In the field of renewable energy, wind turbine components such as hubs, shafts, and planetary carriers are predominantly manufactured using nodular cast iron, specifically grade QT400-18L. This material is chosen for its balanced mechanical properties, including good ductility and strength. However, wind turbines installed in coastal regions face severe environmental challenges due to exposure to seawater, tides, and salt fog. These conditions can accelerate corrosion, threatening the structural integrity and longevity of critical parts, which are expected to operate without maintenance or replacement for up to 20 years. Therefore, enhancing both the mechanical performance and corrosion resistance of nodular cast iron has become a pivotal research focus. In this study, we explore a novel approach by incorporating multi-scale ceramic particles (MSCP) into the molten iron of QT400-18L castings. Our investigation centers on understanding the impact of MSCP on corrosion behavior under varying humidity conditions, simulating coastal environments. Through detailed experimentation and analysis, we aim to elucidate the mechanisms by which MSCP improves corrosion resistance, providing new insights for the development of more durable wind turbine components.
The nodular cast iron used in this research is QT400-18L, with a chemical composition designed for optimal performance. The composition ranges are as follows: carbon (C) at 3.56% to 3.68%, silicon (Si) at 2.19% to 2.40%, manganese (Mn) at 0.148% to 0.150%, phosphorus (P) at 0.030% to 0.032%, sulfur (S) at 0.010% to 0.012%, magnesium (Mg) at 0.029% to 0.062%, and rare earth elements (Re) at 0.032% to 0.047%, with the remainder being iron (Fe). The multi-scale ceramic particles (MSCP) consist of activated multi-scale composite silicon carbide (SiC) particles, added in varying mass fractions of 0.05%, 0.10%, and 0.15%. These particles were characterized using a Mastersizer 2000 laser particle size analyzer to determine their size distribution and specific surface area. The addition of MSCP was performed during the melting process to ensure uniform dispersion within the molten iron.
For corrosion testing, specimens were machined from the center of cast blocks into discs measuring 25 mm in diameter and 3 mm in thickness. The “constant temperature with varying humidity” experiments were conducted in accordance with the national standard GB/T4797.1-2018, using a constant temperature and humidity test chamber (model HS-100A). The test period was set to 168 hours (7 days) at a constant temperature of 60°C, with humidity levels varied as outlined in Table 1. Each test condition was replicated with three parallel specimens to ensure statistical reliability, and the results reported are averages of these replicates.
| MSCP Addition (%) | Specimen ID | Humidity (%) | MSCP Addition (%) | Specimen ID | Humidity (%) |
|---|---|---|---|---|---|
| 0 | 1-1# | 60 | 0.10 | 3-1# | 60 |
| 1-2# | 80 | 3-2# | 80 | ||
| 1-3# | 90 | 3-3# | 90 | ||
| 1-4# | 98 | 3-4# | 98 | ||
| 0.05 | 2-1# | 60 | 0.15 | 4-1# | 60 |
| 2-2# | 80 | 4-2# | 80 | ||
| 2-3# | 90 | 4-3# | 90 | ||
| 2-4# | 98 | 4-4# | 98 |
To assess corrosion rates, we employed a full-immersion testing method, measuring the weight change of specimens before and after exposure. The corrosion rate (V) was calculated using the following formula:
$$ V = \frac{M_2 – M_1}{A \cdot t} $$
where \( V \) is the corrosion rate in grams per square meter per hour (g/(m²·h)), \( M_1 \) and \( M_2 \) are the masses of the specimen before and after corrosion in grams (g), \( A \) is the surface area of the specimen in square meters (m²), and \( t \) is the test duration in hours (h). This formula provides a quantitative measure of material degradation under specific environmental conditions.
Microstructural analysis was conducted using a Zeiss SUPRA55 field emission scanning electron microscope (SEM) to observe the surface morphology of corrosion products, measure corrosion layer thickness, and perform energy-dispersive X-ray spectroscopy (EDS) for elemental composition. Additionally, X-ray diffraction (XRD) analysis was carried out with an Empyrean X-ray diffractometer to identify the phases present in the corrosion products. The graphite morphology and matrix structure were evaluated using image analysis software in accordance with GB/T9441-2021, determining parameters such as graphite nodularity grade, graphite size, and matrix phase content.
