Designing for CNC machining isn’t just about geometry — it’s about minimizing cost without compromising precision. With decades of experience manufacturing parts for aerospace, audio, and medical sectors, a few small adjustments can dramatically improve both performance and price.
Stainless steel can rust under certain conditions, despite its reputation for corrosion resistance. The key is matching the right grade (304, 316L, 430) to your specific environment and understanding how manufacturing processes affect long-term durability. Factors like chloride exposure, surface contamination, and heat-affected zones from welding all determine whether your parts will perform as expected.
Learn which grades perform best for your application, how manufacturing impacts durability, and specification tips from real-world CNC production.
Table of Contents
What stainless steel grade performs best in my specific environment?
For most indoor applications, 304 stainless steel provides excellent corrosion resistance at the lowest cost. For chloride exposure (coastal, pools, cleaning chemicals), 316L is essential and worth the 20-30% price premium. High-temperature applications above 800°C require 310 or 321 grades to prevent oxidation scaling.
Quick Selection Guide:
Environment/Application Grade Reason
Indoor/Office 304 Cost-effective, 10+ year performance
Within 5 miles of ocean 316L Salt spray protection
Chemical cleaning 316L Chloride resistance required
Above 400°C operating temp 310/321 Oxidation resistance
Food processing (dry) 304 Adequate for dry conditions
Food processing (washdown) 316L Better cleanability
From our experience with both CNC machined components and sheet metal fabrication, environment assumptions cause the most costly specification mistakes. We’ve reviewed designs where 304 was specified for coastal enclosures and brackets—resulting in brown pitting within 6-12 months across both machined housings and formed panels. Conversely, 316L specified for indoor electronics enclosures adds $40-60 per part whether CNC machined or fabricated, with zero performance benefit.
According to ASTM A240 standards, 304 contains 18-20% chromium while 316L adds 2-3% molybdenum for enhanced chloride resistance. This applies equally to precision machined parts and sheet metal components exposed to road salt, pool chemicals, or marine environments.
Design Takeaway: Use the selection guide above for both machined and fabricated stainless parts, then specify your exact environment in your RFQ to prevent costly over-specification or corrosion failures.
Is 316L worth the extra cost over 304 for my application?
316L is worth the 20-30% cost premium only when chloride exposure is confirmed—coastal environments, pool equipment, chemical cleaning, or medical washdown applications. For indoor electronics, furniture, or general industrial use, 304 provides identical performance at significantly lower cost.
The wrong choice costs more than the material premium. We’ve seen product recalls when 304 was used in mildly corrosive environments, while companies waste budget over-specifying 316L for indoor applications with zero chloride exposure. The key question: what’s your cost if the parts fail vs the upfront premium?
For obvious cases, the decision is easy—indoor office equipment uses 304, marine hardware needs 316L. For borderline applications (warehouses near coast, light chemical exposure), consider your warranty exposure and replacement complexity. Failed parts in the field cost far more than the material upgrade through service calls, customer dissatisfaction, and potential recalls.
Design Takeaway: Calculate your failure cost scenario—if replacement/warranty costs significantly exceed the 316L premium, upgrade the material. For obvious indoor applications, stick with 304 and allocate the savings to other design improvements.
Does welding or machining affect stainless steel's corrosion resistance?
Yes—welding creates heat-affected zones that reduce corrosion resistance, while machining can embed iron particles that cause rust spots. Both processes require passivation treatment to restore full corrosion performance, but only specify this for parts with confirmed chloride exposure.
This is where many developers over-specify. For indoor applications, welding discoloration and minor machining marks are cosmetic issues—not corrosion failures. However, for coastal installations or chemical environments, untreated welds fail within months while the base material survives.
The practical decision: if your parts face confirmed chloride exposure, specify “passivate per ASTM A380” on your drawings. For indoor applications, save the money unless appearance is critical. Most fabricators know when passivation is required, but clear specification prevents confusion and ensures proper treatment.
Design Takeaway: Only specify passivation for confirmed corrosive environments. Add this note to your drawings: “Passivate per ASTM A380 for marine/chemical applications only.” This gives your supplier clear guidance while avoiding unnecessary processing costs for indoor parts.
Can surface scratches or contamination make stainless steel rust?
Yes—surface scratches can trap contaminants and create corrosion initiation points, while iron contamination from carbon steel tools causes immediate rust spots. Proper handling, stainless-only tooling, and surface protection during fabrication prevent most contamination issues.
Prevention Requirements by Application:
- Indoor applications: Standard fabrication practices – no special handling needed
- Coastal/marine: Specify “protective film until installation”
- Chemical exposure: Require “stainless-dedicated tooling only”
- Medical/food: Add surface finish requirements (Ra values) for cleanability
Surface damage breaks the protective chromium oxide layer that gives stainless steel its corrosion resistance. A scratch you can catch with your fingernail (roughly 0.1mm deep) becomes problematic in salt spray environments but remains cosmetic indoors. Iron contamination from carbon steel cutting tools or shared work surfaces creates immediate rust spots that spread corrosion.
