Ductile iron castings, such as those used in wind turbine components like hubs, shafts, and planetary carriers, are predominantly manufactured from ductile iron grades like QT400-18L. These ductile iron castings are often deployed in coastal regions where harsh environmental conditions, including seawater, tides, and salt fog, accelerate corrosion. Ensuring the longevity of these ductile cast iron parts for up to 20 years without maintenance poses significant challenges, necessitating enhanced mechanical properties and corrosion resistance. In this study, we investigate the incorporation of multi-scale ceramic particles (MSCP) into the melt of QT400-18L ductile iron to improve its corrosion resistance under varying humidity conditions. We explore the mechanisms by which MSCP influences the corrosion behavior, aiming to provide novel insights for enhancing the durability of ductile iron castings in corrosive coastal environments.
The chemical composition of the ductile iron QT400-18L used in this work is detailed in Table 1. The MSCP, comprising activated multi-scale composite SiC particles, was added in mass fractions ranging from 0.05% to 0.15%. Particle size and surface area were characterized using a Mastersizer 2000 laser granulometer. Specimens were machined into ø25 mm × 3 mm discs from the center of cast test blocks. The “constant temperature variable humidity” tests were conducted in accordance with GB/T4797.1-2018 standards in a HS-100A constant humidity and temperature chamber, with a total test duration of 168 hours at 60°C. The humidity variations are summarized in Table 2, with three parallel specimens per condition, and average values reported.
| 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 |
| MSCP Addition (wt.%) | Specimen ID | Humidity (%) |
|---|---|---|
| 0 | 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 |
Corrosion rates were evaluated using a full immersion test, where weight changes before and after exposure were 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 masses before and after corrosion in grams, \( A \) is the specimen surface area in m², and \( t \) is the test duration in hours. Surface morphology, corrosion layer thickness, and corrosion products were examined using a Zeiss SUPRA55 field emission scanning electron microscope (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS). Phase composition of corrosion products was determined via X-ray diffraction (XRD) on an Empyrean diffractometer. Graphite nodularity, size, and matrix content were analyzed according to GB/T9441-2021 using image analysis software.
The incorporation of MSCP significantly refined the microstructure of the ductile iron. In the baseline ductile iron casting without MSCP, graphite nodules exhibited non-uniform size and poor sphericity, with some vermicular graphite present. Adding MSCP enhanced nodularity, reduced graphite size, and improved sphericity, as summarized in Table 3. The optimum nodularity grade of 2 was achieved at 0.10% MSCP addition. The matrix structure consisted primarily of ferrite with minor pearlite, and MSCP addition increased the ferrite content, as shown in Table 4. For instance, ferrite content rose from 79.16% in the untreated ductile iron to 86.95% at 0.15% MSCP, indicating microstructural refinement that contributes to corrosion resistance.
| MSCP Addition (wt.%) | 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 |
| MSCP Addition (wt.%) | Ferrite Content (%) |
|---|---|
| 0 | 79.16 |
| 0.05 | 86.42 |
| 0.10 | 85.84 |
| 0.15 | 86.95 |
Corrosion rates under varying humidity conditions are presented in Table 5. As humidity increased, corrosion rates accelerated, with a notable inflection point at 80% humidity. Below this threshold, corrosion rates were similar regardless of MSCP addition. Above 80%, however, MSCP-modified specimens showed lower corrosion rates, with the 0.15% addition yielding the best performance. At 98% humidity, corrosion rates decreased by 10.10%, 21.21%, and 29.29% for 0.05%, 0.10%, and 0.15% MSCP additions, respectively, compared to the untreated ductile iron. This behavior is attributed to the formation of continuous thin liquid films above 80% humidity, which facilitate electrochemical corrosion. The relationship between humidity and corrosion rate can be modeled as:
$$ V = k_1 \cdot e^{k_2 H} $$
where \( V \) is the corrosion rate, \( H \) is humidity, and \( k_1 \) and \( k_2 \) are constants dependent on MSCP content.
| MSCP Addition (wt.%) | Humidity 60% (g/(m²·h)) | Humidity 80% (g/(m²·h)) | Humidity 90% (g/(m²·h)) | Humidity 98% (g/(m²·h)) |
|---|---|---|---|---|
| 0 | 0.0021 | 0.0045 | 0.0078 | 0.0099 |
| 0.05 | 0.0020 | 0.0043 | 0.0072 | 0.0089 |
| 0.10 | 0.0019 | 0.0040 | 0.0065 | 0.0078 |
| 0.15 | 0.0018 | 0.0038 | 0.0060 | 0.0070 |
Analysis of corrosion products revealed that they predominantly accumulated around graphite nodules, forming distinct “double-ring” patterns. EDS and XRD analyses identified the corrosion products as iron oxides, including FeO, Fe₂O₃, and Fe₃O₄, with elements C, O, Fe, and Si present. XRD diffraction peaks were lower in MSCP-added specimens, indicating reduced corrosion product formation. The “double-ring” morphology arises from micro-droplet phenomena, where moisture preferentially adsorbs at interfaces between graphite and the matrix, leading to localized electrochemical cells. The corrosion process can be described by the anodic dissolution reaction:
$$ \text{Fe} \rightarrow \text{Fe}^{2+} + 2e^- $$
and the cathodic reaction:
$$ \text{O}_2 + 2\text{H}_2\text{O} + 4e^- \rightarrow 4\text{OH}^- $$
The enhanced corrosion resistance in MSCP-modified ductile iron castings is mechanistically explained by several factors. First, activated MSCP particles serve as nucleation sites, refining the graphite and matrix structure. This refinement reduces the interfacial energy between graphite and ferrite, minimizing sites for corrosion initiation. Second, MSCP increases ferrite content while decreasing pearlite, thereby reducing micro-galvanic couples between ferrite and cementite in pearlite. The electrochemical potential difference \( \Delta E \) between graphite and the matrix is lowered, as approximated by:
$$ \Delta E = E_{\text{cathode}} – E_{\text{anode}} $$
where \( E_{\text{cathode}} \) is the potential of graphite and \( E_{\text{anode}} \) is that of the matrix. With MSCP, \( E_{\text{anode}} \) increases due to solid solution effects, reducing \( \Delta E \) and corrosion rate. Additionally, MSCP may promote the formation of protective oxide films, further impeding corrosion.
In summary, the addition of multi-scale ceramic particles to QT400-18L ductile iron significantly improves its corrosion resistance under high humidity conditions. Microstructural refinements, including enhanced graphite nodularity and increased ferrite content, play crucial roles in mitigating corrosion. These findings underscore the potential of MSCP for enhancing the durability of ductile iron castings in demanding environments like coastal wind power applications. Future work could focus on optimizing MSCP composition and distribution for even greater performance gains in ductile cast iron components.