Steel Manufacturing Blog: Keeping it Steel

Galvanizing : Hot Dip vs. Cold Galvanizing

Posted on Mon, May 08, 2023 @ 10:54 AM

Hot Dip Galvanizing and Cold Galvanizing each have advantages.

Hot Dip Galvanizing is the most durable and effective zinc coating available. It is a multi-step process that is more involved than it appears to be. In order, the steps are:

  • Caustic Bath to remove any surface contaminants, grease, salt, markings, etc.
  • Water Rinse to was away the residual caustic acid
  • Hydrochloric Acid refines the cleaning process and delivers a surface equivalent to virgin steel.
  • Flux Bath stops the oxidation process that begins immediately after removal from the Hydrochloric Acid and prepares the steel surface to bond with the liquid zinc.
  • Molten zinc, typically between 835° and 850°F. The steel will need to stay immersed in the zinc bath until it has reached ambient temperature. This establishes the metallurgical bond between the steel and the zinc coating. This same bond is what is interrupted when part of the steel needs to remain non-galvanized by using our Maskote Zinc Stop-Off

Picture1-3 The advantages to Hot Dip Galvanizing are the permanence and longevity of the coating and thorough coverage inside any enclosed area such as tube steel. Internal coverage is very important in that enclosed areas, due to humidity and condensation, will tend to decay faster than exposed surfaces. Estimates vary regarding life cycle but it is not uncommon for the coating to last 50 years even in demanding environments.

The disadvantages are location, time and cost. Hot dip galvanizing is done in regionalized factories often 100’s of miles from the fabricator or jobsite so transportation time and cost can be considerable. It is also not uncommon for the process to take 1 to 2 weeks including transportation.

Cold Galvanizing provides a convenient method of applying a zinc coating in the fabricating shop orCold-Galvanize-Compound-93---65 at the jobsite. It is often used to repair weld burns or other damage to newly galvanized steel but can be specified as the primary galvanizing system. It is important to note there are two type of Cold Galvanize.

  • Zinc Rich which provides 93% zinc in the dry film. It is designed to be applied directly to uncoated steel or on top of previously galvanized steel. It dries to a dull gray finish.
  • Bright Zinc Galvanize has the advantage of drying to a bright finish, very well matched with newly galvanized steel. When cured it leaves 65% zinc in the dry film.

Cold galvanizing has a shorter lifespan than Hot Dip largely due to the lack of the metallurgical bond that occurs with Hot Dip Galvanizing. It is important to note like all paints and coatings surface preparation controls performance. Wire brushing loose rust and contaminants is always required however for the best longevity the steel should also be cleaned with a compatible solvent.

Tags: Hot Dip Galvanizing