A surface treatment changes only the outer layer of a part, because corrosion, wear and fatigue cracks all start at the surface. It can add a barrier such as paint, oil or tar, add a sacrificial metal such as zinc, grow a protective oxide as in anodising, or harden the outer skin of steel while the core stays tough. Choose by the failure you are trying to stop, since a coating cannot repair a bad part and no coating works on a rusty surface.
Part of Materials Science
Surface treatments are processes that protect or strengthen the outer layer of a part: coatings, plating, conversion layers, diffusion hardening and galvanising.
The Surface Is Where Most Failures Begin
Stress is highest at the surface in a bent or twisted part, fatigue cracks begin at scratches and corners (see Failure Modes), moisture and oxygen attack the outside first, and wear removes material from the outer face. So a treatment that changes only the outer layer can change a part’s life without changing its bulk.
Rust Prevention gives the scale of the corrosion problem: rust needs iron, water and oxygen together, so a barrier coating that keeps water and air off the metal stops it, and unprotected iron outdoors in a temperate climate loses 0.1-0.3 mm of metal per year. It adds that surface preparation matters more than the coating, because no coating works on a rusty surface.
Four Ways To Treat A Surface
| Approach | What it does | Examples | Article |
|---|---|---|---|
| Barrier | Keeps water and air off | Paint, oil, tar, pitch, wax | Protective Coatings |
| Sacrificial coating | A more reactive metal corrodes instead of the steel | Zinc by hot dipping | Galvanizing |
| Deposited or grown layer | Adds metal or thickens the oxide | Plating, anodising | Electroplating, Anodizing |
| Diffusion hardening | Puts carbon into the steel skin | Pack carburising | Case Hardening |
Heat treatment of the whole part, as in Heat Treatment, is not a surface treatment, though the two combine: a case-hardened part is tempered afterwards.
Pick The Treatment By The Failure
- Rusting steel outdoors: zinc coating, or paint and oil kept up.
- Aluminium in wear or damp: anodising, which converts the surface to oxide.
- Sliding or edge wear on steel: case hardening, or hardening the whole tool.
- Electrical contacts: plating with copper or silver for conductivity.
- Appearance and food contact: stainless steel needs only a clean surface (see Stainless Steel).
Worked Example: How Long A Coating Buys You
Taking the corpus loss rate of 0.1-0.3 mm per year for bare iron, the Advanced Materials article’s zinc coating of 50-100 micrometres (0.05-0.10 mm) is thinner than a single year of the fastest bare loss, and yet the Advanced Materials article says it lasts 20-50 years. The reason is that zinc corrodes far more slowly than iron in air, because it forms its own protective layer, so the coating thins over decades rather than months. The comparison shows that coating thickness divided by iron’s loss rate is not a service life.
Common Mistakes
- Coating over rust — the rust keeps working underneath.
- Skipping surface preparation — the Advanced Materials article warns plating over grease or oxide peels immediately.
- Expecting a coating to fix a design fault — a crack under a coating still grows.
What’s Next
- Learn next: Galvanizing — the main protection for steel outdoors
- Combine with: Case Hardening — a hard skin on a tough core