Corrosion-resistant metals aren’t always better for CNC parts. After machining thousands of stainless, titanium, and aluminum components, expensive alloys often create more problems than they solve.
Corrosion-resistant metals make parts worse when you over-spec for the environment (316L in mild conditions), choose soft alloys for load-bearing applications (316L for bearings), or select materials that gall during assembly (titanium threads). Match corrosion needs actual exposure conditions.
Learn when pricey materials backfire, which ones cause machining issues, and how to balance corrosion resistance with function—based on real shop examples.
Table of Contents
What's the most cost-effective corrosion-resistant metal for general use?
For most CNC applications, 6061-T6 aluminum with anodizing offers the best corrosion resistance per dollar. Material costs run $2-3 per pound versus $8-12 for 304 stainless, delivering comparable corrosion protection for 80% of general industrial applications.
Quick Decision Framework:
- Indoor/dry environments: 6061 aluminum (no coating needed)
- Outdoor/humid: 6061 + Type II anodizing
- Chemical contact: 304 stainless steel + passivation
- Marine/chloride exposure: 316L stainless steel
From our experience manufacturing enclosures for audio and industrial equipment, anodized 6061 consistently outlasts 10-year specifications in outdoor installations. We’ve measured identical corrosion resistance between anodized aluminum and 304 stainless in salt spray testing for splash-zone applications.
Total project cost comparison (4″ x 6″ enclosure):
| Material | Raw Cost | Finishing | Total |
|---|---|---|---|
| 6061 + Anodizing | $25 | $20 | $45 |
| 304 + Passivation | $65 | $25 | $90 |
| 316L Stainless | $85 | $15 | $100 |
The cost advantage becomes even larger for complex geometries where aluminum’s superior machinability reduces production time significantly compared to stainless steels.
Design Takeaway: Choose 6061 with anodizing unless your application involves direct chemical contact or submersion. The 50-60% cost savings typically outweigh any performance differences in general corrosion resistance applications.
Which corrosion-resistant metals are easiest to machine or form?
stainless steels require significant geometry modifications. If your design has thin walls, deep pockets, or tight tolerances, material choice directly impacts what’s manufacturable without major redesign.
Design constraints by material:
| Material | Min Wall | Tolerance | Internal Radii | Surface Finish |
|---|---|---|---|---|
| 6061 Aluminum | 0.030″ | ±0.005″ | 0.010″ possible | Ra 0.8–1.6 μm |
| 304 Stainless | 0.040″ | ±0.01″ | 0.020″ minimum | Ra 3.2–6.3 μm |
| 316L Stainless | 0.040″ | ±0.01″ | 0.020″ minimum | Ra 3.2–6.3 μm |
| Titanium | 0.060″ | ±0.015″ | 0.030″ minimum | Ra 6.3–12.5 μm |
From our experience, stainless steels work-harden if cutting speeds drop below optimal ranges, making interrupted cuts and complex pockets challenging. Aluminum maintains consistent properties throughout machining, allowing complex internal geometries without tool access issues.
Critical design impacts: Thread engagement increases from 1.0x diameter (aluminum) to 1.5x diameter (stainless) due to material strength differences. Sharp internal corners below 0.020″ radius become impossible with stainless steels due to tool deflection and work hardening.
We’ve redesigned dozens of enclosures when engineers switched from aluminum to stainless mid-project – typical changes include increasing wall thickness by 0.010-0.020″ and opening tolerance bands from ±0.005″ to ±0.01″.
Design Takeaway: Design for your corrosion-resistant material from the start. Aluminum allows maximum design flexibility, while stainless steels require geometry modifications that may affect your assembly clearances or functional requirements.
Can I use 304 with coating instead of 316L?
304 stainless steel with passivation performs identically to 316L in 80% of applications, saving 25-35% on material costs. The difference: 316L contains 2-3% molybdenum for superior chloride resistance, essential only for specific chemical environments, not general corrosion protection.
Environment-specific decision criteria:
Use 304 stainless when:
- Chloride exposure <200 ppm (per ASTM G48 testing)
- Indoor/office environments
- Urban outdoor exposure (rain, humidity, smog)
- Food processing without chlorinated cleaning
- General industrial equipment
Upgrade to 316L when:
- Direct seawater contact (35,000 ppm chlorides)
- Pool equipment with active chlorine exposure
- Chemical processing above 200 ppm chlorides
- Coastal installations within 1000 feet of ocean
- Pharmaceutical cleanrooms using bleach sanitizers
From testing hundreds of parts in accelerated corrosion chambers, passivated 304 and bare 316L show identical performance in ASTM B117 salt spray tests up to 500 hours for non-marine applications. The molybdenum in 316L only provides advantage in high-chloride environments above 500 ppm.
Cost impact: Material cost difference of $4-6 per pound translates to $20-40 savings per typical bracket. For production runs above 100 pieces, 304 selection saves thousands while maintaining identical performance in appropriate environments.
Design Takeaway: Document your chloride exposure levels. If your application sees atmospheric corrosion only (not direct chemical contact), 304 with citric acid passivation provides equivalent protection at significantly lower cost than 316L.
