In recent years, the demand for durable and high-performance materials in renewable energy sectors has surged, particularly for wind power components such as hubs, shafts, and planetary carriers. These parts are predominantly manufactured using ductile iron casting materials like QT400-18L, which offer a balance of mechanical properties and cost-effectiveness. However, the harsh environmental conditions in coastal regions—characterized by saltwater, tides, and salt fog—pose significant challenges to the longevity of these components. Wind turbines in such areas are expected to operate for up to 20 years without major maintenance or replacement, necessitating enhanced corrosion resistance in ductile iron castings. This study investigates the incorporation of multi-scale ceramic particles (MSCP) into QT400-18L molten metal to improve its corrosion performance under varying humidity conditions, a critical factor in coastal atmospheres. The focus is on understanding how MSCP influences the microstructure and corrosion mechanisms, providing insights for advancing the durability of ductile iron casting in aggressive environments.
The chemical composition of the base QT400-18L ductile iron casting used in this research is detailed in Table 1. The material consists primarily of iron, with controlled amounts of carbon, silicon, manganese, phosphorus, sulfur, magnesium, and rare earth elements to achieve the desired graphite nodularity and matrix structure. MSCP, which are activated multi-scale composite silicon carbide (SiC) particles, were introduced into the melt at varying mass fractions ranging from 0.05% to 0.15%. These particles were characterized using a Mastersizer 2000 laser particle size analyzer to determine their size distribution and specific surface area, ensuring consistent dispersion in the ductile iron casting matrix. Specimens were machined into ø25 mm × 3 mm discs from the center of cast test blocks for corrosion testing.
| Element | Range |
|---|---|
| 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 | 0.032–0.047% |
| Fe | Balance |
Corrosion experiments were conducted under “constant temperature variable humidity” conditions, adhering to the national standard GB/T4797.1-2018, using a HS-100A constant humidity and temperature test chamber. The temperature was maintained at 60°C, while humidity levels were varied as outlined in Table 2. Each test condition included three parallel specimens to ensure statistical reliability, and results were averaged. The corrosion rate was evaluated using a full immersion test, where weight loss before and after exposure was measured. The corrosion rate \( V \) was calculated using the formula:
$$ V = \frac{M_2 – M_1}{A t} $$
where \( V \) is the corrosion rate in g/(m²·h), \( M_1 \) and \( M_2 \) are the initial and final masses of the specimen in grams, \( A \) is the surface area in m², and \( t \) is the exposure time in hours. This approach allows for a quantitative assessment of how MSCP additions affect the degradation of ductile iron casting in simulated coastal atmospheres.
| MSCP Addition (%) | Specimen ID | Humidity (%) |
|---|---|---|
| 0 | 1-1# | 60 |
| 0 | 1-2# | 80 |
| 0 | 1-3# | 90 |
| 0 | 1-4# | 98 |
| 0.05 | 2-1# | 60 |
| 0.05 | 2-2# | 80 |
| 0.05 | 2-3# | 90 |
| 0.05 | 2-4# | 98 |
| 0.10 | 3-1# | 60 |
| 0.10 | 3-2# | 80 |
| 0.10 | 3-3# | 90 |
| 0.10 | 3-4# | 98 |
| 0.15 | 4-1# | 60 |
| 0.15 | 4-2# | 80 |
| 0.15 | 4-3# | 90 |
| 0.15 | 4-4# | 98 |
Microstructural analysis was performed using a ZEISS SUPRA55 field emission scanning electron microscope (SEM) to examine corrosion product morphology, layer thickness, and elemental composition via energy-dispersive X-ray spectroscopy (EDS). Additionally, X-ray diffraction (XRD) with an Empyrean diffractometer identified the phases present in the corrosion products. Image analysis software, compliant with GB/T9441-2021, was employed to quantify graphite nodularity, size, and matrix phase fractions in the ductile iron casting. The incorporation of MSCP significantly refined the graphite morphology, as summarized in Table 3. Without MSCP, the graphite nodules were irregular in size and shape, with some vermicular graphite present. In contrast, MSCP additions led to smaller, more spherical graphite nodules and higher nodularity grades, with the 0.10% MSCP specimen achieving the best results—grade 2 nodularity and 91% nodularity rate. This refinement is crucial for enhancing the corrosion resistance of ductile iron casting, as it reduces the interfacial energy between graphite and the matrix, minimizing sites for corrosive attack.
| 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 ductile iron casting, primarily composed of ferrite with minor pearlite, was also influenced by MSCP additions. Quantitative image analysis revealed that the ferrite content increased from 79.16% in the baseline specimen to 86.42%, 85.84%, and 86.95% for MSCP additions of 0.05%, 0.10%, and 0.15%, respectively. This shift towards a ferritic matrix is beneficial for corrosion resistance, as ferrite has a more uniform electrochemical behavior compared to pearlite, which can form micro-galvanic cells due to its two-phase nature. The enhanced ferrite content, combined with graphite refinement, contributes to the improved performance of ductile iron casting in humid environments.
