Hot-dip galvanizing is one of the most common forms of galvanizing. The process involves coating an iron or steel object with molten zinc in a bath at temperatures of around 449°C. Once removed from the bath, the zinc coating on the iron or steel’s exterior reacts with oxygen in the atmosphere to form zinc oxide.
Zinc oxide further reacts with carbon dioxide to form the protective layer known as zinc carbonate. This dull grayish film is relatively stable and adheres tightly to the surface of the iron or steel. In hot-dip galvanizing, the zinc chemically bonds and becomes part of the steel.
Cold galvanizing is simply the application of a zinc-rich paint to the surface of a steel to protect it from corrosion. As such, the term “cold galvanizing” is considered to be a misnomer among some professionals in the coating industry.
Zinc paints may be applied by brushes, rollers, spray guns, etc. Coatings may also be applied by the electrogalvanizing method as well. The zinc-rich paints used in cold galvanizing are different from conventional coatings due to the presence of a binding compound. These binders allow the zinc to mechanically bind to the steel to offer an effective level of protection.
Like hot-dip galvanizing, cold galvanizing can provide barrier protection and also some degree of cathodic protection. However, the zinc dust present in the paint or coating must be in high enough concentrations to promote electrical conductivity between the steel and the zinc.
Comparing Hot-dip and Cold Galvanizing
Although hot-dip and cold galvanizing both serve similar purposes, their method of application and performance differ significantly. Cold galvanizing, unfortunately, does not offer the same level of protection as its hot-dip counterpart. Because cold galvanizing is simply a coating, it cannot bond with the metal on a chemical level and, as such, does not have the same durability, abrasion resistance and cathodic protection capabilities as hot-dip galvanizing.
While cold galvanizing does not live up to the performance of hot-dip galvanizing, it does have its benefits. Cold galvanizing is ideal for cost-effective and rapid application on smaller structures and components. Hot-dip processes are more expensive and better suited for larger structures, typically for heavy-duty industrial uses.