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

What is the color and finish of hot dip galvanising as per Standards

I need to know what is the allowable limit for surface finish and any hot dip galvanising. Also color of HDG after the job has been taken either from zinc bath or from dichromate solution

Ramesh Ramesh
QA - Pondicherry, India
May 5, 2008

---

May 5, 2008

Hi, Ramesh. Although there are industry standards for many things in metal finishing, in most metal finishing situations it is usually the buyer (I assume that's you?) who sets appearance standards because, for example, drip marks may be fine on one surface but unacceptable on the other side. This can be done with "sample boards" which show what is okay and what isn't. Can you make this a little less abstract and tell us what is going on? Thanks.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

---

May 15, 2008

Sir:
A poorly, and improperly maintained dichromate quench can give very ugly galvanizing. Brown, yellow, green, and black coatings will result if the pH, the dichromate concentration, and impurities are not properly controlled.
Aluminum, lead, bismuth, and tin in the zinc all play a part in the appearance (brightness and spangle) of the galvanized product. Also the formation of "white" rust largely depends on the concentrations of each of these elements in the zinc.
Personally I like leaded or non-leaded zinc with aluminum between 0.0005% to 0.001% with IQM. This provides a very bright coating with little tendency to "white" rust. I do not prefer any other elements added to the zinc including bismuth, tin, or nickel for various reasons (including "white" rust, cracking of stressed steel, toxic to workers, and cost).
Regards,

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