1. Avoid Crevice Corrosion and Pitting Traps at the Design Stage

2. Design Rules for Cost‑Effective CNC Machining in Corrosion‑Resistant Materials

3. Surface Finish and Passivation Requirements on the Drawing

4. Expert Insight: The Worst Corrosion Starts in a Gap You Did Not Notice

5. Frequently Asked Questions (FAQ)

1. Avoid Crevice Corrosion and Pitting Traps at the Design Stage

Corrosion loves small gaps and stagnant liquid. Remove the gaps, and you remove the problem.

Eliminate Sharp Internal Corners and Dead Zones – Let Fluid Drain

A sharp internal corner (like a 90° inside edge) is a trap. Liquid sits there. Oxygen runs out. The metal starts to corrode under the deposit. The same happens in blind holes or deep pockets that cannot drain.

• Design fix: Add a radius (R0.5‑1.5mm) to all internal corners. Tilt the part or add a small drain hole so fluid cannot pool.

• Rule of thumb: Any feature that can hold a drop of water for more than an hour is a corrosion risk.

Weld Design – Eliminate Root Gaps and Ensure Full Penetration

Welded joints are common corrosion sites. The gap between two plates before welding – called the root gap – traps moisture. Also, incomplete penetration leaves a crack‑like opening.

• Design fix: Use a J‑groove or U‑groove weld preparation instead of a V‑groove. This allows full penetration. Specify that welds be ground smooth and inspected (dye penetrant or X‑ray) for critical parts.

Specify Surface Roughness on Sealing and Mating Faces

A rough sealing face (Ra > 1.6µm) can leak. It also traps corrosive media in the valleys. But a very smooth face (Ra < 0.2µm) costs more to machine.

• Trade‑off: For static seals (gaskets, O‑rings), Ra 0.4‑0.8µm is enough. For dynamic seals (rotating shafts), Ra 0.2‑0.4µm. For non‑sealing faces, Ra 1.6‑3.2µm is fine. Do not over‑specify.

The table below summarizes design rules.

2. Design Rules for Cost‑Effective CNC Machining in Corrosion‑Resistant Materials

Corrosion‑resistant materials (stainless steel, titanium, Inconel) are expensive and hard to machine. Your design must minimize waste and cutting time.

Use Standard Bar and Tube Sizes – Reduce Material Removal

Machining a 50mm diameter part from a 60mm bar wastes 30% of the material. Starting from a 52mm bar wastes much less.

• Design fix: Choose outer diameters and bore sizes that match standard bar and tube stocks. For stainless, common bar sizes are 6, 8, 10, 12, 16, 20, 25, 30, 40, 50, 60mm. Design to these numbers.

• Benefit: Less material cost, less machining time, less scrap.

Provide Proper Underreliefs and Fillets – Allow Tools to Reach and Polish

Corrosion‑sensitive areas need good surface finish. But if the design has a sharp step or a very narrow groove, the tool cannot get in to polish it.

• Design fix: Add an underrelief (a small groove) at the bottom of threads or at the end of a bearing seat. Use generous fillets (R ≥ 1mm) wherever possible.

• Why: A tool with a 0.4mm corner radius cannot reach a 0.2mm internal fillet. You will get a rough corner that traps fluid.

Standardize Material Grades Across Your Part Family

One project may use 316L stainless. Another uses 17‑4PH. Another uses titanium. Each material needs different tools, speeds, and coolants.

• Design fix: For a product family, pick one corrosion‑resistant grade that meets the worst condition. Use it for all parts.

• Benefit: Your CNC shop can batch parts together. Tooling and programming costs drop.

3. Surface Finish and Passivation Requirements on the Drawing

The drawing must tell the shop exactly how to treat the surface.

Specify Passivation (ASTM A967) and Test Method

Passivation removes free iron from the surface of stainless steel. It restores the chromium oxide layer. Without passivation, a stainless part can still rust.

• Drawing note example: “Passivate per ASTM A967, Method 2 (citric acid). Verify by water immersion test – no rust after 24 hours.”

• Do not just write ‘passivate’. Write the standard. Write the test.

Electropolishing for Max Corrosion Resistance

Electropolishing removes a thin layer of metal. It leaves a smooth, bright, and passive surface. It also removes burrs.

• Surface roughness after electropolishing: Ra 0.1‑0.4µm (starting from Ra 0.6‑0.8µm machined finish).

• Drawing note: “Electropolish all wetted surfaces. Final Ra ≤ 0.2µm. Passivate after electropolishing.”

The table below shows finish recommendations.

4. Expert Insight: The Worst Corrosion Starts in a Gap You Did Not Notice

“I once inspected a failed stainless steel valve. The outside looked perfect. Inside, there was a tiny crevice between the seat and the body. The sealing surface was rough (Ra 1.2µm). The fluid sat there. After six months, the valve leaked. The design had a sharp corner and no drain. We changed the design: added a radius, smoothed the seat to Ra 0.4µm, and specified passivation. The next batch lasted five years. Small details in the drawing make a big difference in real life.”
— Corrosion Engineer, DongGuan YiTai Electronic Technologies

Get a Free DFM Report with Corrosion Environment Analysis

At DongGuan YiTai Electronic Technologies , we review your design for corrosion risks. We check for sharp corners, weld gaps, and drain issues. We recommend the right material, surface finish, and passivation method.

Send us your drawing and tell us the environment (indoor, marine, chemical, etc.). We will reply with a free DFM report that includes corrosion‑focused recommendations.

5. Frequently Asked Questions (FAQ)

Q: What is the difference between passivation and electropolishing?
A: Passivation is a chemical bath that removes surface iron. It does not change roughness. Electropolishing removes metal electrically. It smooths the surface and passivates at the same time. For high corrosion resistance, electropolish then passivate.

Q: Do I need to passivate 316L stainless steel?
A: 316L is more corrosion resistant than 304, but it still benefits from passivation. Machining leaves embedded iron particles. Passivation removes them. Without passivation, you may see rust spots in salt or humid environments.

Q: Can I design a part with no internal corners at all?
A: Not for most parts. But you can make all corners with a radius of at least 0.5mm. Use a ball end mill or a corner radius end mill. Sharp inside corners (0mm radius) are impossible to machine anyway.

Q: What is the best material for seawater applications?
A: Titanium (Grade 2 or 5) is best. Next is super duplex stainless steel (UNS S32750) . Then 316L with electropolish. Standard 304 will pit quickly in seawater.

Q: How do I specify a drain hole on a drawing?
A: Add a note: “Drill Ø2mm drain hole at lowest point of pocket. Deburr both ends.” Also mark the location on the view. The hole should be small enough not to leak, but large enough to let water out.

Q: Does DongGuan YiTai Electronic Technologies offer electropolishing?
A: Yes. We offer electropolishing for stainless steel parts. We also offer passivation per ASTM A967. We can provide surface roughness measurement (Ra) before and after. Contact us for a quote.

DongGuan YiTai Electronic Technologies Co.,Ltd

DongGuan YiTai Electronic Technologies Co.,Ltd is a manufacturing service provider located in Dongguan, China.

YiTai specialized in CNC turning machining and sheet metal fabrication mainly. As a member of Hung Mou Group, we focus on the overseas marketing development. And based on our parent company’s manufacturing capability and resources, YiTai also expended machining services such as die casting, injection molding, aluminum profile extrusion, 3D printing, which are committed to providing customers with one-stop purchasing services and experience.

 CNC MACHINING , CNC MILLING , CNC TURNING , SHEETMETAL , FASTENER , OTHERS