Titanium Surface Finishes Guide
Titanium doesn't need a coating to resist corrosion — its TiO₂ oxide layer handles that automatically. Surface treatments are specified for fatigue life, biocompatibility, contamination removal, or cosmetics. Here's how to choose the right option for your part.
Key data: as-machined Ra 32–63 µin. (0.8–1.6 µm) is standard. Anodizing (AMS 2488), passivation (AMS 2488), and electropolishing (ASTM B912 / Ra ≤ 4 µin (0.1 µm).) are available for specific functional requirements.
Every machined titanium surface has a texture — a pattern of microscopic peaks and valleys left behind by the cutting tool. The coarser the surface (higher Ra number), the more visible those marks are. The finer the surface (lower Ra), the smoother the feel and appearance.
For most structural titanium parts, the as-machined surface is entirely adequate. Titanium's self-forming TiO₂ oxide layer provides corrosion resistance without any additional treatment. The question becomes: does your application require something more? This guide answers that by walking through each surface treatment option, what it achieves, and when it's actually necessary.
Key Takeaway
Titanium is self-passivating. Unless your application has specific fatigue, biocompatibility, contamination, or cosmetic requirements, the as-machined surface finish (Ra 32–63 µin (0.81–1.6 µm).) is appropriate and no additional treatment is needed.
Choosing the Right Titanium Surface Finish
Titanium’s native TiO₂ passivation provides inherent corrosion resistance, making additional surface treatments unnecessary for most applications. The decision to specify a post-machining surface treatment is driven by: functional performance requirements (fatigue life, biocompatibility, wear resistance), regulatory requirements (FDA, AMS, MIL specifications), or cosmetic needs (color, uniformity).
| Finish Type | Ra Range | Standard | Primary Application | Cost Premium |
|---|---|---|---|---|
| Rough machined | 125–250 µin. (3.2–6.3 µm) | ASME B46.1 N10–N11 | Structural non-visible surfaces | 1× (baseline) |
| Standard machined | 32–63 µin. (0.8–1.6 µm) | ASME B46.1 N7–N8 | General-purpose features | 1× (baseline) |
| Fine machined | 16–32 µin. (0.4–0.8 µm) | ASME B46.1 N6–N7 | Mating and seating surfaces | 1.3–1.5× |
| Ground | 4–8 µin. (0.1–0.2 µm) | ASME B46.1 N4–N5 | Precision bearing bores, journals | 2–3× |
| Electropolished | ≤ 4 µin. (0.1 µm) | ASTM B912 / ISO 15730 | Medical implants, fatigue-critical | 3–5× |
| Anodized (Type II) | Same as pre-anodize | AMS 2488 | Structural, color coding | 1.5–2× |
| PEO (hard anodize) | Slightly increased | Proprietary / ASTM-referenced | Wear resistance, industrial | 3–4× |
| Passivated | No change | AMS 2488, ASTM B600 | Medical, iron-contamination removal | 1.2–1.5× |
As-Machined Titanium Surface Roughness
As-machined surface roughness is determined by cutting parameters (feed rate, step-over, tool geometry) and cutting speed. Titanium’s tendency to produce BUE at low SFM can degrade surface finish unpredictably — this is why maintaining 80–120 SFM (24–37 m/min) with sharp tooling is critical to consistent Ra values.
