In the field of renewable energy, ductile iron castings play a critical role in components such as wind turbine hubs, shafts, and planetary carriers due to their excellent mechanical properties and castability. These ductile iron castings are often manufactured using grades like QT400-18L, which offers a balance of strength and ductility. However, when deployed in coastal regions, these ductile iron castings face severe environmental challenges, including exposure to seawater, tidal effects, and salt spray, which accelerate corrosion. Ensuring the longevity of these ductile iron castings for up to 20 years without maintenance is paramount, necessitating enhanced corrosion resistance. This study investigates the incorporation of multi-scale ceramic particles (MSCP) into QT400-18L ductile iron castings to improve their performance under varying humidity conditions, a key factor in coastal atmospheres. The focus is on understanding how MSCP influences the microstructure and corrosion behavior of ductile iron castings, providing insights for industrial applications.
The corrosion of ductile iron castings in humid environments is primarily an electrochemical process, where the formation of thin liquid films on the surface facilitates reactions between the iron matrix and oxygen. The corrosion rate can be described by the following equation, which accounts for mass loss over time: $$ V = \frac{M_2 – M_1}{A \cdot t} $$ where \( V \) is the corrosion rate in g/(m²·h), \( M_1 \) and \( M_2 \) are the initial and final masses in grams, \( A \) is the surface area in m², and \( t \) is the time in hours. This formula is central to evaluating the performance of ductile iron castings under corrosive conditions. In this work, we applied this to samples with and without MSCP additions to quantify improvements.
The base material used in this study was QT400-18L ductile iron, with a chemical composition ranging from 3.56% to 3.68% carbon, 2.19% to 2.40% silicon, 0.148% to 0.150% manganese, 0.030% to 0.032% phosphorus, 0.010% to 0.012% sulfur, 0.029% to 0.062% magnesium, and 0.032% to 0.047% rare earth elements, balanced with iron. The MSCP consisted of activated multi-scale silicon carbide particles, added in weight percentages of 0.05%, 0.10%, and 0.15% to the molten iron. These particles were characterized using laser granulometry to determine their size distribution and specific surface area, which are critical for understanding their role in refining the microstructure of ductile iron castings. Samples were machined into discs of 25 mm diameter and 3 mm thickness from the center of cast blocks, ensuring consistency in testing.
To simulate coastal humidity variations, we conducted constant temperature-variable humidity tests according to standard GB/T4797.1-2018, using a climate chamber set at 60°C with humidity levels of 60%, 80%, 90%, and 98%. The test duration was 168 hours, with three parallel samples for each condition to ensure statistical reliability. The corrosion rates were calculated using the mass loss method, and the results were analyzed to identify trends. Additionally, we employed scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) to examine the morphology and composition of corrosion products. Microstructural analysis was performed based on standard GB/T9441-2021 to assess graphite nodularity, size, and matrix phase distribution in the ductile iron castings.
The addition of MSCP significantly altered the microstructure of the ductile iron castings. In the untreated samples, graphite nodules exhibited irregular shapes and sizes, with some vermicular graphite present, leading to lower nodularity grades. However, with MSCP incorporation, the graphite became finer and more spherical, as summarized in Table 1. For instance, at 0.10% MSCP addition, the nodularity grade improved to level 2, with a nodularity rate of 91% and graphite size of grade 7. This refinement is attributed to the heterogeneous nucleation promoted by MSCP, which reduces the undercooling required for graphite formation. The enhanced nodularity directly contributes to the improved corrosion resistance of ductile iron castings by minimizing localized corrosion sites.
| 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 |
Furthermore, the matrix microstructure of the ductile iron castings was predominantly ferritic, with minor pearlite content. Image analysis revealed that MSCP additions increased the ferrite fraction from 79.16% in the baseline to approximately 86% across all MSCP-treated samples, as shown in Table 2. This shift is crucial because ferrite, being a single-phase structure, reduces the galvanic coupling between phases, thereby lowering the corrosion susceptibility. The increase in ferrite content can be modeled using the lever rule in phase diagrams, where the equilibrium between ferrite and pearlite is influenced by cooling rates and nucleation sites provided by MSCP. The relationship can be expressed as: $$ f_{\alpha} = \frac{C_{\gamma} – C_0}{C_{\gamma} – C_{\alpha}} $$ where \( f_{\alpha} \) is the ferrite fraction, \( C_0 \) is the overall composition, and \( C_{\gamma} \) and \( C_{\alpha} \) are the carbon concentrations in austenite and ferrite, respectively. This microstructural optimization enhances the durability of ductile iron castings in corrosive environments.
