Engineers specifying MIM 316L for marine, outdoor, or automotive underbody applications frequently ask one question: how does it perform in salt spray testing compared to wrought 316L?
Salt spray performance (ASTM B117, 35°C, 5% NaCl):| Material & Condition | Hours to First Red Rust | Rating |
|---|---|---|
| Wrought 316L (polished + passivated) | 800-1200+ hours | Excellent |
| MIM 316L (>97% density, passivated) | 200-500 hours | Good |
| MIM 316L (95-97% density, passivated) | 100-300 hours | Moderate |
| MIM 316L (<95% density, no passivation) | 24-72 hours | Poor |
| Wrought 304 (passivated) | 200-400 hours | Moderate |
| MIM 17-4PH (aged, passivated) | 72-200 hours | Limited |
- Residual porosity: MIM parts at 96-98% density contain microscopic pores (2-4% by volume) that can act as initiation sites for crevice corrosion
- Surface roughness: As-sintered MIM surfaces (Ra 1.6-3.2 µm) are rougher than wrought/rolled surfaces (Ra 0.2-0.8 µm), providing more nucleation sites for corrosion
- Grain structure: MIM has a finer grain structure than wrought, which typically improves corrosion resistance slightly — but the porosity effect dominates
| Method | Improvement | Cost Impact |
|---|---|---|
| Electropolishing (removes 5-10 µm surface layer) | 2x improvement (closes surface pores) | +$0.05-0.15/part |
| Increase sintered density (>98%) | 2-3x improvement | Higher sintering temperature or HIP (+$2-8/part) |
| Citric acid passivation (per ASTM A967) | Essential — adds 50-100% improvement | Negligible (batch process) |
| Reduce surface roughness (polished mold cavity) | 1.5-2x improvement | Mold polishing cost |
For MIM 316L at >96% density with proper passivation: yes, 48 hours is readily achievable. At >97% density with electropolishing: 200+ hours is achievable. Without passivation: results are unpredictable and frequently below 48 hours.
For marine-grade applications requiring 500+ hours to red rust, specify >97% sintered density, electropolishing, and citric acid passivation on the MIM part drawing.