Why “galvanized” does not always mean hot dip galvanized
Many fasteners arrive labelled “galvanized.”
But when inspectors check zinc thickness, the coating fails.
Why?
Because not all galvanizing is hot dip galvanizing.
Electro-galvanized and mechanically plated bolts look similar but perform very differently outdoors.
What makes hot dip galvanized fasteners different
Hot dip galvanizing coats fasteners by dipping them into molten zinc.
This produces:
- Thick zinc layers
- Metallurgical bonding
- Strong corrosion resistance
- Long outdoor life
Electro-plating only coats the surface thinly.
What standards actually require
For structural fasteners:
- ISO 1461 specifies minimum zinc thickness
- IS 4759 defines coating classes
- Thickness typically ranges from 45–85 microns depending on section
Anything below this is not true HDG.
Why coating thickness fails
- 1. Electro-galvanizing used
- 2. instead of HDG
- 3. Under-dipping time
- 4. Incorrect bath chemistry
- 5. Post-coating thread damage
- 6. No thickness testing
What happens when thickness is low
- Early rust
- White corrosion
- Audit failure
- Rejection at site
- Reduced service life
How to verify HDG quality
Ask for coating thickness report
Ask for galvanizing standard
Use magnetic thickness gauge
Inspect threads for bare steel
Demand MTC with coating data
Final thought
Hot dip galvanized fasteners protect structures for decades.
Anything less is a temporary coating pretending to be permanent.