The relentless pursuit of sustainable energy has positioned wind power as a cornerstone of global energy strategies. Critical components within wind turbine systems, such as hubs, main shafts, and planetary carriers, are predominantly fabricated from ductile iron, specifically grade QT400-18L, due to its excellent castability, good mechanical properties, and cost-effectiveness. The operational environment for many of these turbines, particularly offshore installations, presents a severe challenge. Constant exposure to marine atmospheres laden with salt spray, high humidity, and cyclical wet-dry conditions accelerates corrosion processes. With design lifetimes exceeding 20 years and demanding minimal maintenance for key components, the traditional performance envelope of nodular cast iron must be expanded. The imperative is not only to retain high mechanical properties like elongation and impact toughness but also to significantly enhance long-term atmospheric corrosion resistance. This study investigates a novel approach to achieving this dual objective by inoculating the melt of QT400-18L with multi-scale ceramic particles (MSCP). We focus specifically on the corrosion behavior under varying humidity conditions, simulating the critical environmental factor in coastal regions, and elucidate the underlying mechanisms by which MSCP modifies the microstructure to impart superior durability.
The atmospheric corrosion of metals, including nodular cast iron, is an electrochemical process that occurs under a thin layer of electrolyte formed by moisture condensation. The rate and morphology of attack are profoundly influenced by the microstructure. In nodular cast iron, the graphite spheroids are cathodic relative to the ferritic matrix, establishing numerous micro-galvanic cells. The interfacial region between the graphite and the iron matrix acts as a preferred site for corrosion initiation. Furthermore, the geometry of the graphite—its size, shape (nodularity), and distribution—directly affects the continuity and protective nature of the rust layer that forms. Any microstructural refinement that promotes a more uniform, spherical graphite morphology and a higher fraction of the less-noble phase (ferrite) can potentially decelerate corrosion kinetics. The principle of inoculation is well-established in foundry practice for improving graphite nucleation. This work employs a novel, activated multi-scale inoculant designed not only to refine the as-cast structure but also to chemically influence the electrochemical properties of the matrix.
1. Experimental Materials and Methodology
1.1 Base Material and Inoculant
The base material for this investigation was a commercial grade QT400-18L nodular cast iron. Its chemical composition, determined via optical emission spectroscopy, is presented in Table 1. The key characteristics are a high carbon equivalent for good castability and low levels of tramp elements like phosphorus and sulfur.
| Element | C | Si | Mn | P | S | Mg | RE | Fe |
|---|---|---|---|---|---|---|---|---|
| Content | 3.56-3.68 | 2.19-2.40 | 0.148-0.150 | 0.030-0.032 | 0.010-0.012 | 0.029-0.062 | 0.032-0.047 | Bal. |
The inoculant, designated Multi-Scale Ceramic Particles (MSCP), consisted of surface-activated, composite SiC-based particles with a deliberately broad size distribution. Laser diffraction analysis (Mastersizer 2000) confirmed a multi-modal particle size distribution, providing a high total surface area for interaction with the molten iron. The particles were added to the ladle during the late stages of treatment in four different addition levels: 0 wt.% (reference), 0.05 wt.%, 0.10 wt.%, and 0.15 wt.%.
1.2 Casting, Specimen Preparation, and Microstructural Analysis
Standard Y-block castings were produced for each inoculant level under controlled foundry conditions to ensure consistent melting, spheroidization (using a Fe-Si-Mg-RE alloy), and inoculation practice. Specimens for corrosion testing and microstructural analysis were machined from the sound sections of these castings. For corrosion studies, disc-shaped specimens with dimensions of ø25 mm × 3 mm were prepared. Their surfaces were ground sequentially to a 1200-grit finish using SiC paper, degreased with acetone, ultrasonically cleaned in ethanol, dried, and precisely weighed before testing.
Microstructural analysis was conducted on polished and etched (4% Nital) samples using optical microscopy (OM) and scanning electron microscopy (SEM). Quantitative image analysis was performed according to the standard GB/T 9441-2021 (equivalent to ISO 945) to determine key parameters: nodularity, nodule count, graphite size grade, and the area fraction of metallic phases (ferrite and pearlite).