If I use 316L for chemical resistance, will it be too soft for my bearing surfaces?
316L stainless steel is limited to 50 PSI contact pressure for sliding bearing applications due to its low hardness of 70-90 HRB. For rotating shafts, avoid speeds above 100 RPM to prevent excessive wear. If your application requires both corrosion resistance and mechanical durability, 17-4 PH stainless steel provides hardness up to 40-47 HRC.
Load limits for 316L: Static loads remain safe up to 5,000 PSI contact stress, but sliding contact should stay below 50 PSI with lubrication. Rotating shafts above 100 RPM will experience excessive wear, while threaded fasteners work only for permanent assembly applications.
From ASTM G99 wear testing on precision equipment components, 316L shows 10x higher wear rates than hardened steel in sliding applications. We’ve measured 0.010″ wear on 316L bushings in 6 months where 17-4 PH lasted 5+ years without measurable wear.
The decision comes down to application requirements: low-load pivots under 50 PSI can use 316L acceptably, while high-load bearings need 17-4 PH stainless steel. For applications requiring both chemical and wear resistance, 17-4 PH costs 30% more than 316L but eliminates wear issues entirely. Nitrided 316L provides another option, adding surface hardening to 55+ HRC for $15-25 per part.
Design Takeaway: Choose 17-4 PH stainless over 316L for any sliding, rotating, or high-load applications. The 30% material cost increase prevents expensive field failures and eliminates wear-related warranty claims.
Can titanium threads handle corrosion without galling?
Titanium threads gall in 15-20% of assemblies even with proper anti-seize compound, making them unsuitable for removable fasteners. The excellent corrosion resistance comes at the cost of severe cold-welding tendencies during threading operations. Use stainless steel fasteners with titanium components for reliable serviceability.
Galling increases dramatically with fine threads – components with thread pitches above 20 TPI show 25-30% failure rates compared to 10-15% for coarse threads below 8 TPI. Anti-seize compound reduces these rates by approximately 30%, but failures still occur frequently enough to cause production delays and field service problems.
Titanium threads fail completely in removable fastener applications due to high disassembly failure rates. Production assembly lines experience delays from seized fasteners, while field maintenance often requires drill-out and retapping when fasteners cold-weld during service.
Better alternatives include using 316L fasteners into titanium components for removable panels, or stainless steel thread inserts in titanium parts for service access. Weight-critical applications may justify permanent joining methods like welding or bonding instead of threaded connections.
Per MIL-HDBK-60 threading guidelines, titanium-to-titanium threaded connections require 50% larger engagement lengths to compensate for galling-induced strength reduction. Each galled fastener adds $25-50 in labor costs for drill-out and retapping during production or field service.
Design Takeaway: Eliminate titanium threads from any connection requiring disassembly. Specify stainless steel fasteners for serviceable connections – the slight weight penalty prevents costly field failures and maintenance delays.
Can corrosion-resistant Inconel hold tight tolerances?
Most product developers don’t actually need Inconel – if your application operates below 1000°F without extreme chemical exposure, use 17-4 PH stainless steel instead. Inconel’s work hardening limits achievable tolerances to ±0.015-0.020″, making it unsuitable for precision assemblies. The 5-8x machining cost is rarely justified for general corrosion resistance.
Do you actually need Inconel?
- Temperature above 1000°F: Inconel required
- Extreme chemical processing: Possibly Inconel
- Aerospace critical applications: Maybe Inconel
- General outdoor corrosion: NO – use 316L stainless instead
- Precision assemblies: NO – tolerance requirements rule out Inconel
Tolerance reality check: Inconel 625 typically achieves ±0.015″ on simple geometries, while complex features require ±0.020″ tolerance bands. If your design requires tolerances tighter than ±0.01″, Inconel will force a complete redesign with looser clearances and larger fasteners to accommodate dimensional variations.
The fundamental issue is heat buildup during machining causing parts to move 0.005-0.010″ between operations. What costs $100 to machine in aluminum runs $800-1200 in Inconel for equivalent precision, making it economically viable only for extreme environments where other materials fail catastrophically.
Design decision framework:
Your Environment → Material Choice → Expected Tolerance
General corrosion → 316L stainless → ±0.005″ achievable
High temperature (>800°F) → Inconel → ±0.015″ realistic
Precision assembly → 17-4 PH stainless → ±0.005″ achievable
Cost-sensitive → Aluminum → ±0.005″ standard
Design Takeaway: Question whether you actually need Inconel’s extreme properties. If your application doesn’t involve high temperatures or severe chemical attack, choose 17-4 PH stainless steel for both precision and corrosion resistance at much lower cost.
Do I still need a surface treatment if I use a corrosion-resistant metal?
Yes, but the real question is whether your local shop can do the treatments properly – many CNC shops skip passivation or do it incorrectly. Stainless steel without proper passivation will rust within 30 days, while aluminum needs anodizing for any outdoor application. Plan for 3-5 additional days lead time and verify your supplier’s treatment capabilities.
