Wind turbine components like hub assemblies and planetary carriers increasingly utilize ductile iron casting QT400-18L due to its balanced mechanical properties. This study investigates the humidity-dependent corrosion behavior of QT400-18L enhanced with multi-scale ceramic particles (MSCP), particularly focusing on coastal applications where salt spray and moisture accelerate degradation.
Experimental Methodology
The base ductile iron casting composition was maintained within:
$$3.56\% \leq C \leq 3.68\%, \quad 2.19\% \leq Si \leq 2.40\%, \quad 0.148\% \leq Mn \leq 0.150\%$$
$$0.029\% \leq Mg \leq 0.062\%, \quad 0.032\% \leq Re \leq 0.047\%$$
MSCP additives (0.05-0.15 wt%) were introduced through controlled melt treatment. Accelerated corrosion testing followed GB/T4797.1-2018 standards using cyclic humidity conditions:
| MSCP Content (%) | Specimen ID | Humidity (%) |
|---|---|---|
| 0 | 1-1# | 60 |
| 1-2# | 80 | |
| 1-3# | 90 | |
| 1-4# | 98 | |
| 0.15 | 4-1# | 60 |
| 4-2# | 80 | |
| 4-3# | 90 | |
| 4-4# | 98 |
Corrosion Kinetics Analysis
The corrosion rate was calculated using:
$$V = \frac{M_2 – M_1}{A \cdot t}$$
Where \( V \) = corrosion rate (g/m²·h), \( M \) = mass change, \( A \) = surface area, and \( t \) = exposure time.
Microstructural Modifications
MSCP addition significantly improved graphite morphology in ductile iron casting:
| MSCP (%) | Nodularity (%) | Graphite Size | Ferrite Content (%) |
|---|---|---|---|
| 0 | 79 | Grade 6 | 79.16 |
| 0.05 | 82 | Grade 7 | 86.42 |
| 0.10 | 91 | Grade 7 | 85.84 |
| 0.15 | 86 | Grade 7 | 86.95 |
Humidity Threshold Effect
Corrosion rate progression revealed critical humidity thresholds for ductile iron casting:
$$RH_{critical} = 80\%$$
Below this threshold, MSCP-modified specimens showed comparable corrosion resistance to baseline. Above 80% RH:
$$V_{0\%} = 0.012\ \text{g/m²·h} \quad vs \quad V_{0.15\%} = 0.0085\ \text{g/m²·h}\ (\text{at 98\% RH})$$
Corrosion Product Characterization
XRD analysis identified predominant corrosion products:
$$FeO + H_2O \rightarrow Fe(OH)_2$$
$$4Fe(OH)_2 + O_2 \rightarrow 2Fe_2O_3 \cdot H_2O + 2H_2O$$
MSCP-containing specimens exhibited reduced oxide peak intensities, confirming enhanced protection.
Mechanistic Insights
The improved performance in ductile iron casting derives from:
- Graphite spheroidization refinement reducing galvanic couples
- Ferrite phase increase (79% → 87%) lowering electrochemical activity
- MSCP-induced oxide layer stabilization
Conclusion
This study demonstrates that 0.15% MSCP addition optimizes corrosion resistance in ductile iron castings for coastal wind applications, achieving 29.29% corrosion rate reduction at 98% RH through microstructural refinement and electrochemical modification.