There are many ways to protect your metal fabrication against corrosion. Two of the most widely used are powder coating and galvanizing. Each has its merits, and it’s seldom immediately obvious which one to use.
In this blog, we compare and contrast the two. By the end, you should have a clearer idea of what to consider and perhaps which will work best for your application. And if not, we’re always happy to talk.
The points we’ll cover here are:
- Basics of powder coating
- Basics of galvanizing
- Corrosion protection and durability
- Appearance factors
- Environmental considerations
- Process selection
- Real-world examples
Basics of Powder Coating
Powder coating is used to put a protective polymer skin over the surface of the metal. It’s done by spraying dry epoxy powder onto metal that’s been cleaned thoroughly. The coated fabrication is then put in an oven where heat fuses the particles to form an uninterrupted surface.
To ensure even coating and minimize waste, the spraying process is usually done with the aid of electrostatics. This is where the powder is given an electric charge as it leaves the spray gun, and the fabrication being coated is connected to a circuit giving it the opposite charge. The result is to attract powder particles onto every exposed surface.
Any metal can be powder coated, although some require careful surface preparation to ensure good adhesion. For best results, it’s common to apply an epoxy primer first and follow up with a polyester top coat.
Basics of Galvanizing
Galvanizing is one of the oldest methods of corrosion protection but is still widely used today. It entails adding a layer of zinc to a ferrous metal substrate, usually by dipping it in a tank of molten metal. The zinc reacts with the iron to form a hard surface.
Galvanizing protects against corrosion by the zinc corroding preferentially. In other words, in a corrosive environment, the zinc oxidizes before the iron.
Corrosion Protection and Durability
Given adequate surface preparation, both methods protect steel against corrosion for many years. Galvanizing, however, lasts far longer than powder coating.
Powder coating has three limitations in terms of durability:
- Any damage, like a scratch that extends through the coating to the metal, will let moisture penetrate. Once this happens corrosion is going to spread under the epoxy. In contrast, scratching galvanized steel doesn’t create a point for corrosion to start. Instead, the neighboring zinc corrodes first.
- At a microscopic level, the epoxy coating has enough porosity that moisture will eventually reach the steel underneath. While this can take years, it eventually leads to the epoxy coming away from the steel.
- Ultra-violet light gradually breaks down polymers, reducing their flexibility and fading any coloring.
The epoxy materials used in powder coating are produced in a very wide range of colors, and custom shades are available to order. As noted, when placed into service outdoors this coloring will fade.
With galvanizing only one finish is possible: a shiny, silver with a spangled appearance. Over time this fades to a dull mid-gray.
Powder coating is generally considered a “green” process because it doesn’t require any solvents and doesn’t release any VOCs. In addition, the oven bake is performed at around 400°F, so it’s not notably energy-intensive.
In galvanizing the molten zinc is held at around 840°F, which requires a significant amount of energy, but that’s not the biggest environmental concern. Larger issues are the use of hydrochloric acid for surface cleaning, (a process known as “pickling”), emission of zinc fumes when the metal is first submerged, and waste solids and water. Together, these mean the galvanizing process requires close supervision and Environmental, Health and Safety (EHS) attention.
In support of galvanizing though, the process can extend the life of a metal structure as much as five times that of powder coating. In addition, galvanized steel can be recycled and the zinc recovered. You might therefore argue that lower resource consumption gives galvanizing some “green” credentials.
In general, a galvanized finish is more expensive than a powder coating. This results from the time and complexity of surface preparation, the energy required, waste disposal costs, and the price of zinc. However, the benefit of increased durability can shift the total lifecycle cost equation in favor of the zinc finish.
Another factor is how material selection influences the need for corrosion protection, and how that in turn affects cost. For example, instead of galvanized steel, how about using stainless steel? The material is more expensive but will save on galvanizing. Likewise, is aluminum an option instead of powder coating? Again, you’re substituting a more expensive, corrosion-resistant material for a coating/protection process.
Deciding which of the two corrosion protection processes to use on a metal fabrication starts with considering the application. Questions to ask are:
- Will it be used indoors or out?
- What is the required life?
- Would a galvanized finish be acceptable?
If the fabrication is going into a hostile environment, (for example, outdoors, perhaps in a coastal area), corrosion resistance is an important consideration, particularly in conjunction with the required life. Conversely, if it will be sheltered for its entire life, corrosion resistance takes a lower priority and appearance may be more important.
Galvanizing is generally used when appearance takes a lower priority than corrosion resistance and durability. Examples of products typically galvanized are:
- Posts and poles: For example, for outdoor signage.
- Supports and brackets for agricultural equipment
- Fence posts
Powder coating is often preferred when corrosion is of limited concern and there are advantages in applying colored finishes. For example:
- Indoor storage racks: These can be made to conform to “house” colors for a unified appearance.
- Bins for material/product storage: Color-coding can convey information about the contents.
- Workbenches: Finish in the “house” colors.
- Railings: Color red or yellow to indicate hazardous areas.
The Right Finish Completes the Project
With our extensive metal fabrication experience we’re well-versed in corrosion resistance and prevention techniques. We’re always happy to explain the options and discuss how these impact cost, appearance, durability, and material selection. Contact us with your questions.