Practical Drawing Specifications: For critical applications, add: “Maintain protective film during fabrication. Use stainless-dedicated tooling only.” For standard indoor parts, no special callouts are needed.
Most fabricators understand contamination risks, but clear specifications prevent costly assumptions. Protective film adds minimal cost, while dedicated stainless tooling may increase fabrication costs 5-10% for complex parts.
Design Takeaway: Don’t over-specify handling for indoor parts. For corrosive environments, use the specification language above and verify your fabricator has dedicated stainless capabilities before ordering.
Do I need passivation after machining to restore full corrosion resistance?
Passivation is required only for parts exposed to corrosive environments—coastal, chemical, or medical applications. Indoor parts rarely need passivation unless cosmetic appearance is critical, as the natural chromium oxide layer reforms over time.
Treatment Decision Tree:
- Indoor office/residential: No passivation – natural oxide layer sufficient
- Coastal (within 5 miles): Yes – specify passivation for salt spray protection
- Chemical cleaning/medical: Yes – required for cleanability and performance
- Appearance-critical indoor: Optional for uniform matte finish
Passivation removes surface contamination and promotes uniform chromium oxide formation per ASTM A380. The process adds 2-3 days to lead time and 10-15% to part cost, but provides measurable corrosion improvement where environmental exposure justifies the expense.
Exact Drawing Note: For parts requiring treatment: “Passivate per ASTM A380, Method A (citric acid).” For conditional treatment: “Passivate if specified for corrosive service.”
Properly passivated parts have uniform, matte appearance without machining marks or heat discoloration. Most fabricators offer passivation services, though regulated industries may require certified processes with documentation.
Design Takeaway: Use the decision tree above and copy the specification language to your drawings. Define your operating environment clearly to avoid both unnecessary processing costs and inadequate corrosion protection.
How do I specify surface finish requirements that support corrosion resistance?
For corrosion resistance, specify Ra 3.2 μm or finer for general applications, Ra 1.6 μm for chemical exposure, and Ra 0.8 μm for medical/food contact surfaces. Smoother finishes reduce crevice corrosion but increase machining costs—balance performance needs with budget constraints.
Surface Finish Decision Matrix:
Application Ra Requirement Cost Impact Supplier Capability
Indoor/general Ra 3.2 μm Baseline Any machine shop
Coastal/chemical Ra 1.6 μm +15–25% machining time Most precision shops
Medical/food Ra 0.8 μm +40–60% machining time Certified medical suppliers
High-end appearance Ra 0.4 μm +100%+ machining time Specialized finishing shops
Surface roughness affects corrosion by creating crevices where contaminants accumulate. Ra 3.2 μm feels smooth to touch but shows visible machining marks. Ra 1.6 μm eliminates most tool marks. Ra 0.8 μm requires polishing operations and feels mirror-smooth.
Practical Implementation:
- Feature-specific finishing: Specify tight finishes only on critical surfaces—”Ra 1.6 μm on sealing surfaces, Ra 3.2 μm elsewhere”
- Supplier qualification: Ask potential fabricators: “What’s your standard achievable surface finish?” and “Do you have surface roughness measurement capability?”
- Inspection requirements: Add “Document surface finish verification with calibrated profilometer” for critical applications
Drawing Specification Examples:
- “Surface finish Ra 3.2 μm max unless noted”
- “Ra 1.6 μm on surfaces marked △”
- “Verify finish with profilometer, provide certification for Ra 0.8 μm surfaces”
Design Takeaway: Start with Ra 3.2 μm baseline and upgrade only critical surfaces. Verify your supplier can measure and document the specified finish before ordering. Reserve tight finishes for functional surfaces—appearance surfaces can often use standard finishes with proper cleaning.
Conclusion:
Stainless steel won’t rust when properly matched to your environment—use 304 for indoor applications and 316L for chloride exposure. Specify surface finishes and passivation only where justified to control costs. Contact us to explore manufacturing solutions tailored to your stainless steel component requirements.
Frequently Asked Questions
Threaded holes typically use standard Ra 3.2 μm unless specified otherwise. Internal channels in medical or food applications may require Ra 1.6 μm for cleanability—specify only where functional performance demands it.
Yes, but factor in redesign costs and lead time delays. For borderline applications, consider the total cost of potential field failures versus upfront 316L specification—early material upgrade often costs less than retrofit.
Within 5 miles of coastline, salt spray makes 316L essential for long-term performance. Beyond 10 miles inland, 304 typically performs adequately unless other chloride sources exist like road salt or industrial chemicals.
Ask for their contamination prevention procedures and request photos of dedicated stainless work areas. Quality suppliers maintain separate tool sets and can document their processes to prevent iron contamination.
Natural chromium oxide reformation occurs within days to weeks in normal air exposure. However, this natural layer is thinner than chemical passivation and may not provide adequate protection for aggressive environments.
Laser cutting creates heat-affected zones similar to welding, potentially reducing corrosion resistance at cut edges. For critical applications, specify edge finishing or passivation of laser-cut parts exposed to corrosive environments.
