Sheet metal products serve a vast array of purposes in modern society and are a common fixture in the manufacturing, industrial, and transportation industries. Among other beneficial properties, sheet metal displays excellent strength, durability, and longevity. Yet in certain environments, those properties can be compromised by corrosion and other forms of structural degradation.
Manufacturers frequently apply a surface coating to protect sheet metal from unwanted oxidation. Two of the most common protective strategies are zinc plating and powder coating. This article takes a closer look at what each of these methods entails, as well as the relative merits to consider when choosing between the two.
Zinc plating, one type of galvanization, involves applying a thin layer of zinc metal onto the surface of another metal through a process known as electrodeposition. The zinc forms a protective layer, physically preventing oxidative forces from corroding the surface of the base metal; this is often referred to as the substrate.
Many people mistakenly assume that the zinc itself remains completely immune to corrosive action. This is not the case, however. Zinc instead offers what is known as cathodic protection. In other words, the zinc makes itself available to corrosion, thus protecting the underlying substrate.
That said, zinc has a higher degree of natural corrosion resistance than steel. This resistance has to do with zinc’s ability to form basic carbonate films. The actual corrosion rate of zinc depends on environmental factors such as the amount of moisture present, how quickly the surface dries, and any industrial pollution that may be present.
The Zinc Plating Process
Zinc plating is a complex, multi-step process requiring a large amount of specialized equipment. The first step involves preparing the substrate with a thorough surface cleaning. If any debris, rust, or contaminants remain present, the zinc may fail to adhere properly. Substrate cleaning is usually accomplished with an alkaline detergent solution followed with an acid treatment to remove any corrosion.
The actual coating process takes place in an electrolyte solution known as the plating bath. This bath consists of the zinc itself—present as an ionic solution—as well as various other needed chemicals. The substrate is introduced to the plating bath in one of two ways. So-called barrel plating involves placing loose pieces directly in a barrel, where they encounter the plating bath.
Barrel plating offers quick and consistent results. Yet some parts are either too large or too fragile for this method. In that case, the substrate pieces are affixed to a rack plating system and immersed in the bath. In either case, an electrical current is then introduced to the plating bath, causing the electrodeposition process to lay down a layer of zinc on the substrate.
Powder coating takes a much different approach to protect a substrate against corrosion. Whereas zinc plating works through cathodic protection, powder coating utilizes barrier layer protection. In other words, powder coating involves the formation of an impenetrable outer layer, which prevents water and other oxidizing materials from accessing the susceptible metal.
Powder coat is made up of a paint-like, organic material consisting of pigment and resin. These finely ground particles are given an electrostatic charge and then sprayed onto the substrate. The charge allows the particles to adhere to the surface. Heat is then applied in order to melt the particles, fusing them together in a continuous, uniform layer.
The Powder Coating Process
As with zinc plating, the powder coating process begins with a thorough chemical cleaning of the substrate. The goal is to remove any dust, grease, oil, rust, or other contaminants and ensure optimal adherence. Once the substrate is clean, the powder is applied using an electrostatic spray gun.
A pumping device sends powder from the feed hopper, through a series of hoses, to the spray gun. Not only does the spray gun direct the flow of powder, but it also imparts the electrostatic charge that allows the particles to stick to the metal. The application process usually happens inside a spray booth. A built-in powder recovery system collects any overspray and routes it back to the feed hopper.
Once the sheet metal substrate has been completely coated, the powder must be cured. This commonly takes place inside either convection or an infrared oven. At a certain temperature, the powder melts, flowing together into a continuous film. Chemical crosslinking soon follow before the coating hardens into a stable layer.
Zinc Plating and Powder Coating Compared
With the above overviews of the zinc plating and powder coating processes in mind, it’s time to take a closer look at the particular advantages and disadvantages of each method relative to one another.
Of the two methods, powder coating tends to be less expensive—at least in terms of material costs. Simply put, the organic materials used in powder coating don’t carry as high a price tag as the zinc used in plating. The process itself is also somewhat simpler, thus helping to keep equipment and maintenance costs down.
If durability and longevity of the finish are of primary importance, zinc plating proves to be the long-term cost-effective solution. This ultimately reduces future costs associated with either re-coating or outright component replacement.
Time of the Process
Zinc plating has a slight advantage over powder coating when it comes to processing time. Both techniques require similar time investments in terms of substrate preparation. Yet once the base metal is ready to go, the zinc plating process can proceed from start to finish in just a few minutes. After the component has been dipped in the zinc bath, it only needs to be rinsed and dried before it is ready to go.
Powder coating, by contrast, does require more of a time investment. Not only does the powder have to apply to the entire surface carefully, but the sheet metal must then be cured. Curing alone tends to require between 10 and 15 minutes. While this may not seem like a huge amount of time, it can significantly increase processing time over the course of a large batch of components.
Service Life and Durability
Neither powder coating nor zinc plating will prove ideal for every application. In protected environments, powder coating can provide protection nearly indefinitely. Yet if powder coating becomes cracked or otherwise damaged, corrosion will quickly set in. Even a small scratch will leave the underlying steel without any protection.
Zinc plating by contrast will continue to provide protection even if the surface becomes scratched or damaged. The nearby zinc will continue performing its role of cathodic protection, attracting corrosive agents and sparing the steel.
Does your application involve steel in high-temperature situations? In such cases, powder coating may deliver more reliable protection. To learn more about what it takes to design, manufacture, and protect high-quality sheet metal products, please contact the industry pros at Staub Manufacturing.