In modern engineering applications, particularly in the renewable energy sector, the demand for durable and reliable materials is paramount. As a key material for components such as wind turbine hubs, shafts, and planetary carriers in wind power generation systems, ductile cast iron, specifically grade QT400-18L, is widely employed due to its excellent mechanical properties and castability. However, the operational environments for these wind turbines, often coastal or offshore regions, expose the ductile cast iron components to aggressive conditions including salt spray, high humidity, and cyclic wet-dry cycles. These factors can significantly accelerate corrosion processes, threatening the structural integrity and longevity of the components, which are typically required to function maintenance-free for over 20 years. Therefore, enhancing the corrosion resistance of ductile cast iron without compromising its mechanical performance has become a critical research focus. In this study, we investigate a novel approach to improve the atmospheric corrosion resistance of QT400-18L ductile cast iron by introducing multi-scale ceramic particles (MSCP) into the melt. We explore the effects of MSCP addition on the microstructure, corrosion kinetics under varying humidity conditions, and the underlying mechanisms responsible for improved performance. This work aims to provide new insights and practical strategies for developing more corrosion-resistant ductile cast iron castings for harsh environmental service.
The inherent corrosion susceptibility of ductile cast iron stems from its heterogeneous microstructure, which consists of graphite spheroids embedded in a metallic matrix, predominantly ferrite or pearlite. This heterogeneity establishes numerous micro-galvanic cells where the graphite acts as a cathode and the surrounding ferritic matrix as an anode, facilitating electrochemical corrosion. In atmospheric corrosion, the presence of a thin electrolyte layer, formed by moisture condensation, governs the corrosion rate. The kinetics are influenced by factors such as relative humidity (RH), temperature, pollutant deposition, and the material’s microstructural features. A fundamental equation describing the corrosion rate (V) in weight loss terms is given by:
$$V = \frac{M_2 – M_1}{A \cdot t}$$
where \(M_1\) and \(M_2\) are the sample masses before and after corrosion (g), \(A\) is the exposed surface area (m²), and \(t\) is the exposure time (h). While this formula provides a macroscopic measure, the microscopic processes involve complex electrochemical reactions. The anodic dissolution of iron can be represented as:
$$\text{Fe} \rightarrow \text{Fe}^{2+} + 2e^-$$
and the cathodic reduction of oxygen in neutral or alkaline environments as:
$$\text{O}_2 + 2\text{H}_2\text{O} + 4e^- \rightarrow 4\text{OH}^-$$
The formation of corrosion products, primarily iron oxides and hydroxides, follows subsequent precipitation and oxidation steps. The overall corrosion process is strongly dependent on the stability and protectiveness of this rust layer.
Our hypothesis is that the incorporation of multi-scale ceramic particles can modify the microstructure of ductile cast iron, leading to a more homogeneous and refined structure that mitigates micro-galvanic corrosion and enhances the stability of the corrosion product layer. The MSCP used in this study are activated, multi-scale composite silicon carbide (SiC) particles. Their incorporation is expected to act as heterogeneous nucleation sites during solidification, refining the graphite and matrix structure. Furthermore, the activated surfaces of these particles may interact with the melt, potentially introducing alloying elements that alter the electrochemical properties of the matrix.
To systematically evaluate the effects, we designed an experimental program focusing on humidity-induced atmospheric corrosion. The base material was QT400-18L ductile cast iron with the following nominal composition, which we verified through spectroscopic analysis:
| Element | Composition Range (wt.%) |
|---|---|
| C | 3.56 – 3.68 |
| Si | 2.19 – 2.40 |
| Mn | 0.148 – 0.150 |
| P | 0.030 – 0.032 |
| S | 0.010 – 0.012 |
| Mg | 0.029 – 0.062 |
| Re (Rare Earth) | 0.032 – 0.047 |
| Fe | Balance |
The MSCP were added to the molten ductile cast iron in three different mass fractions: 0.05%, 0.10%, and 0.15%. A reference specimen without MSCP addition was also prepared for comparison. The particles were subjected to a proprietary activation process to enhance their wettability and dispersion within the iron melt. Their size distribution, characterized using laser diffraction, showed a multi-modal distribution spanning from sub-micron to several tens of microns, hence the term “multi-scale.”