1.3 Corrosion Testing Methodology
To simulate the critical humidity fluctuations encountered in coastal environments, a “constant temperature-variable humidity” test was employed. Testing was conducted in a programmable climate chamber (HS-100A) following the general guidelines of environmental testing standards. The temperature was held constant at a representative 60°C to accelerate processes, while relative humidity (RH) was varied systematically across a wide range: 60%, 80%, 90%, and 98%. The test matrix is shown in Table 2. The total exposure duration for each condition was 168 hours (7 days). For statistical reliability, triplicate specimens were tested for each unique combination of MSCP level and humidity, with results reported as the average.
| MSCP Addition (wt.%) | Specimen ID (60°C) | Relative Humidity Level (%) |
|---|---|---|
| 0.00 (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 |
Post-exposure, corrosion products were carefully removed from the specimens using a standardized chemical cleaning solution (inhibited hydrochloric acid). The specimens were then rinsed, dried, and weighed again. The corrosion rate, \( V \), was calculated using the weight loss method according to the formula:
$$ V = \frac{M_1 – M_2}{A \cdot t} $$
where:
\( V \) is the corrosion rate in g·m⁻²·h⁻¹,
\( M_1 \) and \( M_2 \) are the mass before and after exposure (in grams),
\( A \) is the total exposed surface area (in m²),
\( t \) is the exposure time (in hours).
1.4 Corrosion Product Characterization
The morphology and distribution of corrosion products formed during testing were examined using a field-emission scanning electron microscope (ZEISS SUPRA55) equipped with an energy-dispersive X-ray spectroscopy (EDS) system. This allowed for high-resolution imaging of corrosion features and semi-quantitative elemental analysis of specific regions. The phase composition of the corrosion products (rust layer) was identified using X-ray diffraction (XRD, Empyrean model) with Cu-Kα radiation. Cross-sectional analysis of selected specimens was also performed to measure the approximate thickness of the corrosion-affected zone.
2. Results and Discussion
2.1 Microstructural Evolution Induced by MSCP
The addition of MSCP exerted a profound and beneficial influence on the microstructure of the QT400-18L nodular cast iron. Quantitative analysis data is consolidated in Table 3.
| MSCP (wt.%) | Nodularity (%) | Graphite Size Grade | Nodule Count (mm⁻²) | Ferrite Area (%) | Pearlite Area (%) |
|---|---|---|---|---|---|
| 0.00 | 79 | 6 | ~120 | 79.2 | 20.8 |
| 0.05 | 82 | 7 | ~185 | 86.4 | 13.6 |
| 0.10 | 91 | 7 | ~210 | 85.8 | 14.2 |
| 0.15 | 86 | 7 | ~195 | 87.0 | 13.0 |
Graphite Morphology: The reference sample (0% MSCP) exhibited a mix of graphite spheroids with moderate nodularity, alongside some irregular (vermicular) graphite and a relatively low nodule count. The introduction of MSCP dramatically improved graphite formation. The particles acted as potent, heterogeneous nucleation sites. This led to a significant increase in nodule count (refinement) and a marked improvement in nodularity and spheroidal shape uniformity. The optimum graphite refinement and shape were observed at the 0.10% MSCP addition, achieving a nodularity of 91%. The relationship between inoculant particle surface area (\(S_{MSCP}\)), undercooling (\(\Delta T\)), and nodule count (\(N\)) can be conceptually framed as the particles reducing the energy barrier for nucleation, leading to a higher number of stable nuclei:
$$ N \propto f(S_{MSCP}, \frac{1}{\Delta T}) $$
Matrix Structure: The matrix of all specimens was predominantly ferritic, consistent with the annealed state of QT400-18L. A key finding was that MSCP addition consistently increased the ferrite area fraction by approximately 7-8 percentage points, concurrently reducing the pearlite content. This microstructural shift is critical because it alters the electrochemical character of the matrix. The increased ferrite fraction reduces the number of ferrite/cementite interfaces within pearlite colonies, which themselves can act as local micro-galvanic couples.
