What is the Process for Coating Metal with Zinc?
The process for coating metal like iron or steel with a thin zinc layer is known as galvanization. The zinc layer serves to protect the metal from rusting. There are three different processes that can be used for galvanization. The two less-common processes are electrodeposition and electrochemical, while the one used the most often is hot dip galvanization. The basic idea behind the hot dip method is immersing the metal in a zinc bath.
Here's the step by step process for how it works:
- The metal is cleaned in order to remove grease, oil, soluble markings, and dirt. This step involves immersing the metal in a heated alkaline solution designed to remove these items. However, the solution can't remove other residues like varnish, so sometimes mechanical cleaning is required.
- Next the metal is pickled by being immersed into a solution of sulfuric or hydrochloric acid, which removes mill scale and surface rust.
- Then it's put into a solution of zinc ammonium chloride, which gets rid of oxides and helps to keep the oxidation process from beginning. At the end of this step, the metal is totally clean and ready to move on to the galvanizing step.
- Liquid zinc is heated up to temperatures higher than 800 degrees Fahrenheit and the metal is immersed in it. During the immersion, the zinc actually bonds to the metal and builds several layers of alloy before finally topping off the metal with a later of pure zinc that's highly resistant to impacts.
- When the metal is pulled out of the zinc bath, it's then drained and vibrated or centrifuged to remove all of the extra zinc.
- The metal is then put out to air dry if it's going to receive a powder coating or put into a cooling liquid if powder coating isn't the ultimate plan for the metal. When ordering galvanized metal, you should remember to tell them if you want the galvanized metal quenched in the cooling liquid because this deposits contaminants on the surface of the metal, which can hinder certain processes done to it later.
- After it cools, the galvanized metal is then inspected. Experts examine the thickness of the zinc coating and the condition of the finished surface. Often there are gashes or other imperfections in the surface of the metal. They need to be smoothed out in some cases, like if you wish to powder coat the metal.
While the galvanizing process was once used as an alternative to a powder coating, today the industry standard is becoming a combination of the two processes. In many cases, metal is first galvanized and then powder coated. This is known within the industry as duplex coating.
Powder coating galvanized metal involves cleaning the surface to remove oil, grease, or dirt, being careful not to remove the galvanized coating. A zinc bath is typically used. Then it is profiled in preparation for the powder to be applied. Sealers in the form of zinc phosphate treatments are applied to protect the underlying zinc layer. This is because although powder coating offers many attractive qualities like a non-stick surface, it is also porous. In order to prevent the zinc underneath it from corroding, you need that extra sealer. Then the powder is sprayed on and heated up so that the powder adheres to the surface.
Toefco is a leading provider of zinc and powder coatings. Their experts will help you figure out if galvanization is the right process for you and if duplex coating is the appropriate finish for the metal you are working with.