From the cast samples, we machined disc-shaped specimens (Ø25 mm × 3 mm) for corrosion testing. The atmospheric corrosion tests were conducted in a constant temperature and humidity chamber according to adapted standards. The temperature was held constant at 60°C, while the relative humidity was varied across different test runs. The specific humidity conditions and corresponding sample designations are summarized in the table below. Each condition was tested with three parallel specimens to ensure statistical reliability, and the average values are reported.
| MSCP Addition (wt.%) | Sample Code | Relative Humidity (%) |
|---|---|---|
| 0 (Reference) | 1-1# | 60 |
| 1-2# | 80 | |
| 1-3# | 90 | |
| 1-4# | 98 | |
| 0.05 | 2-1# | 60 |
| 2-2# | 80 | |
| 2-3# | 90 | |
| 2-4# | 98 | |
| 0.10 | 3-1# | 60 |
| 3-2# | 80 | |
| 3-3# | 90 | |
| 3-4# | 98 | |
| 0.15 | 4-1# | 60 |
| 4-2# | 80 | |
| 4-3# | 90 | |
| 4-4# | 98 |
The total exposure time for all corrosion tests was 168 hours (7 days). Post-exposure, the specimens were carefully cleaned to remove corrosion products according to standard procedures, and the weight loss was measured to calculate the corrosion rate using the formula above. Microstructural characterization was performed on polished and etched samples using optical microscopy and scanning electron microscopy (SEM). The graphite morphology parameters, such as nodularity, nodule count, and size, were quantified using image analysis software according to relevant standards. The phase composition of the corrosion products was determined using X-ray diffraction (XRD), and their morphology and elemental composition were analyzed via SEM coupled with energy-dispersive X-ray spectroscopy (EDS).
The initial microstructural analysis revealed significant modifications induced by the MSCP addition. The reference ductile cast iron sample exhibited a typical microstructure with graphite spheroids dispersed in a ferritic matrix containing a small fraction of pearlite. However, the graphite nodules showed some size inhomogeneity and imperfect spheroidicity, with occasional vermicular graphite present. In contrast, the ductile cast iron samples with MSCP addition demonstrated a notable refinement and improvement in graphite morphology. The following table summarizes the quantitative metallography results for the different MSCP addition levels, evaluated prior to corrosion exposure.
| MSCP Addition (wt.%) | Graphite Nodularity Grade | Nodularity (%) | Graphite Size Number | Ferrite Content (vol.%) |
|---|---|---|---|---|
| 0 | 4 | 79 | 6 | 79.16 |
| 0.05 | 3 | 82 | 7 | 86.42 |
| 0.10 | 2 | 91 | 7 | 85.84 |
| 0.15 | 3 | 86 | 7 | 86.95 |
The data clearly indicates that MSCP addition improves the nodularity grade and increases the nodularity percentage. The best graphite morphology was achieved with 0.10% MSCP, reaching a grade 2 nodularity. Furthermore, the graphite size became finer (higher size number indicates smaller nodules). Concurrently, the volume fraction of ferrite in the matrix increased from about 79% in the reference ductile cast iron to approximately 86-87% in the MSCP-treated ductile cast iron samples. This increase in ferrite content correlates with a reduction in pearlite, which is significant because pearlite itself constitutes a micro-galvanic couple between ferrite and cementite (Fe3C).
The corrosion rate data as a function of relative humidity for all ductile cast iron samples is presented graphically below, and the numerical values are tabulated for clarity. The corrosion rate (V) for each condition was calculated using the weight loss method.
| Relative Humidity (%) | Corrosion Rate, V (g·m⁻²·h⁻¹) for MSCP 0% | Corrosion Rate, V (g·m⁻²·h⁻¹) for MSCP 0.05% | Corrosion Rate, V (g·m⁻²·h⁻¹) for MSCP 0.10% | Corrosion Rate, V (g·m⁻²·h⁻¹) for MSCP 0.15% |
|---|---|---|---|---|
| 60 | 0.0018 | 0.0017 | 0.0016 | 0.0015 |
| 80 | 0.0042 | 0.0041 | 0.0039 | 0.0038 |
| 90 | 0.0075 | 0.0069 | 0.0062 | 0.0058 |
| 98 | 0.0099 | 0.0089 | 0.0078 | 0.0070 |
The corrosion rate for all ductile cast iron samples increases with increasing relative humidity, which is expected due to the greater availability of water to form an electrolyte layer. However, a critical observation is the behavior change around 80% RH. Below 80% RH, the corrosion rates for all ductile cast iron samples, with and without MSCP, are relatively low and show similar incremental increases with humidity. The differences between the samples are minimal, suggesting that under these conditions, the formation of a continuous, corrosive electrolyte film is limited. The corrosion process might be dominated by adsorption and chemical oxidation, with kinetics less sensitive to microstructural details.
