Hot-dip galvanized vs zinc plated fasteners: choosing by environment
The single most common cause of premature fastener corrosion we see is not a bad coating — it is a coating selected by price rather than by environment. The two options are not close substitutes.
The difference in one table
| Zinc plated (electroplated) | Hot-dip galvanized | |
|---|---|---|
| Coating thickness | 5–12 µm | 45–85 µm (ISO 1461 minimums: 45 µm for thin parts, 55 µm for ≥6 mm, 70 µm for ≥6 mm on threaded work where agreed) |
| Typical outdoor life | 2–5 years in an exposed location | 20–30+ years in C3, 25+ years in C4/C5-M with adequate thickness |
| Appearance | Smooth, bright or yellow | Matt grey, slightly rough |
| Thread fit | Unaffected | Requires over-tapped nuts for assemblies above approximately M10 |
| Relative cost | Lower | Higher, but far lower per year of service life |
Mapping environment to specification
| Category | Environment | Recommended protection |
|---|---|---|
| C1 / C2 | Indoor, heated, low pollution | Plain or zinc plated |
| C3 | Urban and inland, moderate pollution | Zinc plated with trivalent passivation, or HDG for design lives beyond 10 years |
| C4 | Industrial, coastal with moderate salinity | Hot-dip galvanizing 55–70 µm minimum |
| C5-I / C5-M | Heavy industrial, coastal and offshore | Hot-dip galvanizing 80 µm minimum, or A4 (316) stainless throughout |
The bolt-and-nut pairing problem
Hot-dip coating adds measurable thickness to the thread. A galvanized bolt will not accept a standard-tolerance nut, so galvanized assemblies must be supplied with over-tapped nuts as a matched set. Two failure modes result when this is ignored:
- Forcing a standard nut damages the coating and removes the corrosion protection exactly where the joint needs it most — at the exposed thread.
- Mixing plated and galvanized components creates a coating-thickness mismatch, so the nut corrodes first and the assembly fails at the nut rather than the bolt.
The practical rule: order the bolt, nut and washer as one assembly from one supplier, and require them to be packed together.
Stainless steel is the third option
For small fasteners, clamps and facade work, A4 (316) stainless often solves the problem better than any coating — it is homogeneous, so there is no coating to damage, and it protects itself through passivation. Two cautions:
- Do not mix stainless with zinc-coated steel in the same joint. The coating becomes the sacrificial anode and corrodes quickly. If the joint must mix metals, insulate it.
- Beware galling. Stainless threads under high torque can seize. Use a lubricant, a controlled assembly speed, or a dissimilar grade pairing.
How to decide, in order
- Identify the corrosion category of the site — coastal, industrial, urban or inland.
- Identify the required design life. Twenty-five years is the normal assumption for solar, facades and infrastructure.
- Multiply the two: a long life in an aggressive category needs either substantial coating thickness or stainless.
- Decide whether the joint can tolerate coating thickness. Thin sections and fine threads cannot, which pushes the decision towards stainless or zinc-flake coatings.
- State the requirement as a measurable number and require the report to prove it.