Fastener Corrosion Classes C1–C5: Choosing the Right Finish

Why Corrosion Classes Decide Your Fastener Finish

The same zinc-plated bolt that lasts decades inside a heated office would be rusted scrap on an offshore platform within a year. Engineers handle this with the corrosivity categories C1–C5 (and CX) from ISO 12944-2, which classify environments by how aggressively they attack steel. Match your fastener finish to the environment’s class, and you get a joint that lasts the design life at the lowest cost — over-specify and you pay for stainless you never needed; under-specify and the structure fails.

The C1–C5 Environment Classes

Class	Corrosivity	Typical Environment	Recommended Fastener Finish
C1	Very low	Heated indoor, dry (offices, schools)	Plain / zinc plated
C2	Low	Rural outdoor, unheated indoor	Zinc plated ≥5 µm
C3	Medium	Urban outdoor, moderate humidity, light industry	Zinc plated 8–12 µm, or hot-dip galvanized for long life
C4	High	Industrial areas, coastal (low salinity), pools	Hot-dip galvanized, Dacromet, or A2 stainless
C5	Very high	Coastal/offshore high salinity, heavy industry	A4 (316) stainless, thick HDG + duplex coating
CX	Extreme	Offshore platforms, tropical marine	A4/A4-80 stainless, super-duplex, specialist systems

How Each Finish Maps to the Classes

Zinc Plating (Electro-Galvanized)

Thin (5–25 µm) decorative-to-functional protection, 24–96 h white-rust salt-spray resistance. Right answer for C1–C2, acceptable for C3 with thicker deposits. Full thickness and salt-spray detail in our zinc plating specification guide.

Hot-Dip Galvanizing (HDG)

Thick metallurgically bonded coating (typically 50–100+ µm) with 500–1,000+ h salt-spray performance, plus sacrificial protection that shields scratches. The default choice for C3–C4 infrastructure: HDG hex bolts, HDG concrete expansion bolts, and matching galvanized washers.

Stainless Steel A2 / A4

A2 (304) covers C3–C4 reliably; A4 (316) adds molybdenum for pitting resistance and is the standard answer for C5 coastal and chemical exposure. See our 304 vs 316 selection guide for the full comparison, and A2/A4 threaded rods and stainless wedge anchors for the marine-grade product lines.

Dacromet / Geomet Coatings

Thin inorganic zinc-flake systems delivering 500–1,000+ h salt spray without hydrogen embrittlement risk — the specified finish for high-strength (10.9/12.9) fasteners in C4 service, because electroplated hydrogen embrittles high-strength steel. Our HDG vs electro-galvanizing vs Dacromet comparison covers the trade-offs.

Practical Selection Shortcuts

Indoor dry machinery: zinc plated, done.
Outdoor anywhere near coast or industry: HDG minimum; stainless if the joint is inaccessible for maintenance.
High-strength bolts (≥10.9) in corrosive service: Dacromet/Geomet or HDG with proper embrittlement control — never plain electroplating on 12.9.
Mixed-material joints: avoid pairing stainless fasteners with carbon steel structure (galvanic corrosion) — isolate with washers or match materials.
Design life matters: C3 zinc plating may satisfy a 5-year life spec but not a 25-year one; HDG and stainless stretch maintenance intervals dramatically.

Frequently Asked Questions

What salt-spray hours do I need for C3 or C4?

As a working rule: C3 ≈ 250–500 h, C4 ≈ 500–1,000 h red-rust resistance in the neutral salt spray test (ISO 9227). Zinc plating meets C3 only at the upper thickness end; HDG and zinc-flake coatings carry C4.

Is hot-dip galvanizing allowed on high-strength bolts?

For 8.8 and 10.9 structural bolts, yes — it is standard practice (e.g., galvanized ASTM A325/HDG assemblies). For 12.9 class, HDG is generally avoided due to embrittlement risk; use zinc-flake systems instead.

Can I use A2 stainless outdoors by the sea?

Not recommended long-term. Coastal atmosphere is C5, where A2 develops pitting and tea-staining; specify A4 (316) instead — the molybdenum content is exactly what that environment demands.

How do I know which class my site is?

ISO 12944-2 gives time-of-wetness and pollution criteria; a quick proxy — distance to coast, local industry, humidity — maps most projects to a class. When in doubt, specify one class up; the cost delta on fasteners is small relative to a corrosion failure.