2.2 Corrosion Rate as a Function of Humidity and MSCP Content
The calculated corrosion rates for all specimens across the humidity spectrum are graphically summarized in Figure 1 and key data is presented in Table 4. The overarching trend confirms the dominant role of atmospheric humidity: corrosion rate increases monotonically with rising RH for all material conditions.
| Humidity (%) | Corrosion Rate, V (×10⁻³ g·m⁻²·h⁻¹) | |||
|---|---|---|---|---|
| 0% MSCP | 0.05% MSCP | 0.10% MSCP | 0.15% MSCP | |
| 60 | 2.05 | 2.01 | 1.98 | 1.92 |
| 80 | 4.15 | 4.08 | 3.95 | 3.82 |
| 90 | 8.90 | 8.25 | 7.40 | 7.05 |
| 98 | 11.90 | 10.70 | 9.38 | 8.41 |
A critical observation is the presence of a distinct “transition humidity” at approximately 80% RH. Below this threshold (at 60% and 80% RH), the corrosion rates for all four material variants are very similar, with only minor improvements visible for the MSCP-containing samples. The corrosion process in this regime is likely limited by the availability of a continuous, thick enough electrolyte film on the metal surface. The thin adsorbed moisture layers result in high ionic resistance, restricting the oxygen reduction reaction, which is the common cathode reaction: \( O_2 + 2H_2O + 4e^- \rightarrow 4OH^- \).
Above the 80% RH transition point, the corrosion rate curves diverge significantly. The detrimental effect of high humidity is markedly attenuated in the MSCP-inoculated nodular cast iron samples. At 98% RH, the sample with 0.15% MSCP addition showed a corrosion rate of \(8.41 \times 10^{-3}\) g·m⁻²·h⁻¹, which represents a reduction of approximately 29.3% compared to the uninoculated reference material (\(11.90 \times 10^{-3}\) g·m⁻²·h⁻¹). This demonstrates that the microstructural benefits conferred by MSCP become electrochemically decisive under conditions where a persistent, conductive electrolyte film is readily formed—precisely the condition prevalent in marine atmospheres.
2.3 Morphology and Characteristics of Corrosion Products
SEM examination of corroded surfaces revealed a distinct and reproducible corrosion morphology centered on the graphite nodules. In all samples, but most prominently in the reference material, corrosion products accumulated preferentially at the periphery of graphite spheroids, often forming distinctive “double-ring” structures. EDS point analysis on these ring-shaped products confirmed they were iron oxides/hydroxides, with signals from carbon (from the underlying graphite) and occasional silicon.
XRD analysis of the rust layers formed at 98% RH identified the corrosion products primarily as a mixture of lepidocrocite (γ-FeOOH), goethite (α-FeOOH), and magnetite (Fe₃O₄). The relative peak intensities from the XRD patterns indicated that the total amount of crystalline corrosion products was lower for the MSCP-inoculated specimens, consistent with their lower weight loss.
The formation of the “double-ring” pattern is a direct consequence of the localized electrochemical cell and moisture adsorption dynamics. The graphite nodule, being cathodic, promotes the oxygen reduction reaction around it. The surrounding ferritic matrix serves as the anode, dissolving as Fe²⁺ ions: \( Fe \rightarrow Fe^{2+} + 2e^- \). The region of highest electrochemical activity and thus highest concentration of dissolved species (Fe²⁺, OH⁻) is the graphite/matrix interface. This creates a concentration gradient. According to the micro-droplet phenomenon observed in atmospheric corrosion, a main droplet formed at this active site can develop secondary, satellite micro-droplets in its immediate vicinity due to solute spreading and condensation, leading to the observed concentric corrosion rings. The more irregular the graphite/ferrite interface, the more intense and widespread this localized attack becomes.
2.4 Mechanism of Corrosion Resistance Enhancement by MSCP
The superior atmospheric corrosion resistance of MSCP-inoculated QT400-18L nodular cast iron, particularly under high humidity, can be attributed to a synergistic combination of microstructural modifications induced by the particles.
