"Galvanized" is one of the most overloaded words in fastener procurement. It can mean electroplated zinc a few micrometres thick, or a hot-dip coating several times thicker. The two are not interchangeable, and the difference only becomes visible after a few years outdoors.
Electroplated zinc
The parts are plated in a bath at low temperature. The coating is thin, uniform and visually bright.
Strengths
- Consistent appearance.
- Low cost.
- Suitable for internal and sheltered applications.
- Minimal effect on thread fit.
Limitations
- Thin coating — limited outdoor life.
- Less resistance to abrasion and handling damage.
- Not normally specified for exposed structural steelwork.
Hot-dip galvanizing
The parts are dipped in molten zinc, which reacts with the steel to form a metallurgically bonded coating. The coating is much thicker and harder.
Strengths
- Long service life in external conditions.
- Coating is bonded to the steel rather than deposited on it.
- More resistant to mechanical damage.
Limitations
- Higher cost.
- Coating thickness varies slightly across the part.
- Threads must be cut or tapped to allow for coating thickness — over-tapping is normal and necessary.
- Nuts may be supplied galvanized or dyed to a different colour to prevent mix-ups.
A comparison in practice
| Factor | Zinc plating | Hot-dip galvanizing |
|---|---|---|
| Coating thickness | Thin | Substantially thicker |
| Typical environment | Internal, sheltered | External, exposed |
| Thread fit | Standard | Adjusted for coating |
| Appearance | Bright, uniform | Matte, slightly variable |
| Relative cost | Lower | Higher |
Choosing the finish
Ask three questions:
- Where is the part installed? Internal dry space, sheltered external, or fully exposed.
- What does the project specification say? Coating class is usually defined by the corrosion category, not by preference.
- Does the thread need to run freely? Hot-dip coating changes thread fit; the nut and bolt must be produced as a matched pair.
For coastal, industrial or humid environments, the specification will usually push towards hot-dip galvanizing or stainless steel. For internal fixing and dry service voids, zinc plating is normally adequate.
Common procurement errors
- Writing "galvanized" without a coating requirement. Specify hot-dip or electroplated, and state the coating mass or thickness if the project requires it.
- Mixing coated and uncoated nuts and bolts. Produce and supply as a matched set.
- Assuming thicker is always better. A heavy coating on a fine thread can cause fit problems if the threads were not prepared for it.
- Ignoring handling damage. Coated parts still need protection in transit; bundles should be separated and threads capped.
What to put in the enquiry
State: part, size, material, finish type, coating requirement if specified, quantity and destination. If the project has a corrosion class or a coating standard, quote it directly. A supplier can then confirm what is achievable rather than offering a generic "galvanized" answer.
This note is general technical guidance prepared for the product library. It is not a project-specific approval and does not replace the specification issued by the project engineer. Values requiring test evidence are not published.