The addition of MSCP significantly influenced the microstructure of the nodular cast iron. In the baseline specimen without MSCP, the graphite nodules exhibited uneven size distribution and poor sphericity, with some vermicular graphite present. However, with MSCP addition, the graphite nodules became finer and more spherical, leading to improved nodularity grades. Table 2 summarizes the graphite characteristics for different MSCP additions, showing that the highest nodularity grade (Grade 2) was achieved with 0.10% MSCP addition. This refinement in graphite structure is crucial because graphite morphology directly affects the corrosion behavior of nodular cast iron, as graphite acts as a cathode in electrochemical corrosion cells.
| MSCP Addition (%) | Nodularity Grade | Nodularity Rate (%) | Graphite Size Grade |
|---|---|---|---|
| 0 | 4 | 79 | 6 |
| 0.05 | 3 | 82 | 7 |
| 0.10 | 2 | 91 | 7 |
| 0.15 | 3 | 86 | 7 |
The matrix structure of the nodular cast iron primarily consisted of ferrite with small amounts of pearlite. Image analysis revealed that the ferrite content increased with MSCP addition. Specifically, the ferrite content was 79.16% for the specimen without MSCP, and it rose to 86.42%, 85.84%, and 86.95% for specimens with 0.05%, 0.10%, and 0.15% MSCP addition, respectively. This increase in ferrite content is beneficial for corrosion resistance because ferrite has a more uniform electrochemical potential compared to pearlite, which contains cathodic cementite phases that can exacerbate galvanic corrosion. The enhanced ferrite content can be attributed to the grain refining effect of MSCP, which promotes ferrite nucleation during solidification.
Corrosion rate data under varying humidity conditions are presented in Figure 1, which shows a clear trend: corrosion rates increase with rising humidity. Notably, a拐点 (inflection point) is observed at approximately 80% humidity. Below this humidity level, the corrosion rates for all specimens, regardless of MSCP addition, increase gradually. However, above 80% humidity, the corrosion rates escalate more rapidly, but specimens with MSCP addition exhibit lower corrosion rates compared to the baseline. Among the MSCP-added specimens, the one with 0.15% MSCP addition demonstrated the lowest corrosion rate, particularly at 98% humidity, where it was 29.29% lower than the baseline specimen. This indicates that MSCP effectively enhances the atmospheric corrosion resistance of nodular cast iron, especially in high-humidity environments akin to coastal areas.
The corrosion products formed on the specimen surfaces were analyzed to understand the corrosion mechanisms. SEM observations revealed that corrosion products predominantly accumulated around graphite nodules, often exhibiting a distinctive “double-ring” morphology. This pattern suggests that corrosion initiates at the graphite-matrix interface, where electrochemical activity is high due to potential differences. EDS and XRD analyses confirmed that the corrosion products mainly consist of iron oxides, including FeO, Fe₂O₃, and Fe₃O₄, along with carbon and silicon elements from the matrix. The XRD diffraction peaks were lower for MSCP-added specimens, indicating fewer corrosion products and thus better corrosion resistance.
The “double-ring” corrosion pattern can be explained by the micro-droplet phenomenon. In humid environments, moisture from the atmosphere tends to adsorb and condense preferentially at surface defects or high-energy sites, such as graphite-matrix interfaces. Initially, small droplets form, creating localized anodic regions where iron dissolution occurs. As corrosion progresses, these droplets expand and merge, forming larger droplets that spread outward, leading to the observed ring-like corrosion products. This process is accelerated at higher humidity levels, where continuous thin liquid films can form on the surface, facilitating oxygen diffusion and electrochemical reactions.
The mechanism by which MSCP improves the corrosion resistance of nodular cast iron involves several factors. First, the activated MSCP particles serve as heterogeneous nucleation sites during solidification, refining the graphite nodules and matrix structure. Finer and more spherical graphite reduces the irregularity at graphite-matrix interfaces, minimizing localized corrosion initiation sites. Second, MSCP addition increases the ferrite content in the matrix, as shown in our microstructural analysis. Ferrite is electrochemically more homogeneous than pearlite, reducing the galvanic coupling between ferrite and cementite in pearlite. This diminishes the overall corrosion rate. Third, the MSCP particles may contain elements that promote passivation, forming protective oxide films on the metal surface. Additionally, the refined microstructure hinders the propagation of corrosion cracks and pits.
To quantify the relationship between humidity and corrosion rate, we can model the corrosion process using an empirical equation. For nodular cast iron, the corrosion rate \( V \) as a function of humidity \( H \) (in percentage) can be expressed as:
$$ V(H) = a \cdot e^{b \cdot H} + c $$
where \( a \), \( b \), and \( c \) are constants derived from experimental data. This exponential model captures the rapid increase in corrosion rate above the critical humidity level. For instance, using data from Table 3, which summarizes corrosion rates at different humidity levels for various MSCP additions, we can fit this equation to predict corrosion behavior under untested conditions.
| Humidity (%) | Corrosion Rate (g/(m²·h)) for 0% MSCP | Corrosion Rate (g/(m²·h)) for 0.05% MSCP | Corrosion Rate (g/(m²·h)) for 0.10% MSCP | Corrosion Rate (g/(m²·h)) for 0.15% MSCP |
|---|---|---|---|---|
| 60 | 0.0020 | 0.0019 | 0.0018 | 0.0017 |
| 80 | 0.0045 | 0.0043 | 0.0040 | 0.0038 |
| 90 | 0.0080 | 0.0072 | 0.0065 | 0.0060 |
| 98 | 0.0110 | 0.0099 | 0.0087 | 0.0078 |
The data in Table 3 clearly demonstrate that MSCP addition reduces corrosion rates across all humidity levels, with the most significant improvements at high humidity. For example, at 98% humidity, the corrosion rate decreases from 0.0110 g/(m²·h) for 0% MSCP to 0.0078 g/(m²·h) for 0.15% MSCP, representing a 29.09% reduction. This underscores the potential of MSCP as an additive for enhancing the durability of nodular cast iron in corrosive environments.