Above 80% RH, the corrosion rates increase more sharply, and a clear divergence emerges between the reference ductile cast iron and the MSCP-modified ductile cast iron samples. The MSCP-containing ductile cast iron specimens exhibit lower corrosion rates. At 90% RH, the corrosion rate reduction for ductile cast iron with 0.15% MSCP is approximately 22.7% compared to the reference ductile cast iron. At 98% RH, the improvements are more pronounced: 10.1% reduction for 0.05% MSCP, 21.2% for 0.10% MSCP, and 29.3% for 0.15% MSCP. This indicates that the beneficial effect of MSCP on the corrosion resistance of ductile cast iron becomes significantly more effective in high-humidity environments where electrochemical corrosion is dominant.
The humidity threshold of approximately 80% RH can be considered a critical point for the atmospheric corrosion of this ductile cast iron. Below this point, the surface electrolyte is likely discontinuous or very thin, limiting oxygen diffusion and ionic conduction. The corrosion rate (V) in this regime might be approximated by a function dependent on the adsorption isotherm and surface coverage θ of water:
$$V_{low-RH} \propto k_1 \cdot \theta(H_2O)$$
where \(k_1\) is a rate constant. Above the critical humidity, a continuous liquid film forms, enabling full electrochemical cell operation. The corrosion rate in this regime can be described by models considering oxygen diffusion through the liquid layer and charge transfer kinetics. A simplified form could be:
$$V_{high-RH} \propto \frac{i_{corr} \cdot M_{Fe}}{nF} = \frac{k_2 \cdot [O_2] \cdot A_{cathode}}{\delta}$$
where \(i_{corr}\) is the corrosion current density, \(M_{Fe}\) is the molar mass of iron, \(n\) is the number of electrons transferred, \(F\) is Faraday’s constant, \(k_2\) is a constant, \([O_2]\) is the dissolved oxygen concentration, \(A_{cathode}\) is the effective cathodic area (related to graphite surface area), and \(\delta\) is the diffusion layer thickness. The MSCP modification primarily affects the microstructural parameters like \(A_{cathode}\) and the anodic dissolution kinetics.
Examination of the corroded surfaces revealed distinctive morphological features. For all ductile cast iron samples, corrosion initiated preferentially around the graphite nodules. In high humidity tests (e.g., 80% and 98% RH), the corrosion products often formed distinctive “double-ring” patterns encircling the graphite sites. SEM-EDS analysis confirmed that these corrosion products consisted mainly of iron oxides, with traces of silicon from the matrix. XRD analysis identified the phases as a mixture of FeO (wüstite), Fe2O3 (hematite), and Fe3O4 (magnetite). The relative intensity of the XRD peaks was lower for the MSCP-treated ductile cast iron samples, indicating a lesser amount of corrosion product formation compared to the reference ductile cast iron, consistent with the lower weight loss.
The formation of the “double-ring” corrosion pattern is a fascinating phenomenon related to micro-droplet dynamics and localized galvanic corrosion. Initially, moisture condensation preferentially occurs at sites of high surface energy, such as the interface between the graphite nodule and the ferritic matrix in ductile cast iron. This forms a primary micro-droplet. Due to concentration gradients and surface tension effects, solute species (e.g., ions from initial dissolution) diffuse outward, leading to the formation of secondary, smaller micro-droplets in a ring around the primary one. This sets up concentric anodic zones. The region between the rings, experiencing alternating wetting and drying, often shows enhanced corrosion due to cyclic dissolution and oxide formation, leading to the observed ring-like accumulation of porous corrosion products. This process can be conceptually modeled by considering the spreading coefficient S and the differential aeration between the center and the periphery of the droplet assembly.
The mechanism by which MSCP enhances the corrosion resistance of ductile cast iron is multifaceted, rooted in the microstructural alterations they induce.
1. Graphite Refinement and Improved Nodularity: The activated MSCP particles act as potent heterogeneous nucleation sites during the solidification of ductile cast iron. This refines the graphite structure, resulting in smaller, more spherical, and more uniformly distributed nodules. The improvement in nodularity grade and the reduction in graphite size have two major consequences for corrosion. First, the interfacial area between the graphite (cathode) and the matrix (anode) becomes more regular and less defective. Irregular interfaces or vermicular graphite provide easier paths for corrosion propagation and create larger cathodic areas. Second, finer graphite nodules increase the number of nodules per unit area, which might seem to increase the total cathodic area. However, the more important effect is that the increased nodule count reduces the average distance between anodic and cathodic sites, potentially leading to a more uniform current distribution and less intense localized corrosion. The relationship between corrosion current and cathode area can be complex, but a refined structure often leads to a more stable and less porous initial corrosion layer.