1. Refinement and Spheroidization of Graphite: The primary action of MSCP is to dramatically improve graphite morphology. The increase in nodule count and nodularity has several anti-corrosion consequences:
• Reduced Cathodic Area: While the total graphite volume fraction remains constant, it is distributed over a larger number of smaller, more isolated spheroids. This decreases the effective cathodic area per graphite unit and disrupts the continuity of cathodic sites, impeding the overall cathodic reaction rate.
• Smoothened Interface: More perfectly spherical graphite presents a smoother, less defective interface with the ferrite matrix. This reduces the density of high-energy sites that preferentially adsorb moisture and initiate localized anodic dissolution. The corrosion “ring” formation is therefore less pronounced.
• Barrier Effect: A finer and more uniform distribution of graphite nodules can contribute to a more tortuous path for corrosion front propagation through the matrix.
2. Increased Ferrite Fraction and Matrix Homogenization: The shift in matrix phase balance towards a higher ferrite content is highly beneficial. Pearlite is a two-phase lamellar structure of ferrite and cementite (Fe₃C). Cementite is cathodic to ferrite, creating numerous, localized micro-cells within each pearlite colony that can accelerate corrosion. By reducing the pearlite content, MSCP inoculation effectively eliminates these internal galvanic couples. The matrix becomes more electrochemically homogeneous, dominated by ferrite. This reduces the overall driving force for micro-galvanic corrosion within the matrix itself. The relationship between corrosion current (\(i_{corr}\)) and the potential difference (\(\Delta E\)) between micro-constituents can be conceptually simplified (ignoring polarization effects) as:
$$ i_{corr} \propto \frac{\Delta E}{R} $$
where \(R\) is the solution resistance. Reducing the number of high \(\Delta E\) couples (graphite/ferrite and ferrite/cementite) by improving graphite morphology and increasing ferrite fraction directly contributes to lowering the overall corrosion current.
3. Potentiation and Solid Solution Effects: The surface activation of the MSCP particles likely involves elements that can enter solid solution within the ferrite matrix during solidification. Certain alloying elements (e.g., Cr, Cu, Ni) are known to elevate the corrosion potential of ferrite. Even in trace amounts from the dissolving inoculant, they can slightly nobilize the ferritic anode, thereby reducing the potential difference (\(\Delta E\)) between the graphite cathode and the iron matrix anode. This reduction in the galvanic driving force is a fundamental way to decelerate corrosion kinetics. Furthermore, these elements may promote the formation of a more protective, dense inner layer of corrosion products.
3. Conclusions
This investigation demonstrates that the inoculation of QT400-18L nodular cast iron melt with multi-scale ceramic particles (MSCP) is a highly effective strategy for enhancing its resistance to atmospheric corrosion, a property critical for long-term service in demanding environments like offshore wind farms. The key findings are:
1. MSCP addition significantly refines the microstructure of nodular cast iron. It increases graphite nodule count, improves nodularity and spheroidization grade (with an optimum at 0.10% addition), and increases the volume fraction of ferrite in the matrix while reducing pearlite.
2. The corrosion performance shows a strong dependence on ambient humidity, with a critical transition point near 80% RH. Below this, corrosion rates are low and similar for all materials. Above 80% RH, where corrosive electrolyte films become stable, the benefits of MSCP inoculation become pronounced.
3. At 98% RH, the MSCP-inoculated nodular cast iron exhibits substantially lower corrosion rates. The sample with 0.15 wt.% MSCP showed a 29.3% reduction in corrosion rate compared to the uninoculated reference material.
4. Corrosion initiates and propagates preferentially at the graphite-ferrite matrix interface, often forming characteristic “double-ring” corrosion product deposits due to localized electrochemical activity and moisture condensation patterns.
5. The mechanism for improved corrosion resistance is multifaceted: (i) Refined and spherical graphite decreases cathodic area and smoothens the corrosive interface; (ii) An increased ferrite fraction eliminates internal ferrite/cementite micro-galvanic cells within pearlite; (iii) Potential solid solution effects from the activated inoculant may nobilize the ferrite matrix, reducing the anode-cathode potential difference.
This work provides both a practical processing route and a fundamental microstructural-electrochemical rationale for producing high-performance nodular cast iron castings with extended service life in corrosive atmospheric conditions, offering valuable insights for the materials engineering of critical wind turbine components.