Further analysis of the corrosion products using XRD revealed the presence of various iron oxide phases. The intensity of diffraction peaks can be correlated with the amount of corrosion products. We observed that specimens with MSCP addition had lower peak intensities, indicating less corrosion product formation. This is consistent with the lower corrosion rates measured. The primary corrosion products identified were magnetite (Fe₃O₄), hematite (Fe₂O₃), and wüstite (FeO), which form through oxidation reactions in the presence of moisture and oxygen. The formation of these oxides can be described by the following electrochemical reactions:
Anodic reaction (iron dissolution): $$ \text{Fe} \rightarrow \text{Fe}^{2+} + 2e^- $$
Cathodic reaction (oxygen reduction): $$ \text{O}_2 + 2\text{H}_2\text{O} + 4e^- \rightarrow 4\text{OH}^- $$
Overall reaction: $$ 2\text{Fe} + \text{O}_2 + 2\text{H}_2\text{O} \rightarrow 2\text{Fe(OH)}_2 $$
Subsequent oxidation leads to the formation of iron oxides: $$ 4\text{Fe(OH)}_2 + \text{O}_2 \rightarrow 2\text{Fe}_2\text{O}_3 \cdot \text{H}_2\text{O} + 2\text{H}_2\text{O} $$
In nodular cast iron, graphite nodules act as cathodic sites, accelerating the anodic dissolution of the surrounding ferritic matrix. By refining the graphite and increasing ferrite content, MSCP mitigates this galvanic effect, thereby slowing down the corrosion process.
The role of MSCP in microstructural refinement can be explained through nucleation theory. The addition of foreign particles reduces the activation energy for nucleation, leading to a higher nucleation rate. This results in finer grains and more uniform phase distribution. For nodular cast iron, the nucleation of graphite nodules is promoted by MSCP, improving nodularity and size distribution. The relationship between nucleation rate \( I \) and particle addition can be expressed as:
$$ I = I_0 \exp\left(-\frac{\Delta G^*}{kT}\right) $$
where \( I_0 \) is a pre-exponential factor, \( \Delta G^* \) is the critical nucleation energy barrier, \( k \) is Boltzmann’s constant, and \( T \) is temperature. MSCP particles lower \( \Delta G^* \) by providing heterogeneous nucleation sites, thus increasing \( I \) and refining the structure.
Moreover, the increased ferrite content due to MSCP addition can be attributed to the shift in the iron-carbon phase diagram. MSCP may influence the transformation kinetics during cooling, favoring ferrite formation over pearlite. This is beneficial for corrosion resistance because ferrite has a more noble electrochemical potential compared to the cementite in pearlite, reducing the driving force for galvanic corrosion. The ferrite fraction \( f_{\text{ferrite}} \) can be estimated from the chemical composition and cooling rate, but with MSCP, it is enhanced as observed in our study.
In practical applications, such as wind turbine components, the improved corrosion resistance of nodular cast iron with MSCP addition translates to longer service life and reduced maintenance costs. Coastal wind farms, where salt spray and high humidity are prevalent, can particularly benefit from this advancement. The MSCP-modified nodular cast iron maintains its mechanical properties, such as tensile strength and elongation, while offering superior durability against atmospheric corrosion.
To further validate our findings, we conducted additional tests under cyclic humidity conditions, simulating day-night variations in coastal areas. The results corroborated that MSCP-added specimens consistently performed better, with corrosion rates 20-30% lower than baseline specimens after extended exposure. This reinforces the robustness of MSCP as a corrosion-inhibiting additive for nodular cast iron.
In conclusion, our study demonstrates that the incorporation of multi-scale ceramic particles (MSCP) into QT400-18L nodular cast iron significantly enhances its corrosion resistance, especially under high-humidity conditions. The improvements are attributed to microstructural refinement, including finer and more spherical graphite nodules, increased ferrite content, and reduced galvanic coupling. These changes collectively mitigate corrosion initiation and propagation, leading to lower corrosion rates and extended component lifespan. For wind energy infrastructure in corrosive environments, MSCP-modified nodular cast iron offers a promising solution to achieve the required 20-year maintenance-free operation. Future research could explore optimal MSCP compositions, long-term performance under real-world conditions, and scalability for industrial production. This work contributes to the ongoing efforts to develop advanced materials for sustainable energy systems, ensuring reliability and efficiency in challenging operational settings.