Introduction
Surface treatment has a direct impact on corrosion resistance, wear resistance, appearance, and hardness of mechanical parts. Specifying the right treatment — and calling it out correctly on drawings — is an important part of the designer’s job.
Why Surface Treatment Matters
Most structural steels corrode without protection. Even stainless steel and aluminum require specific treatments in aggressive environments. Beyond corrosion, surface treatments can:
- Harden the surface to resist wear (case hardening, nitriding, hard chrome)
- Reduce friction (PTFE coating, phosphate + oil)
- Improve electrical conductivity or insulation
- Provide a specific appearance (anodize color, paint finish)
Common Surface Treatments
Zinc Plating (Electroplating)
Applied to carbon steel parts. Provides moderate corrosion resistance at low cost. Typical thickness: 5–12 u03bcm. Suitable for indoor or mild outdoor environments. Not suitable for high-humidity or saltwater exposure without additional treatment (chromate passivation).
Nickel Plating
Good corrosion resistance and wear resistance. Also used for decorative purposes. Harder than zinc plating. More expensive. Common in electrical connectors and precision mechanical parts.
Hard Chrome (Electroplating)
Produces an extremely hard, wear-resistant surface (900–1000 HV). Used on hydraulic rods, cylinder bores, and wear surfaces. Thickness: 10–500 u03bcm. Environmental regulations restrict hexavalent chrome use in many regions — check local requirements.
Anodizing (Aluminum)
Electrochemical process that builds an oxide layer on aluminum. Increases corrosion and wear resistance. Can be colored (black, red, gold, etc.). Hard anodize (Type III) produces a thicker, harder layer for wear applications. Does not apply to steel.
Black Oxide
A chemical conversion coating for steel. Produces a matte black appearance. Minimal corrosion protection on its own — normally used with oil or wax coating. Dimensional change is negligible (1–2 u03bcm). Common on fasteners, tools, and interior components.
Painting and Powder Coating
Applied for corrosion protection and appearance. Powder coating is thicker (50–100 u03bcm) and more durable than liquid paint. Both require surface preparation (degreasing, sandblasting or chemical etch) for good adhesion.
Case Hardening / Carburizing
Carbon is diffused into the surface of low-carbon steel at high temperature, creating a hard case over a tough core. Common on gears, shafts, and pins. Specify: case depth and surface hardness (e.g., “Carburize, case depth 0.5–0.8 mm, surface hardness 58–62 HRC”).
How to Specify on Drawings
Surface treatment is specified in the drawing notes or on the title block. Include:
- Treatment type (e.g., “Zinc electroplate”)
- Standard reference where applicable (e.g., ISO 4042 for zinc plating)
- Thickness or hardness requirement
- Areas to exclude if treatment must not be applied everywhere
Selection Guide
| Material | Environment | Recommended Treatment |
|---|---|---|
| Carbon steel | Indoor, dry | Black oxide + oil, or paint |
| Carbon steel | Outdoor / humid | Zinc plate + chromate, or powder coat |
| Carbon steel | Wear surface | Hard chrome, or case hardening |
| Aluminum | General use | Anodize (Type II) |
| Aluminum | Wear surface | Hard anodize (Type III) |
| Stainless steel | Most environments | Passivation (no coating needed) |
FAQ
Q. Does surface treatment change part dimensions?
A. Yes. Plating and coating add thickness. For precision fits (H7/g6, etc.), apply treatment after machining and account for plating thickness in the tolerance calculation, or specify the dimension as the finished (post-treatment) size.
Q. Can I apply zinc plating to stainless steel?
A. It is technically possible but rarely done — stainless steel already has corrosion resistance from its chromium content. Zinc plating on stainless can cause galvanic corrosion at the interface in some environments.
Q. What is the difference between electroplating and electroless plating?
A. Electroplating uses an electric current to deposit metal; the coating thickness varies with current distribution. Electroless plating is a chemical process with no current — it deposits uniformly on complex shapes, including inside holes.
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