| MSCP Addition (%) | Ferrite Content (%) | Pearlite Content (%) |
|---|---|---|
| 0 | 79.16 | 20.84 |
| 0.05 | 86.42 | 13.58 |
| 0.10 | 85.84 | 14.16 |
| 0.15 | 86.95 | 13.05 |
Corrosion rate analysis under different humidity levels revealed a distinct trend, as illustrated in Figure 1. Below 80% humidity, the corrosion rates for all ductile iron castings samples increased gradually, with minimal difference between MSCP-treated and untreated specimens. However, above 80% humidity, a divergence occurred: MSCP-containing samples showed significantly lower corrosion rates, with the 0.15% addition yielding the best performance. At 98% humidity, the corrosion rates for 0.05%, 0.10%, and 0.15% MSCP additions were reduced by 10.10%, 21.21%, and 29.29%, respectively, compared to the baseline. This inflection point at 80% humidity correlates with the threshold for continuous liquid film formation on the surface of ductile iron castings, where electrochemical corrosion accelerates. The corrosion rate \( V \) can be further described by an Arrhenius-type equation accounting for humidity effects: $$ V = k \cdot e^{(b \cdot RH)} $$ where \( k \) is a constant, \( b \) is a humidity coefficient, and \( RH \) is the relative humidity. This model highlights the exponential increase in corrosion beyond critical humidity levels, emphasizing the protective role of MSCP in ductile iron castings.
The morphology of corrosion products provided additional insights. In untreated ductile iron castings, extensive corrosion occurred around graphite nodules, forming distinctive “double-ring” patterns due to micro-droplet formation and spreading. EDS and XRD analyses identified these products as iron oxides (FeO, Fe₂O₃, Fe₃O₄), with higher peak intensities in untreated samples indicating greater corrosion. The “double-ring” phenomenon arises from the preferential adsorption of moisture at graphite-matrix interfaces, where activation energy is high. As corrosion progresses, secondary droplets form around primary ones, leading to concentric corrosion zones. This process can be modeled using diffusion equations: $$ \frac{\partial C}{\partial t} = D \nabla^2 C $$ where \( C \) is the concentration of corrosive species, \( D \) is the diffusion coefficient, and \( t \) is time. The refined microstructure in MSCP-treated ductile iron castings reduces the energy gradients at interfaces, slowing down this diffusion and corrosion spread.
The mechanism by which MSCP enhances the corrosion resistance of ductile iron castings involves multiple factors. First, the activated MSCP particles serve as nucleation sites, refining the graphite and matrix structure. This refinement reduces the potential difference between graphite (cathode) and the ferritic matrix (anode), minimizing galvanic corrosion. The electrochemical potential difference \( \Delta E \) can be expressed as: $$ \Delta E = E_{\text{cathode}} – E_{\text{anode}} $$ where a smaller \( \Delta E \) correlates with lower corrosion rates. Second, the increase in ferrite content decreases the volume of pearlite, which consists of ferrite and cementite phases that form micro-galvanic cells. Additionally, MSCP may introduce passivating elements that form protective oxide films, further shielding the ductile iron castings from environmental attacks. The overall corrosion current density \( i_{\text{corr}} \) in a galvanic couple can be described by: $$ i_{\text{corr}} = \frac{\Delta E}{R_{\text{p}}} $$ where \( R_{\text{p}} \) is the polarization resistance, which increases with MSCP addition due to microstructural homogenization.
In summary, this study demonstrates that multi-scale ceramic particles effectively improve the corrosion resistance of ductile iron castings, particularly under high humidity conditions typical of coastal areas. The optimal MSCP addition of 0.15% resulted in the lowest corrosion rates, attributed to enhanced graphite nodularity, increased ferrite content, and reduced electrochemical driving forces. These findings underscore the potential of MSCP as a valuable additive for prolonging the service life of ductile iron castings in demanding environments. Future work could explore the long-term effects and scalability for industrial production of ductile iron castings.