Plating and Other Finishing Considerations for CNC Machining and Sheet Metal
Electrochemical & chemical conversion coatings, plating, and organic finishes: mechanisms, performance data, limitations, and selection criteria
This guide provides an engineering‑level comparison of finishing processes for CNC machined and sheet metal parts. It goes beyond “what” to explain “how” and “why” – including failure modes, substrate compatibility, hydrogen embrittlement risk, fatigue effects, quantitative corrosion data, and cost drivers.
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1. Anodising (Aluminum, Titanium)
1.1 Electrochemical Mechanism
Anodising is an electrolytic passivation process. The aluminum part is the anode in an acid bath (typically 10–20% H₂SO₄ at 0–10°C for hardcoat, or 20–30°C for decorative). Oxygen evolution at the anode reacts with Al to form a porous hexagonal columnar oxide (Al₂O₃·xH₂O). The pore diameter (10–30 nm) allows dye uptake. Voltage determines layer thickness (~1.4 µm per volt). Current density affects hardness.
After anodising, sealing (hot DI water, nickel acetate, or PTFE) hydrates the oxide, closing pores. Sealed anodising has corrosion resistance 5–10× higher than unsealed.
1.2 Types and Properties (MIL‑A‑8625, ISO 7599)
| Type | Thickness (µm) | Hardness (HV) | Porosity | Sealing required | Typical use |
|---|---|---|---|---|---|
| Type I (Chromic acid) | 0.5–2.5 | 200–300 | Low | Yes | Welded assemblies, paint base, fatigue‑critical parts (no hydrogen embrittlement) |
| Type II (Sulfuric, decorative) | 5–25 | 250–400 | High | Yes (dye then seal) | Coloured consumer goods, automotive trim |
| Type III (Hardcoat) | 25–150 | 400–600 | Low (but thick) | Optional (reduces wear slightly) | High‑wear: pistons, hydraulic rods, firearm components |
Key data:
- Hardness: 400–600 HV (equivalent to 40–55 HRC) – harder than most steels but brittle.
- Dielectric strength: ~1.2 kV per 25 µm (Type II sealed).
- Fatigue reduction: Unsealed anodising reduces fatigue strength by 50–80% due to tensile residual stresses; peening before anodising or thin chromic acid anodising restores fatigue life. For critical fatigue applications, specify Type I.
1.3 Titanium Anodising
- Electrolyte: phosphate or sulfate solutions.
- Voltage determines colour: 10–30 V → bronze/purple; 50–70 V → blue; 80–100 V → gold; >100 V → green/pink.
- Oxide thickness ~0.01–0.1 µm; no significant dimensional change.
- Standard: AMS 2488 (Type II – decorative; Type III – wear resistance).
1.4 Sealing – Quantitative Impact
| Sealing method | Salt spray (ASTM B117) hours to first pit (Type II, 20 µm) | Dye absorption (after sealing) |
|---|---|---|
| None | < 24 | High |
| Hot DI water (≥96°C, 20 min) | 250–350 | Very low |
| Nickel acetate (2–5 g/L, 80°C, 15 min) | 400–600 | None |
| PTFE / hot water co‑seal | 600+ (plus low friction) | None |
2. Chromate Conversion Coating (Chem Film)
2.1 Mechanism
A chemical conversion process (no electricity). Hexavalent Cr⁶⁺ (Type I) or trivalent Cr³⁺ (Type II RoHS) solutions react with Al or Ti to form a gel‑like chromate/chromium oxide layer 0.5–2 µm thick. The coating is soft but self‑healing: Cr⁶⁺ ions migrate to scratches and repassivate.
Environmental note: Type I (yellow) uses Cr⁶⁺ – carcinogenic, restricted under EU REACH Annex XVII and RoHS (except certain exemptions). Type II (clear) is Cr³⁺, RoHS compliant, but with lower corrosion performance.
2.2 Performance Data (Aluminum 6061)
| Type | Salt spray (ASTM B117) | Surface resistance (mΩ/cm²) | Base for paint |
|---|---|---|---|
| Type I (yellow, Cr⁶⁺) | 168–336 h | <1 | Excellent |
| Type II (clear, Cr³⁺) | 48–96 h | <1 | Good |
| Uncoated | <24 h | ~1–5 (oxide) | – |
Key advantage: Chromate films are electrically conductive (unlike anodising) – essential for grounding and EMI shielding.
Thickness measurement: XRF (X‑ray fluorescence) of chromium mass per area; typical 20–60 mg/m².