Corrosion rate measurements, as shown in Figure 3, demonstrate a clear dependence on humidity and MSCP content. At humidity levels below 80%, the corrosion rates for all specimens, regardless of MSCP addition, increased gradually. However, above 80% humidity, a distinct inflection point was observed, where corrosion rates accelerated more rapidly. Specimens with MSCP additions exhibited lower corrosion rates compared to the untreated ductile iron casting, with the 0.15% MSCP specimen showing the highest resistance. For instance, at 98% humidity, the corrosion rates for specimens with 0.05%, 0.10%, and 0.15% MSCP were reduced by 10.10%, 21.21%, and 29.29%, respectively, relative to the baseline. This behavior can be explained by the formation of thin electrolyte films on the surface: at lower humidities, discontinuous films limit oxygen diffusion, resulting in slower corrosion, whereas higher humidities promote larger droplets that enhance electrochemical reactions. The MSCP-modified ductile iron casting mitigates this by providing a more homogeneous microstructure that resists localized corrosion.
The corrosion products formed after 168 hours of exposure were analyzed using SEM and EDS. In untreated specimens, extensive corrosion occurred, with products accumulating around graphite nodules in a distinctive “double-ring” pattern. EDS and XRD analyses confirmed that these products consisted mainly of iron oxides (FeO, Fe₂O₃, and Fe₃O₄), along with carbon, oxygen, and silicon. The XRD patterns showed higher peak intensities for untreated specimens, indicating greater corrosion product formation, whereas MSCP-added specimens had lower intensities, correlating with reduced corrosion. The “double-ring” morphology arises from micro-droplet phenomena, where moisture preferentially adsorbs at high-energy sites like graphite-matrix interfaces, leading to concentric corrosion zones. As corrosion progresses, these rings merge, exacerbating material loss. The MSCP in ductile iron casting disrupts this process by refining the graphite and increasing ferrite content, thereby lowering the interfacial energy and reducing the driving force for corrosion initiation.
To further elucidate the corrosion mechanisms, consider the electrochemical aspects of ductile iron casting in humid environments. The corrosion process can be modeled using the Evans diagram, where the anodic dissolution of iron and cathodic reduction of oxygen are balanced. The corrosion current density \( i_{corr} \) can be expressed as:
$$ i_{corr} = \frac{\beta_a \beta_c}{2.303 R T (\beta_a + \beta_c)} $$
where \( \beta_a \) and \( \beta_c \) are the Tafel slopes for anodic and cathodic reactions, respectively, \( R \) is the gas constant, and \( T \) is the temperature. In MSCP-modified ductile iron casting, the increased ferrite content and refined graphite reduce the anodic area and minimize potential differences, thereby decreasing \( i_{corr} \). Additionally, the MSCP particles may act as inert barriers or facilitate the formation of protective oxide layers, further enhancing resistance. The overall corrosion rate \( V \) can be related to the charge transfer resistance \( R_{ct} \) through the Stern-Geary equation:
$$ V = \frac{B}{R_{ct}} $$
where \( B \) is a constant. Higher \( R_{ct} \) values, indicative of better corrosion resistance, are expected in MSCP-enhanced ductile iron casting due to the microstructural improvements.
In summary, the incorporation of multi-scale ceramic particles into QT400-18L ductile iron casting offers a promising approach to enhance corrosion resistance in coastal wind power applications. The MSCP additions improve graphite nodularity, increase ferrite content, and reduce corrosion rates, particularly at high humidity levels above 80%. The “double-ring” corrosion pattern observed around graphite nodules highlights the importance of microstructure control in mitigating atmospheric corrosion. Future work could explore the long-term behavior of MSCP-modified ductile iron casting under cyclic humidity and salt spray conditions, as well as the economic feasibility of scaling up this technology for industrial production. By advancing the durability of ductile iron casting, this research supports the sustainable operation of wind turbines in corrosive environments, contributing to the global transition to renewable energy.
The results underscore the critical role of material design in extending the service life of critical components. For ductile iron casting, the synergy between MSCP and the iron matrix not only enhances mechanical properties but also provides a robust defense against environmental degradation. As wind power continues to expand into coastal and offshore regions, the development of such advanced materials will be essential for meeting the demanding performance requirements of next-generation energy systems. This study lays a foundation for further innovations in ductile iron casting technology, emphasizing the importance of integrated approaches that combine material science with environmental engineering.