing, N.A.R.R. and D.G.N.; Supervision, J.S.G.V. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The research data and methods used in the investigation are presented in sufficient detail in the paper to allow other researchers to reproduce the work.

Acknowledgments

The authors are grateful to the TECNM (project: 11548.21-P) and the Universidad Autónoma de Coahuila for the facilities granted.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Y-block schematic representation and samples extraction region.
Figure 1. Y-block schematic representation and samples extraction region.
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Figure 2. SEM micrographs of (a) commercial DI, (b) U-HSDI, (c) HSDI-0.3% Nb, (d) HSDI-0.6% Nb, (e) HSDI-0.9% Nb.
Figure 2. SEM micrographs of (a) commercial DI, (b) U-HSDI, (c) HSDI-0.3% Nb, (d) HSDI-0.6% Nb, (e) HSDI-0.9% Nb.
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Figure 3. Distributions of Nb and NbC carbides in samples: (a) HSDI-0.3% Nb, (b) HSDI-0.6% Nb, (c) HSDI-0.9% Nb, obtained by EDX at 1000×.
Figure 3. Distributions of Nb and NbC carbides in samples: (a) HSDI-0.3% Nb, (b) HSDI-0.6% Nb, (c) HSDI-0.9% Nb, obtained by EDX at 1000×.
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Figure 4. Effect of Nb content on the size and number of NbC carbides on HSDI.
Figure 4. Effect of Nb content on the size and number of NbC carbides on HSDI.
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Figure 5. (a) SEM micrograph of HSDI-0.9% Nb sample showing the NbC carbides distribution, (b) EDX of NbC carbide inside the graphite nodule, (c) EDX of NbC carbide.
Figure 5. (a) SEM micrograph of HSDI-0.9% Nb sample showing the NbC carbides distribution, (b) EDX of NbC carbide inside the graphite nodule, (c) EDX of NbC carbide.
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Figure 6. Distributions of C, Si, and Cu in samples: (a) HSDI-0.3% Nb, (b) HSDI-0.6% Nb, (c) HSDI-0.9% Nb, obtained by EDX at 1000×.
Figure 6. Distributions of C, Si, and Cu in samples: (a) HSDI-0.3% Nb, (b) HSDI-0.6% Nb, (c) HSDI-0.9% Nb, obtained by EDX at 1000×.
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Figure 7. Effect of Nb on the nodularity and nodule count of HSDI samples.
Figure 7. Effect of Nb on the nodularity and nodule count of HSDI samples.
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Figure 8. OM microstructures of samples: (a) commercial DI, (b) U-HSDI, (c) HSDI-0.3% Nb, (d) HSDI-0.6% Nb, (e) HSDI-0.9% Nb.
Figure 8. OM microstructures of samples: (a) commercial DI, (b) U-HSDI, (c) HSDI-0.3% Nb, (d) HSDI-0.6% Nb, (e) HSDI-0.9% Nb.
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Figure 9. Variation of volume fraction of phases in commercial DI and HSDI alloyed with Nb.
Figure 9. Variation of volume fraction of phases in commercial DI and HSDI alloyed with Nb.
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Figure 10. Hardness profile of commercial DI and HSDI with different Nb content.
Figure 10. Hardness profile of commercial DI and HSDI with different Nb content.
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Figure 11. Potentiodynamic curves of commercial DI and HSDI with different Nb contents.
Figure 11. Potentiodynamic curves of commercial DI and HSDI with different Nb contents.
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Figure 12. Corrosion rates of commercial DI and HSDI with different Nb contents.
Figure 12. Corrosion rates of commercial DI and HSDI with different Nb contents.
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Figure 13. SEM microstructures of all corroded surfaces of HSDI with different Nb contents: (a) commercial DI, (b) U-HSDI, (c) HSDI-0.3% Nb, (d) HSDI-0.6% Nb, (e) HSDI-0.9% Nb.
Figure 13. SEM microstructures of all corroded surfaces of HSDI with different Nb contents: (a) commercial DI, (b) U-HSDI, (c) HSDI-0.3% Nb, (d) HSDI-0.6% Nb, (e) HSDI-0.9% Nb.
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Figure 14. The schematic mechanism on HSDI corrosion exposed to H2SO4. (a) unaffected HSDI; (b) HSDI is initially exposed at H2SO4 solution; (c) galvanic pairs were formed between the graphite and HSDI matrix; (d) formation of a protective layer of silicon oxide (SiO2) on HSDI surface.
Figure 14. The schematic mechanism on HSDI corrosion exposed to H2SO4. (a) unaffected HSDI; (b) HSDI is initially exposed at H2SO4 solution; (c) galvanic pairs were formed between the graphite and HSDI matrix; (d) formation of a protective layer of silicon oxide (SiO2) on HSDI surface.
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Table
Table 1. Chemical composition for HSDI-Nb alloys studied (wt.%).
Table 1. Chemical composition for HSDI-Nb alloys studied (wt.%).
CSiMnSCuPNbMgCeCrVMoFe
Commercial DI3.252.600.850.0060.710.018-0.0530.031---bal.
U-HSDI3.403.760.420.0061.120.030.0040.0260.0330.070.0130.011bal.
HSDI-0.3% Nb3.423.760.420.0051.120.030.2960.0240.0330.080.0110.012bal.
HSDI-0.6% Nb3.393.760.420.0061.120.030.5920.0210.0310.060.0150.011bal.
HSDI-0.9% Nb3.403.750.420.0071.120.030.9210.0200.0310.060.0120.014bal.
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Muñiz Valdez, C.R.; García Navarro, D.; Galindo Valdés, J.S.; Montes González, F.A.; Almanza Casas, E.; Rodríguez Rosales, N.A. Determination of Corrosion Resistance of High-Silicon Ductile Iron Alloyed with Nb. Metals 2023, 13, 917. https://doi.org/10.3390/met13050917

AMA Style

Muñiz Valdez CR, García Navarro D, Galindo Valdés JS, Montes González FA, Almanza Casas E, Rodríguez Rosales NA. Determination of Corrosion Resistance of High-Silicon Ductile Iron Alloyed with Nb. Metals. 2023; 13(5):917. https://doi.org/10.3390/met13050917

Chicago/Turabian Style

Muñiz Valdez, Carlos Rodrigo, Daniel García Navarro, Jesús Salvador Galindo Valdés, Félix Alan Montes González, Efrain Almanza Casas, and Nelly Abigail Rodríguez Rosales. 2023. "Determination of Corrosion Resistance of High-Silicon Ductile Iron Alloyed with Nb" Metals 13, no. 5: 917. https://doi.org/10.3390/met13050917

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