1 Introduction
Precision manufacturing increasingly demands surface treatments that balance mechanical performance, corrosion resistance, and aesthetic quality. Anodizing and passivation are widely employed post-machining techniques for aluminum and stainless steel, respectively. Prior studies focus predominantly on single-material outcomes, leaving limited comparative data on performance metrics across materials and treatment conditions. This research addresses this gap by systematically analyzing surface finish characteristics and corrosion behavior resulting from anodizing and passivation processes applied to CNC machined components.
2 Research Methodology
2.1 Experimental Design
CNC machined samples of aluminum 6061 and stainless steel 304 were prepared with uniform cutting parameters. Each material underwent two surface treatment processes:
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Anodizing: Sulfuric acid-based electrolytic oxidation, varying voltage (12–18 V) and time (20–40 min).
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Passivation: Nitric acid-based chemical treatment, immersion time 15–30 min at ambient temperature.
2.2 Data Collection and Tools
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Surface Roughness: Measured using a Mitutoyo SJ-210 profilometer (Ra, Rz).
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Microscopy: SEM imaging at 500× magnification to observe oxide layer uniformity.
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Corrosion Testing: Electrochemical impedance spectroscopy (EIS) and salt spray testing (ASTM B117 standard).
2.3 Reproducibility Measures
All experiments were repeated in triplicate. Environmental factors (temperature 23±1°C, relative humidity 50±5%) were controlled. Detailed protocols for voltage, immersion duration, and sample preparation are included in Appendix A.
3 Results and Analysis
3.1 Surface Roughness Outcomes
Table 1 presents average Ra values before and after treatment.
| Material | Untreated Ra (µm) | Anodized Ra (µm) | Passivated Ra (µm) |
|---|---|---|---|
| Aluminum 6061 | 0.85 | 0.92 | — |
| Stainless Steel 304 | 0.78 | — | 0.79 |
Figure 1 shows SEM micrographs illustrating the anodic oxide layer on aluminum and passivated film on stainless steel. Anodizing slightly increases surface roughness due to oxide growth, whereas passivation maintains baseline smoothness.
3.2 Corrosion Resistance Evaluation
EIS measurements demonstrate that passivated stainless steel exhibits higher impedance compared to untreated samples (p<0.05), indicating superior corrosion resistance. Aluminum anodized samples show improved resistance in chloride-containing environments relative to untreated controls, though less than stainless steel post-passivation.
3.3 Comparative Discussion
Anodizing predominantly enhances wear resistance and hardness, which benefits aluminum components in mechanical applications. Passivation effectively mitigates corrosion in stainless steel with negligible surface morphology alteration. The trade-off between mechanical enhancement and corrosion protection depends on material-specific properties and functional requirements.
4 Discussion
4.1 Mechanistic Insights
The anodic oxide layer introduces a microhard surface that resists abrasion, whereas the passive chromium oxide film on stainless steel inhibits electrochemical reactions. Variability in layer thickness correlates with process parameters such as voltage and acid concentration.
4.2 Limitations
The study focuses on laboratory-controlled conditions, which may differ from industrial-scale environments. Further investigation under cyclic mechanical loading and real-world exposure is warranted.
4.3 Practical Implications
Selection of surface treatment should consider both material and operational context:
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Aluminum parts: Prefer anodizing for wear-critical components.
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Stainless steel parts: Apply passivation for corrosion-critical applications.
5 Conclusion
The research demonstrates that anodizing and passivation offer complementary benefits depending on the substrate material. Anodizing improves surface hardness for aluminum, while passivation enhances corrosion resistance for stainless steel. These findings inform process selection in CNC machining workflows and provide a standardized reference for surface finishing practices. Further studies under operational stresses are recommended to validate long-term performance.
Post time: Nov-27-2025