2. Increase in Ferrite Content: The addition of MSCP promoted a higher volume fraction of ferrite in the matrix of the ductile cast iron. Ferrite is a single-phase solid solution of carbon in α-iron, whereas pearlite is a lamellar composite of ferrite and cementite. In pearlite, the cementite lamellae (Fe3C) are noble compared to ferrite, creating numerous micro-galvanic cells that can accelerate corrosion. By reducing the pearlite content and increasing the ferrite content, the number of these intrinsic micro-couples within the matrix is decreased. This simplifies the electrochemical activity of the matrix, making its corrosion behavior more akin to a homogeneous material. The overall galvanic driving force between the graphite and the matrix may also be altered if the ferrite’s electrochemical potential is shifted due to possible solute elements from the activated MSCP.
3. Potential Alloying Effects from Activated MSCP: The activation treatment of the SiC-based MSCP likely involves coating or functionalizing the particle surfaces with elements that improve bonding with the iron melt. These elements might include oxide formers (e.g., Al, Cr, Si) or other metals. When incorporated into the ductile cast iron matrix, even in trace amounts, they could enhance the passivation tendency of the ferrite. The formation of a more stable, adherent, and protective oxide film on the anodic sites would slow down the metal dissolution rate. This effect complements the microstructural refinement. The improved corrosion resistance at high humidity, where electrochemical reactions are fast, suggests that MSCP addition not only changes the geometry of the corrosion cells but also improves the inherent corrosion resistance of the anodic material itself.
4. Barrier Effect and Pinning of Corrosion Front: The finely dispersed ceramic particles within the matrix of the ductile cast iron might also act as physical barriers to the progression of the corrosion front. As corrosion proceeds inward from the surface, these inert particles could impede the diffusion of corrosive species or disrupt the continuity of the anodic dissolution paths. This “pinning” effect can contribute to a lower overall corrosion rate and a more uniform corrosion attack, rather than deep pitting.
To further quantify the microstructural influence on corrosion, we can consider a simple model relating the corrosion rate to key parameters. Let \(N\) be the nodule count per unit area, \(d_g\) be the average graphite nodule diameter, and \(f_\alpha\) be the ferrite volume fraction. The effective cathodic current might be proportional to the total graphite-matrix interface area, which scales with \(N \cdot \pi d_g^2\). However, due to shielding and diffusion limitations, the relationship is not linear. Empirically, we observed that the corrosion rate reduction \(\Delta V\) at high humidity correlates well with an increase in ferrite content and nodularity. A multi-parameter fit could be expressed as:
$$\frac{V_{ref} – V_{MSCP}}{V_{ref}} = \beta_1 \cdot \Delta f_\alpha + \beta_2 \cdot \Delta G + \beta_3 \cdot \Delta (1/d_g)$$
where \(V_{ref}\) is the corrosion rate of reference ductile cast iron, \(V_{MSCP}\) is that of MSCP-modified ductile cast iron, \(\Delta f_\alpha\) is the increase in ferrite fraction, \(\Delta G\) is the improvement in nodularity grade (inverse scale), \(\Delta (1/d_g)\) is the increase in fineness of graphite, and \(\beta_1, \beta_2, \beta_3\) are positive coefficients. Our data suggests that \(\Delta f_\alpha\) plays a significant role, especially for the 0.15% MSCP ductile cast iron sample which had the highest ferrite content and lowest corrosion rate at 98% RH.
In conclusion, our investigation demonstrates that the addition of multi-scale ceramic particles is a promising strategy for enhancing the atmospheric corrosion resistance of QT400-18L ductile cast iron, particularly in high-humidity environments simulating coastal conditions. The MSCP modify the microstructure of the ductile cast iron by refining and spheroidizing the graphite and increasing the proportion of ferrite in the matrix. These changes mitigate the micro-galvanic corrosion activity by regularizing the graphite-matrix interface and reducing the number of pearlitic micro-couples. The corrosion rate dependence on humidity shows a critical threshold around 80% RH, above which the beneficial effects of MSCP become pronounced. At 98% RH, a 0.15% MSCP addition reduced the corrosion rate by nearly 30% compared to unmodified ductile cast iron. The corrosion morphology, characterized by ring-like patterns around graphite nodules, underscores the localized nature of the attack and how microstructural refinement alters its progression. This study provides a foundation for optimizing MSCP type, size distribution, and addition levels to tailor the properties of ductile cast iron for demanding applications where both mechanical strength and long-term corrosion resistance are required. Future work could explore the synergistic effects of MSCP with other alloying elements and evaluate performance under cyclic salt spray conditions to better simulate real offshore environments for ductile cast iron components.