Required treatments and supplier reality:
- Stainless steel passivation: Required for all machined parts, but 30% of shops skip it
- Aluminum anodizing: Essential for outdoors, most shops subcontract (adds lead time)
- Titanium: Usually bare metal acceptable, anodizing rarely available locally
What happens when you skip treatments to save cost: We’ve seen stainless steel parts develop rust spots within weeks when customers eliminated passivation. The $15-25 passivation cost becomes hundreds in warranty claims and customer complaints.
How to specify treatments on your drawings:
- Stainless steel: “Passivate per ASTM A967 using citric acid”
- Aluminum: “Type II anodize, natural finish, 0.0007″ minimum thickness”
- Include treatment in your material callout, not as a separate note
Lead time impact: Anodizing typically adds 3-5 days to delivery, while passivation can be done same-day if properly equipped. Many shops subcontract anodizing, adding cost and schedule risk – verify capabilities during supplier selection.
Quality control requirements: Request certificates of compliance for treatments. Passivation quality varies dramatically between suppliers, and improper anodizing thickness affects both appearance and corrosion resistance.
Design Takeaway: Include surface treatments in your initial cost estimates and supplier qualification. Verify your chosen shop can perform treatments in-house or has reliable subcontractors – don’t discover treatment limitations after placing orders.
Can I still get the surface finish I need for sealing?
Change your seal design rather than fighting for impossible surface finishes on hard-to-machine materials. While aluminum achieves sealing-quality Ra 0.8-1.6 μm as-machined, stainless steel typically produces Ra 3.2-6.3 μm requiring expensive secondary finishing. Consider different seal types or materials instead of costly surface treatments.
Seal type vs. required surface finish:
- Static O-rings: Ra 1.6 μm (aluminum works as-machined)
- Dynamic seals: Ra 0.8 μm (aluminum + light polishing)
- Face seals: Ra 0.4 μm (expensive on stainless, easy on aluminum)
- Metal-to-metal: Ra 0.2 μm (avoid with stainless steel)
Design alternatives when finishes conflict with materials: Instead of grinding stainless steel to Ra 0.8 μm for $40-60 per surface, consider switching to face seal designs that work with Ra 3.2 μm as-machined finishes. Alternative approaches include using separate sealing inserts made from aluminum in stainless steel housings.
Material-specific finish capabilities: Aluminum machines to Ra 0.8-1.6 μm directly, making it ideal for sealing applications. Stainless steels require secondary grinding or polishing for finishes below Ra 1.6 μm, adding $20-40 per sealing surface. Inconel and titanium rarely achieve sealing-grade finishes economically.
Cost-driven decision matrix:
Application → Surface Finish → Recommended Approach
Static seal → Ra 1.6 μm → Use aluminum, as-machined
Dynamic seal → Ra 0.8 μm → Aluminum + polishing or redesign seal
High pressure → Ra 0.4 μm → Consider aluminum inserts in stainless housing
Critical seal → Ra 0.2 μm → Redesign with different seal technology
Design Takeaway: Evaluate seal design changes before specifying expensive surface finishes. Often a different O-ring compound or seal configuration works better than grinding stainless steel to unrealistic surface requirements.
Conclusion
Corrosion-resistant metals often compromise part performance through reduced strength, machining difficulties, or excessive costs. Match material selection to actual environmental requirements rather than over-specifying expensive alloys. Consider aluminum with anodizing or 304 stainless before jumping to exotic materials. Contact us to explore manufacturing solutions tailored to your corrosion-resistant part requirements.
Frequently Asked Questions
Avoid titanium threads for any removable fastener application due to 15-20% galling rates even with anti-seize compound. Use stainless steel fasteners with titanium components instead. Reserve titanium fasteners only for permanent installations where weight savings justify the assembly complexity and potential galling failures.
Yes, passivation removes iron contamination from cutting tools that creates corrosion initiation sites. Without passivation, even “stainless” steel can develop rust spots within 30 days. Specify citric acid passivation per ASTM A967 on your drawings and verify your supplier performs this step – many shops skip it to save time.
304 stainless works for atmospheric corrosion and general outdoor exposure. Upgrade to 316L only when facing direct seawater contact, chemical processing above 200 ppm chlorides, or pool equipment with active chlorine exposure. Document your chloride exposure levels – if it’s just rain and humidity, 304 with passivation provides identical performance to 316L.
Aluminum easily achieves ±0.005″ tolerances, while 304/316L stainless typically holds ±0.01″ with standard setups. Inconel and titanium are limited to ±0.015-0.020″ due to work hardening and thermal effects. If your design requires tighter tolerances, choose aluminum or 17-4 PH stainless steel rather than fighting material limitations.
Over-specifying expensive alloys for environments that don’t require them. We see engineers default to 316L stainless for general outdoor exposure when anodized aluminum provides equivalent protection at 60% lower cost. Always match material selection to actual chloride exposure levels and environmental conditions rather than choosing the “best” material regardless of application needs.
Aluminum achieves Ra 0.8-1.6 μm as-machined, suitable for most O-ring applications. Stainless steels typically produce Ra 3.2-6.3 μm, requiring secondary finishing for critical seals. Consider changing seal design to work with achievable finishes rather than specifying expensive grinding operations on hard-to-machine materials.
