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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.) is appropriate and no additional treatment is needed.

Selection Framework

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).

Titanium surface finish selection guide
Finish TypeRa RangeStandardPrimary ApplicationCost Premium
Rough machined125–250 µin. (3.2–6.3 µm)ASME B46.1 N10–N11Structural non-visible surfaces1× (baseline)
Standard machined32–63 µin. (0.8–1.6 µm)ASME B46.1 N7–N8General-purpose features1× (baseline)
Fine machined16–32 µin. (0.4–0.8 µm)ASME B46.1 N6–N7Mating and seating surfaces1.3–1.5×
Ground4–8 µin. (0.1–0.2 µm)ASME B46.1 N4–N5Precision bearing bores, journals2–3×
Electropolished≤ 4 µin. (0.1 µm)ASTM B912 / ISO 15730Medical implants, fatigue-critical3–5×
Anodized (Type II)Same as pre-anodizeAMS 2488Structural, color coding1.5–2×
PEO (hard anodize)Slightly increasedProprietary / ASTM-referencedWear resistance, industrial3–4×
PassivatedNo changeAMS 2488, ASTM B600Medical, iron-contamination removal1.2–1.5×
As-Machined Finish

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 with sharp tooling is critical to consistent Ra values.

As-machined titanium surface roughness by operation
OperationTypical RaISO GradeCutting Parameters
Face mill (rough)125–250 µin. (3.2–6.3 µm)N10–N11Vc 80 SFM (24 m/min), fz 0.006 in. (0.15 mm), ap 0.10 in.
End mill (semi-finish)63–125 µin. (1.6–3.2 µm)N8–N9Vc 100 SFM (30 m/min), fz 0.003 in. (0.076 mm), ap 0.05 in.
End mill (finish)32–63 µin. (0.8–1.6 µm)N7–N8Vc 120 SFM (37 m/min), fz 0.001 in. (0.025 mm), ap 0.010 in.
End mill (fine finish)16–32 µin. (0.4–0.8 µm)N6–N7Vc 130 SFM (40 m/min), fz 0.0005 in. (0.013 mm), ap 0.005 in.
Turning (rough)63–125 µin. (1.6–3.2 µm)N8–N9Vc 100 SFM (30 m/min), f 0.010 in./rev, DOC 0.080 in.
Turning (finish)16–32 µin. (0.4–0.8 µm)N6–N7Vc 130 SFM (40 m/min), f 0.003 in./rev, DOC 0.010 in.
Turning (fine finish)8–16 µin. (0.2–0.4 µm)N5–N6Vc 150 SFM (46 m/min), f 0.001 in./rev, DOC 0.005 in.
CBN grinding4–8 µin. (0.1–0.2 µm)N4–N5Wheel ANSI B74.3; wet grinding, low infeed rate
Titanium Anodizing

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).

Titanium anodizing voltage-to-color guide
Applied VoltageOxide ThicknessColorApplication
5–10 V~10–15 nmGold / ChampagneAMS 2488 Type 1 — low voltage, minimal oxide
15–20 V~20–30 nmBronze / BlueDecorative, part identification
20–25 V~30–35 nmBlue / PurpleColor coding (e.g., Ti fasteners for identification)
25–35 V~35–50 nmPurple / GreenDecorative
35–45 V~50–65 nmGreen / YellowPart ID, medical device color coding
50–70 V~70–90 nmYellow / Pink / TealAMS 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”.
Passivation

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
Electropolishing

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. (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.) per ISO 10993.