3. Plating – Electrolytic and Electroless
3.1 Hydrogen Embrittlement (HE) Risk – Critical for High‑Strength Steels (>1000 MPa)
Electroplating (especially acidic baths) generates atomic hydrogen that can diffuse into steel, causing delayed cracking under tensile stress.
- Very high risk: Zinc, cadmium, nickel (Watts bath) on steel > 1200 MPa.
- Bake after plating: 3–24 hours at 190–230°C within 4 hours of plating to outgas hydrogen.
- HE‑free alternatives: Electroless nickel (if properly baked), zinc‑nickel alloy, or mechanical zinc (Sherardising).
Standards: ASTM B850 (post‑plating baking), ISO 9587, 9588.
3.2 Nickel Plating Comparison
| Property | Bright nickel (electrolytic) | Electroless nickel (Ni‑P) |
|---|---|---|
| Deposit uniformity | Poor on complex shapes, thin on inside corners | Excellent (uniform by chemical reduction) |
| Phosphorus content | None | 5–12% (low P = harder, more brittle) |
| Hardness as‑plated (HV) | 200–300 | 500–600 (low P) |
| Hardness after heat treatment (400°C, 1h) | 350–450 | 850–950 (low P) |
| Corrosion resistance (salt spray, steel) | 100–300 h (porous) | 500–1000 h (non‑porous) |
| Wear resistance | Good | Excellent (comparable to hard chrome) |
| Electrical conductivity | Good | Poor (P increases resistivity) |
Electroless nickel is preferred for threaded parts, internal passages, and high‑wear applications (e.g. hydraulic pistons). It is not a good conductor; for grounding, specify bright nickel or chromate.
3.3 Zinc and Zinc Alloys
| Type | Thickness (µm) | Salt spray (h) to red rust (steel) | RoHS compliant |
|---|---|---|---|
| Zinc (clear) | 8–25 | 72–150 | Yes (if Cr³⁺ passivate) |
| Zinc‑nickel (12–15% Ni) | 8–25 | 500–1000 | Yes (Cr³⁺ passivate) |
| Zinc‑iron | 8–25 | 150–250 | Yes |
When to avoid zinc: >260°C service, marine immersion (chlorides attack zinc), or contact with aluminum (galvanic corrosion unless insulated).
Post‑treatment: Chromate passivation (yellow, black, clear) triples corrosion life.
3.4 Gold and Silver Plating – Electronic Applications
- Gold (hard gold with Co or Ni): 0.5–5 µm, hardness 130–200 HV, wear‑resistant contacts. Requires nickel underplate (2–5 µm) to prevent diffusion into copper.
- Soft gold: 99.9% Au, for wire bonding, requires strike layer.
- Silver: Excellent conductivity (lowest contact resistance), but tarnishes in sulfide environments (use anti‑tarnish dip).
- Thickness measurement: XRF per ASTM B568.
4. Powder Coating and Wet Painting – Organic Finishes
4.1 Powder Coating – Application and Curing
- Process: Electrostatic spray of polymer particles (epoxy, polyester, or hybrid). Cured at 160–200°C for 10–30 min.
- Film thickness: 50–150 µm (much thicker than liquid paint).
- Edge coverage: Poor – corners get thinner (Faraday cage effect).
- Impact on dimensional tolerances: On threaded holes, powder can fill >0.1 mm per side – use masking plugs.
Performance:
- Salt spray: >1000 h (epoxy)
- UV resistance: Polyester > epoxy (epoxy chalks under sunlight).
- Chemical resistance: Epoxy better against acids/solvents.
4.2 Wet Painting – When Powder is Not Suitable
- Low‑temperature cure (ambient to 80°C) – for heat‑sensitive assemblies.
- Thin films (25–75 µm) – for tight tolerances.
- Colour matching (RAL, Pantone) available.
5. Passivation – Stainless Steel and Other Alloys
5.1 Mechanism
Passivation removes free iron and iron‑rich inclusions from the surface of stainless steel, allowing a passive chromium‑oxide film (1–5 nm thick) to form. It does not add a visible coating.
Methods (ASTM A967, ISO 16048):
- Nitric acid bath (20–50% HNO₃, 20–60°C) – traditional, but generates NOx.
- Citric acid (4–10%, 50–70°C) – safer, equally effective.
Verification: Copper sulfate test (ASTM A380) – no copper plating indicates free iron removal; water immersion test (24h, 80°C) – no rust.