| Operation | Typical Ra | ISO Grade | Cutting Parameters |
|---|---|---|---|
| Face mill (rough) | 125–250 µin. (3.2–6.3 µm) | N10–N11 | Vc 80 SFM (24 m/min), fz 0.006 in. (0.15 mm), ap 0.10 in. (2.54 mm) |
| End mill (semi-finish) | 63–125 µin. (1.6–3.2 µm) | N8–N9 | Vc 100 SFM (30 m/min), fz 0.003 in. (0.076 mm), ap 0.05 in. (1.27 mm) |
| End mill (finish) | 32–63 µin. (0.8–1.6 µm) | N7–N8 | Vc 120 SFM (37 m/min), fz 0.001 in. (0.025 mm), ap 0.010 in. (0.25 mm) |
| End mill (fine finish) | 16–32 µin. (0.4–0.8 µm) | N6–N7 | Vc 130 SFM (40 m/min), fz 0.0005 in. (0.013 mm), ap 0.005 in. (0.13 mm) |
| Turning (rough) | 63–125 µin. (1.6–3.2 µm) | N8–N9 | Vc 100 SFM (30 m/min), f 0.010 in. (0.25 mm)/rev, DOC 0.080 in. (2.03 mm) |
| Turning (finish) | 16–32 µin. (0.4–0.8 µm) | N6–N7 | Vc 130 SFM (40 m/min), f 0.003 in. (0.076 mm)/rev, DOC 0.010 in. (0.25 mm) |
| Turning (fine finish) | 8–16 µin. (0.2–0.4 µm) | N5–N6 | Vc 150 SFM (46 m/min), f 0.001 in. (0.025 mm)/rev, DOC 0.005 in. (0.13 mm) |
| CBN grinding | 4–8 µin. (0.1–0.2 µm) | N4–N5 | Wheel ANSI B74.3; wet grinding, low infeed rate |
Titanium Anodizing (AMS 2488)
Titanium anodizing is a voltage-controlled electrochemical process that thickens the native TiO₂ layer. The resulting interference color is determined by the oxide layer thickness, which is set by the applied voltage. Unlike aluminum anodizing, titanium anodizing does not involve dye — the colors are structural (thin-film interference).
| Applied Voltage | Oxide Thickness | Color | Application |
|---|---|---|---|
| 5–10 V | ~10–15 nm | Gold / Champagne | AMS 2488 Type 1 — low voltage, minimal oxide |
| 15–20 V | ~20–30 nm | Bronze / Blue | Decorative, part identification |
| 20–25 V | ~30–35 nm | Blue / Purple | Color coding (e.g., Ti fasteners for identification) |
| 25–35 V | ~35–50 nm | Purple / Green | Decorative |
| 35–45 V | ~50–65 nm | Green / Yellow | Part ID, medical device color coding |
| 50–70 V | ~70–90 nm | Yellow / Pink / Teal | AMS 2488 Type 2 — thicker protective oxide |
Key Specifications
- • AMS 2488: Primary titanium anodizing specification. Type 1 (low voltage) and Type 2 (medium voltage) produce colored oxide layers for identification and limited corrosion protection.
- • Dimensional impact: Virtually none — oxide layers are 10–90 nm thick (negligible vs. drawing tolerances).
- • Surface roughness impact: None — anodizing replicates the pre-anodize Ra exactly.
- • Masking: Areas requiring bare titanium can be masked before anodizing; call out on drawing with “ANODIZE PER AMS 2488 EXCEPT WHERE NOTED”.
Titanium Passivation (AMS 2488)
When to Specify Passivation
- • Medical implants and surgical instruments (FDA requirement for Class III devices)
- • Parts with potential iron contamination from steel tooling or fixturing
- • Parts with visible surface discoloration from embedded carbon or graphite
- • High-purity fluid system components (pharmaceutical, semiconductor)
- • Post-weld treatment to restore the passive layer in HAZ
Passivation Process
- • Specification: AMS 2488 Method 1 (nitric acid), Method 2 (citric acid), or ASTM B600
- • Process: Chemical immersion (HNO₃ 20–40% or citric acid 4–10%) at ambient temperature, rinse, dry
- • Dimensional impact: Negligible — surface treatment, no material removal
- • Testing: Water immersion test, ferroxyl test (iron contamination), or high-humidity test per AMS 2488
- • Cost: 1.2–1.5× baseline; typically $15–40/part for small batches
Titanium Electropolishing (ASTM B912)
Process
Titanium electropolishing uses mixed acid electrolytes (typically H₂SO₄ + HF or proprietary formulations). The part is the anode; preferential dissolution smooths surface asperities. Material removal: typically 0.0001–0.0003 in. (0.0025–0.0076 mm) (2.5–7.5 µm) per surface.
Performance
Starting from Ra 32 µin. (0.8 µm): electropolishing achieves Ra 2–4 µin. (0.05–0.1 µm). Also removes: machining-induced work-hardened surface layer (~20–50 µm depth), residual stress (improves fatigue life 20–40%), surface cracks and tool marks.
Specification
ASTM B912: Standard Specification for Passivation of Stainless Steels Using Electropolishing. ISO 15730: Metallic and other inorganic coatings — electropolishing. FDA guidance for Class II/III implantable devices recommends Ra ≤ 0.1 µm (4 µin (0.1 µm).) per ISO 10993.
Frequently Asked Questions
Related Titanium Machining Resources
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