Common Questions

Frequently Asked Questions

What is "surface finish" and how is it measured on machined parts?
Surface finish describes the texture of a machined surface — specifically the microscopic peaks and valleys left by the cutting tool. The standard measurement is Ra (roughness average, per ASME B46.1 / ISO 4287): the arithmetic mean deviation of the surface profile over a measured length. Units: µin. (microinches) in US practice, µm (micrometers) in SI. Typical references: Ra 125 µin. (3.2 µm) — rough machined, visible tool marks; Ra 32 µin. (0.8 µm) — standard finish milling, fine tool marks; Ra 8 µin. (0.2 µm) — smooth ground surface; Ra 4 µin. (0.1 µm) — electropolished, mirror-like. On engineering drawings, surface finish is called out using the surface texture symbol (√ with a horizontal arm). Finer finishes require slower feeds, sharper tools, and more machining passes — each step adds cost.
Does titanium need a surface treatment or coating to resist corrosion?
No — titanium is inherently corrosion-resistant without any surface treatment. It forms a thin, stable titanium dioxide (TiO₂) passive layer within milliseconds of exposure to air or moisture, which self-repairs when scratched or damaged. This is unlike steel (which rusts without coating) or aluminum (which benefits from anodizing in harsh environments). Titanium's as-machined surface is already corrosion-resistant in most environments — seawater, mild acids, and body fluids. Surface treatments for titanium (anodizing, passivation, electropolishing) are specified for specific functional reasons: improved fatigue life, biocompatibility requirements, removal of embedded machining contamination, or cosmetic color — not for basic corrosion protection.
What surface finish does CNC machined titanium achieve?
CNC milled Ti-6Al-4V achieves the following Ra (arithmetic average roughness per ASME B46.1 / ISO 4287) values: rough milling: Ra 125–250 µin. (3.2–6.3 µm); semi-finish: Ra 63–125 µin. (1.6–3.2 µm); standard finish milling: Ra 32–63 µin. (0.8–1.6 µm); fine finish milling: Ra 16–32 µin. (0.4–0.8 µm). CNC turning achieves: rough turning Ra 63–125 µin.; finish turning Ra 16–32 µin.; fine turning Ra 8–16 µin. (0.2–0.4 µm). Post-machining grinding: Ra 4–8 µin. (0.1–0.2 µm). Electropolishing: Ra ≤ 4 µin. (0.1 µm). The default "as-machined" surface on a standard quote is approximately Ra 63 µin. (1.6 µm), equivalent to a 63-finish callout or N8 per ISO 1302.
Can titanium be anodized? What types are available?
Yes, titanium can be anodized. Titanium anodizing is a different process from aluminum anodizing. There are two primary types: (1) Electrochemical anodizing (Type II equivalent) produces a thin oxide layer (0.5–2 µm / 0.02–0.08 mil) through voltage-controlled anodic oxidation in an electrolyte (typically H₂SO₄ or H₃PO₄). The surface color is determined by the oxide thickness (voltage-controlled): gold (~5V), bronze (~15V), blue (~20V), purple (~25V), green (~35V). This process does NOT significantly change surface Ra and does NOT provide the hard protective layer that aluminum Type III anodizing provides. (2) Hard anodizing (Type III equivalent) is less common on titanium than aluminum but available via plasma electrolytic oxidation (PEO), producing TiO₂ layers to 5–25 µm with hardness of 800–1,200 HV. Titanium anodizing specification: AMS 2488 (Type II, decorative/functional) or MIL-A-8625 analogous specifications from the OEM.
Does titanium require passivation?
Titanium is self-passivating — it spontaneously forms a stable, adherent TiO₂ oxide layer within milliseconds when exposed to oxygen or moisture. This native oxide is 2–7 nm thick and provides excellent corrosion resistance without any treatment. However, post-machining passivation (ASTM B600 or AMS 2488) is specified for: (1) Medical implants and surgical instruments, where particulate titanium contamination from machining must be removed. (2) Surfaces contaminated with iron from tooling (embedded iron particles can cause surface rusting). (3) Parts with embedded carbon from graphite tooling or lubricants. Passivation per AMS 2488 Method 1 (typically nitric acid or citric acid treatment, room temperature) removes surface contamination and restores the passive TiO₂ layer. It does NOT change dimensions measurably (no material removal ± 0.0001 in.).
When is electropolishing required for titanium?
Electropolishing titanium is required or specified for: (1) Load-bearing biomedical implants (acetabular shells, femoral stems, bone screws) — electropolishing per ASTM F86 removes machining-induced surface damage and work-hardened layers that are crack initiation sites under fatigue loading. Target: Ra ≤ 4 µin. (0.1 µm), ISO 4287. (2) Biofilm-critical surgical instruments and endoscopic components — smooth surfaces resist biofilm adhesion and improve sterilization efficacy. (3) Fatigue-critical structural components where surface roughness directly affects fatigue life — electropolishing extends fatigue life by 20–40% vs as-machined by removing tool marks. (4) Ultra-high-purity fluid systems (semiconductor, pharmaceutical) requiring smooth, cleanable inner bore surfaces. Electropolishing titanium uses mixed acid electrolytes (H₂SO₄ + HF or proprietary solutions); controlled process per ASTM B912 or ISO 15730.

Related Titanium Machining Resources

Titanium CNC Machining Guide

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Titanium Machining Tolerances

Achievable tolerance ranges, GD&T guidance, and DFM cost rules.

Titanium CNC Design Guidelines

DFM rules for walls, corners, holes, threads, and cost reduction.

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