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Rusted hot dip galvanized bolt and nuts

Q. Hot dip galvanised bolt and nuts on transmission line tower steel structure became badly rusted and when I measured the galvanizing thickness, it shows the thickness of the galvanizing is still OK i.e. within BS EN ISO1462:1999 allowable. It puzzles me that how galvanized bolt and nuts can get badly rusted (service period less than 20 years)? How come the thickness of the galvanising is not reduced due to rust formed? Does it means that the steel thickness got reduced instead of galvanized layer? Does it impose danger to the tower steel structure?

Thank you.

Mr Azren Othman
- Malaysia

A. The threads in the nut will not be galvanized,because of which you are seeing the rust,as such if the galvanized coating is intact there should not be any problem.


- Riyadh, KSA

A. Mr. Othman,

The problem is that you use HDG coating on bolts and nuts. HDG is traditionally used for this type of coating even though it is not appropriate to use in such application. As Mr. Ameen said, the problem is in the treads of the bolts as well as the nuts. Additionally, most HDG coatings will get scraped off when you drive the bolt into the nut. I suggest you look into different methods of heavy coatings, such as zinc thermo diffusion coating.

Dorian Shifman
- Scarsdale, New York

A. Hot dip galvanizing has been used extensively for nuts and bolts for many decades in many countries very successfully. It is not reasonable to say it is an "inappropriate" coating.

You say rust is visible but the measured thickness of the zinc coating appears to show it as still galvanized?
Several possibilities exist.
1. You are not measuring zinc. Perhaps you are using a device to measure the coating thickness that relies on the magnetic properties of the steel, and actually measures the distance from the probe to the steel. In this case its possible to get a false reading from any non ferrous material between probe and steel. Even corrosion products will do this.

2. The rust is on top of the zinc. In this case the rust is staining from elsewhere, and stains the surface of the zinc. This is a common phenomenon, with end users complaining of rust where what they actually have is rust staining on galvanizing. Try scraping the rust with a pen-knife. Do you see shiny zinc underneath?

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK
crithwood logo

Q. Hi, my question is why is the galvanized bolt showing rusting after assembly, the nut fitment is done by torque wrench.

42842-1d   42842-1b   42842-1c   42842-1a  

Thanks & Regards,

Jitendra Panchal
hot dip galvanization and plant mfg. - Pune, Maharashtra, India
October 25, 2019

simultaneous replies

A. This might not be rust, it might be rust staining o top of the zinc.
You might measure the zinc thickness, or try scraping (even a light touch smoothing file) the stain and see if there's zinc underneath.
It looks more like staining that rust.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK
crithwood logo
November 5, 2019

A. On the first pic with the two nuts, that white marks are paint or from the nuts itself? If it is, then this can be white corrosion (zinc oxide that didn't passivate because of moisture/water).
In the picture (the two nuts one) can see clearly if that corrosion (red one) is from the nuts itself. It has a "splash" shape, usually from another piece of equipment that is showing rust.
Additionally, in the third picture (the one showing the long series of nuts and the floor) the corner of the nut appears to have its protection gone away/chipped.

Biro Liro
Galvanization - Caracas, Catalan, Venezuela
November 2, 2019

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