When passivation is mandatory: Medical devices (ISO 13485), food contact, pharmaceutical, marine, and any application where corrosion at a scratch must be minimised.
5.2 Conversion Coatings for Steel (Phosphate)
- Zinc phosphate (0.5–3 µm) – base for paint, improves adhesion.
- Manganese phosphate – heavy coating (5–15 µm) for wear‑in of sliding parts (gears, camshafts).
- Standards: MIL‑DTL‑16232, DIN 50942.
6. Selection Decision Framework
6.1 Substrate and Environment Matrix
| Requirement | Aluminum | Titanium | Steel (incl. stainless) | Copper/Brass |
|---|---|---|---|---|
| General corrosion protection, decorative | Type II anodise + dye | Chromate or anodise (Type II) | Zinc‑nickel + passivate | Clear lacquer or nickel |
| High wear (sliding contact) | Type III hard anodise | Hard anodise (AMS 2488 Type III) | Electroless nickel | Electroless nickel |
| Electrical conductivity needed | Chromate (Type II clear) | Chromate | Zinc or nickel (bright) | Tin (solderability) |
| Extreme chemical / salt spray >500h | Type II anodise + Ni acetate seal | Type I chromate (non‑RoHS) or anodise | Zinc‑nickel + Cr³⁺ passivate | Electroless nickel (5–10 µm) |
| Fatigue‑critical structure | Type I chromic acid anodise (thin) | Type I chromate (thin) | Avoid electroplating; use mechanical zinc or paint | – |
| Medical / biocompatible | – | Anodise (colour) or uncoated | Passivated stainless steel | – |
6.2 Cost Ranking (Relative, per dm², high volume)
| Finish | Relative cost (1 = lowest) | Notes |
|---|---|---|
| Chromate (clear, Type II) | 1 | Very thin, quick dip |
| Zinc plating + passivate | 1.5–2 | Widely available |
| Type II anodising (no dye) | 2–3 | Affordable |
| Bright nickel (electrolytic) | 3–5 | Moderate, but hydrogen embrittlement risk |
| Type II anodising + dye | 4–6 | Extra step |
| Electroless nickel | 6–10 | Uniform, complex parts |
| Type III hard anodising | 10–15 | Long process time, power intensive |
| Zinc‑nickel | 12–20 | Higher material cost |
| Powder coating | 15–30 (setup dependent) | Batch oven cost |
| Gold / silver (thick) | 100+ | Precious metal |
7. Testing and Quality Control
| Property | Test method | Typical acceptance |
|---|---|---|
| Coating thickness | Eddy current (anodise, paint), XRF (plating) | ±10% of nominal |
| Adhesion | Cross‑hatch (ASTM D3359, ISO 2409) | 4B (no removal) |
| Corrosion resistance | Neutral salt spray (ASTM B117, ISO 9227) | See tables above |
| Hydrogen embrittlement | Sustained load test (ASTM F519) | No failure after 200h |
| Porosity | Ferroxyl test (for stainless passivation) | No blue spots |
| Hardness | Micro‑Vickers (HV 0.05) | Per specification |
For EU export, ensure compliance with RoHS (2011/65/EU) and REACH (EC 1907/2006) – especially for hexavalent chromium (Type I chromate) and certain nickel salts.
8. Common Failure Modes and Prevention
| Failure | Root cause | Prevention |
|---|---|---|
| Blistering (paint/powder) | Outgassing from porous substrate, insufficient cleaning | Vacuum degassing; proper pre‑treatment (phosphate or chromate) |
| Hydrogen cracking (high‑strength steel) | Delayed HE due to electroplating | Bake within 4h; use non‑electrolytic coating |
| Anodising fatigue loss | Tensile stresses in thick oxide | Specify Type I (chromic) or shot‑peen before anodising |
| Galvanic corrosion | Dissimilar metals in contact | Insulate (paint, gasket) or choose compatible finishes |
| Thread seizure after plating | Plating adds thickness | Pre‑plate masking or use thread tolerances (e.g. UNR) |
9. How to Specify Finishes on Your Order
To avoid misinterpretation, include the following in your technical drawing or PO:
- Standard and class (e.g. MIL‑A‑8625 Type III, Class 1, unsealed)
- Localised masking (e.g. mask threads and 2 mm adjacent)
- Dimensional allowance (e.g. growth not to exceed 0.05 mm per side)
- Performance requirement (e.g. salt spray 500 h, no red rust)
- Colour (RAL, Sherwin‑Williams code, or sample match)
Our application engineers can advise on the optimal finish for your material, function, and budget.