Hot dip vs Cold galvanizing: What’s the difference?
What is galvanizing?
In its simplest form, galvanizing refers to the application of a zinc coating to a metal (usually steel or iron) surface. It is mainly used to prevent corrosion and extend corrosion protection measures to useful living protected metal parts. When properly applied to steel, zinc coatings provide two main types of protection: barrier protection and cathodic protection.
Hot dip galvanized
Hot dip galvanizing is one of the most common forms of galvanizing. The process involves dipping an iron or steel object into a molten zinc bath at a temperature of around 840°F(449°C). Once removed from the bath, the zinc layer on the surface of the iron or steel reacts with oxygen in the atmosphere to form zinc oxide nanoparticles.
The zinc oxide further reacts with carbon dioxide to form a protective layer called zinc carbonate (ZnCO3.). The dark gray film is relatively stable and clings to the surface of steel. In hot dip galvanizing, the zinc content of the chemical bond becomes part of the steel to be protected.
Although the concept of hot dip galvanizing seems simple, the process involves several key steps (Figure 2). The steel first passes through three cleaning stages to prepare its surface for galvanizing; These are degreased, pickled and communicated. Degreasing is used to remove dirt, oil and other organic residues; Mild acid baths, or pickling, etch steel and remove factory-scale iron oxide; And flux further removes any oxides and coats the steel with a protective coating that prevents further oxides from forming on the steel surface prior to galvanizing.
After the cleaning process is complete, the structure dries and can be immersed in a molten zinc bath. Molten zinc flows into or around iron or steel, thus covering it thoroughly to protect all surfaces. The coated material is then removed from the bath and air-dried prior to inspection.
Cold galvanizing
Cold galvanizing is simply the application of a zinc-rich coating to the surface of a steel element to protect it from corrosion. As a result, the term “cold galvanizing” is considered a misunderstanding among certain professionals in the coating industry.
Zinc coating can be coated with brush, roller, spray gun, etc. The coating can also be electroplated as well. The zinc-rich coatings used in cold galvanizing are different from conventional coatings due to the presence of binding compounds. These adhesives allow zinc to mechanical bonds to provide effective protection against steel.
In the case of hot dip galvanizing, cold galvanizing may provide barrier protection and may also provide some degree of cathodic protection. However, the zinc dust present in the paint or coating must have a high enough concentration to improve the conductivity between the steel and zinc.
Comparison of hot dip galvanizing and cold galvanizing
Although hot dip galvanizing and cold galvanizing have similar uses, their application methods and properties are significantly different. Unfortunately, cold galvanizing does not provide the same level of protection as hot galvanizing. Since cold galvanizing is a coating, it cannot bond to metal at chemical levels and therefore does not have the same durability, wear resistance and cathodic protection as hot dip galvanizing.
Although cold galvanizing can not achieve the performance of hot galvanizing, but also has its advantages. Cold galvanizing is ideal for low cost and fast applications to smaller structures and components. The hot-dip process is more expensive and more suitable for large structures, and is often used for heavy industrial purposes. (Related reading: galvanizing and its anticorrosion effect.)
The choice of galvanizing method ultimately comes down to finding a balance between cost and coating performance in a given application.
