The authoritative public forum
  for Metal Finishing since 1989
-----

White spots on galvanized hardware

Hi,

I have some problems on my metal hardware which I bought from our suppliers. After few days of exposure to the environment white spots (a sort of oxides) coming out from the surface. I tried to measure the galvanization and recorded at around 14-20 microns. Could it be possible to know why these white spots (oxides) are coming out from the surface? Is there any process of galvanization that could guarantee the attack of rust into the hardware? What is the best galvanization process or materials that can be used for galvanization? Is there any standard thickness for galvanization? Hope to get information about this.

Thanks a lot,

Regards,
Luichito

Luichito Durban
Buyer - Ho Chi Minh City Vietnam
August 8, 2011

---

There are two main systems of galvanizing steel. One is for sheet metal and typically gas about 20-30 microns of zinc coating. That sheet metal (thin sheet) is often made into items for use, such as buckets, containers, ducting, etc. It sounds like your item is this.

The other system is to make the item first then dip the whole item in zinc. This "post fabrication" galvanizing has a much thicker coating. Typically 100 you for steel that's 5-6 mm thick.

The white spots you see, sound like the normal products of oxidation of zinc. In air, with some water, you will get zinc oxide, zinc carbonate and similar products. These will form a white deposit. This shows that the oxygen has consumed zinc, and not steel. Galvanizing is a sacrificial coating. It is consumed in preference to the steel. If no zinc was consumed, then there was no oxidation.

You could make the zinc last longer by also protecting it using another coating, such as powder coating or paint on top.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

August 10, 2011

---

August 12, 2011

Sir:

As already stated the very thin zinc coating is typical of sheet galvanizing or electroplate.

With sheet galvanizing there are two problems: (1)The high aluminum content of the zinc which promotes white rust and (2)The sheared edge producing a bare steel edge which also promotes white rust.

Regards, Dr. Thomas H. Cook, Hot Springs, South Dakota, USA

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA