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.
Stainless steel fasteners can show surface rust in coastal environments despite their corrosion resistance. The most common cause is contamination from embedded carbon steel particles — from grinding, machining, or contact with galvanised steel components — that corrode and stain the stainless surface. This is surface contamination, not bulk material corrosion. Clean with oxalic acid or proprietary stainless passivation solution, then re-passivate. If the staining returns within weeks, the contamination is embedded (passivation alone is insufficient) and the fastener should be replaced.
- Best for
- O&M teams investigating rust staining on SS fasteners or clamps in tower or nacelle, and deciding whether the item needs replacement or passivation treatment
- Not suitable for
- Painting over rust staining on stainless fasteners — paint traps moisture and accelerates localised crevice corrosion under the coating
- Diagnosis steps
- 1 — Check whether staining is surface-only or pitting → 2 — Identify carbon steel contamination sources nearby → 3 — Clean with oxalic acid and re-passivate → 4 — If pitting present, replace fastener → 5 — Remove contamination source to prevent recurrence
- RFQ information
- Fastener type and grade, current condition, replacement grade required (SS 316L for offshore), quantity, passivation standard
§ 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.
Stainless-bolt galling causes and prevention
Evidence and decision boundary
- Direct evidence
- ISO 3506 defines properties and selection guidance for covered stainless fasteners. ISO 12944-2 classifies environments, while ASTM G71 guides galvanic-couple testing; none certifies an unspecified clamp assembly for offshore service.
- Engineering inference
- Select the complete assembly from chloride exposure, wet/dry cycling, crevices, temperature, loads, fabrication, material pairing, drainage, inspection and traceability. Stainless staining is not automatically structural failure, but its cause must be identified.
- Typical or indicative value
- Alloy labels, PREN comparisons and service-life statements are indicative until supported by certificates, geometry-specific assessment and project evidence.
Primary sources checked
- ISO 3506-1:2020 - stainless-steel fastener properties
- ISO 3506-6:2020 - selection of stainless steels and nickel alloys
- ISO 12944-2:2017 - environmental corrosivity classification
- ASTM G71-81(2024) - galvanic corrosion testing
Related commercial route: Compare wind-turbine clamp systems and RFQ inputs.