A common site complaint: "the bolts are stainless, but they're rusting." Stainless steel is not rust-proof — it is corrosion-resistant, and only as long as its protective passive layer stays intact. On a wind turbine, especially near the coast, several everyday factors can break that protection down.
§ 01 Stainless is not rust-proof
Stainless steel resists corrosion because chromium in the alloy forms a thin, self-healing passive oxide film on the surface. That film is what protects the steel — not the bulk metal itself. If the film is damaged faster than it can re-form, or if the environment is too aggressive for the grade, the underlying steel corrodes and you see rust on a part that is genuinely "stainless."
§ 02 How stainless actually fails
Rust on stainless usually takes one of a few recognisable forms:
- Tea-staining — a brown cosmetic surface discolouration, common on 304 in coastal air. Often shallow, but a warning the grade is under-specified.
- Pitting corrosion — localised deep pits where chlorides break through the passive film; far more dangerous than it looks because it concentrates stress.
- Crevice corrosion — attack in tight gaps (under washers, in threads) where oxygen is depleted and the film cannot re-form.
§ 03 Common causes
| Cause | Mechanism | Fix |
|---|---|---|
| Grade too low | 304 in a chloride (coastal) environment | Use 316/A4 or higher |
| Carbon-steel contamination | Steel grinding dust / tooling embeds and rusts | Segregate tools; passivate |
| Chloride pitting | Salt exceeds grade's resistance | Higher PREN grade (316 / duplex) |
| Crevice geometry | Stagnant gaps, debris traps | Design to drain; correct grade |
§ 04 Grade and environment
The single most common root cause is simply specifying 304 (A2) where the chloride load demands 316 (A4) or better. The corrosion resistance of a grade is often summarised by its PREN (pitting resistance equivalent number) — 316 outperforms 304, and duplex grades outperform both. The full comparison is in 304 vs 316 stainless for offshore fasteners, and for the harshest sites, duplex / super duplex.
Matching grade to the site's corrosion category (C5/CX) is the structured way to avoid this mistake.
§ 05 Prevention and remediation
- Specify the right grade for the corrosion category — 316/A4 minimum for coastal, duplex for splash/offshore.
- Avoid cross-contamination — never use carbon-steel brushes or shared grinding tools on stainless; store separately.
- Passivate after fabrication to restore the chromium-rich film.
- Check galvanic pairing — stainless next to a less noble metal can accelerate attack; see preventing galvanic corrosion.
Existing tea-staining can usually be cleaned and passivated; deep pitting means the grade is wrong for the site and the fastener should be upgraded.