UPDATE (4/16/2021): SPC is no longer taking on new business for zinc plating. Please refer to our coatings page to learn about the other coatings we offer.
If you’re contemplating the best way to protect metal surfaces against the relentless forces of corrosion, a simple phrase to keep in mind is “think zinc.” When a zinc coating is electroplated onto the surface of ferrous (iron-containing) metals, it creates a formidable corrosion-resistant barrier — while also providing a number of other important benefits.
The remarkable effectiveness of zinc plating combined with its relatively low cost has made it a popular choice for protecting surfaces in all types of manufacturing processes. You’ll find zinc coatings on nuts, bolts, automotive parts and a wide range of other consumer products.
Zinc: What is Zinc Used For?
Zinc is a naturally occurring element that’s abundant in the Earth’s crust. If you think back to your high school chemistry days, you might remember that zinc (Zn) is one of the chemical elements listed on the periodic table of elements — #30 to be exact.
Long before zinc was officially discovered and isolated as a natural element by a German chemist named Andreas Marggraf in 1746, zinc ores were combined with copper to produce brass. The production of metallic zinc dates all the way back to the 13th century. In its metallic state, zinc is relatively hard and brittle. It’s also noted for its bluish-white color.
Zinc is now the fourth-most widely consumed metal in the entire world. Nearly one half of all zinc produced is used in zinc galvanizing processes to protect steel and iron from rusting. This involves coating the surface of a metal with a thin layer of zinc to create a corrosion-resistant barrier. Alloying zinc with copper to produce brass remains a widespread practice, representing the second-most common form of zinc usage.
If you work in industrial manufacturing, you may be familiar with the term “Commercial Zinc.” This refers to a zinc finishing specification that’s often used to coat metallic parts. It can provide basic protection against corrosion.
How Does Zinc Stop Corrosion?
Zinc has the inherent ability to form corrosion byproducts that can significantly reduce the corrosion rate of ferrous metals. These zinc corrosion products, which are commonly referred to as zinc patina, serve as a protective barrier on metal surfaces. They help keep out the moisture that can significantly speed up the corrosion process. Depending on environmental conditions, zinc can corrode at a rate of up to 100 times slower than other metals.
Zinc Coatings Form a Protective Barrier Against Moisture
You might also be familiar with the concept of oxidation, which is colloquially referred to as rust. It occurs when iron or steel comes in contact with moisture in the air. Iron and steel are particularly susceptible to oxidation. Think about what happens when you leave a steel or iron-based metal object outdoors without protecting it from the rain or snow: the formation of rust is inevitable.
Zinc coating serves as a metallic barrier that keeps moisture from reaching the surface of the coated object. This is not only important in outdoor settings, but many indoor industrial or manufacturing environments are also conducive to the formation of oxidation. Moisture can find its way in through ventilation ducts, and certain chemical-laden atmospheres found in some industrial settings can also promote corrosion on metal surfaces.
Zinc Serves as a Sacrificial Coating
Interestingly, another reason that zinc coating is so effective at stopping corrosion is due to zinc’s own corrosive properties. Through a process known as galvanic corrosion, zinc will “defer” to the metal that it’s protecting. Zinc is more electrochemically active than iron. And when zinc and steel come in contact with each other in an electrolytic solution — which is what occurs during the zinc plating process — the zinc serves as the anode for the steel. This prevents the formation of small anodic and cathodic regions on the surface of the metal, which will significantly reduce the development and spreading of corrosion.
Zinc also acts as a sacrificial coating that protects the steel via galvanization. Steel will not corrode as quickly when covered with a zinc coating, even when a scratch or cut exposes the steel to air or moisture. The zinc coating will always tarnish and corrode first. You could say that a zinc coating “takes one for the team.”
This differs significantly from coatings that consist of materials such as paint or aluminum where the corroding steel undercuts the coating over time and reduces its effectiveness.
The Zinc Plating Process
The zinc plating process can vary depending on the preferences of the metal finishing company, as well as the specific zinc plating application at hand. In general, the process begins by cleaning the surface of the metal with an alkaline detergent solution to remove oil, other materials or debris. The item is then treated with an acidic solution to remove rust or surface scales.
After cleaning, the actual zinc plating process can commence. The item is immersed in an electrolyte solution containing dissolved zinc. A DC electric current is then introduced into the solution, which deposits the zinc onto the cathode — the less chemically active item, or part being plated. This procedure is known as electrodeposition. The zinc serves the role of the anode, or the more active component.
One potential issue with plating zinc is that the metal is likely to come in contact with hydrogen molecules during the plating process. This can result in a condition known as hydrogen embrittlement, where trapped hydrogen causes the metal to weaken and can lead to cracking. Baking the object after plating can diffuse the hydrogen throughout the metal, helping to minimize the risk of embrittlement.
Rack and Barrel Zinc Plating
Plating methods for applying zinc coatings include the use of rack and barrel plating processes. With rack plating, the parts are affixed to a jig during plating to hold them in place and prevent damage. Barrel zinc plating involves placing the parts inside a specially designed vessel containing the plating bath. The barrel is rotated slowly during the plating process to coat the parts.
Rack zinc plating is normally the best choice for smaller, delicate parts that cannot withstand the tumbling caused by a rotating barrel. Barrel zinc plating, on the other hand, is typically more cost effective and less labor intensive. It’s often the better option for plating many parts at once.
Applying a Post-Plate Chromate Treatment
Another way to enhance the corrosion protection properties of zinc plating is through the application of a post-plate chromate treatment. A chromate is a salt that contains a combination of chromium and oxygen. Immersing zinc-plated parts in dichromate or chromic acid solution as part of the post-plating process can make them less susceptible to oxidation. It will also help provide the additional benefit of creating another corrosion-resistant barrier against moisture.
Using Post-Treatment Sealers
Another widely used post-zinc plating process is the application of post-treatment sealers, which takes place after chromate treatment. Sealers can be used on yellow, black, black oxide (olive drab) or clear (blue) zinc coatings. A sealer works by causing the chromate film to harden, while also promoting adhesion to the zinc coating. When used in tandem with a chromate treatment, sealers can also increase corrosion protection by as much as 100 percent.
Testing the Corrosion Resistance of Zinc Coatings
Some metal finishing shops have the capability to test the corrosion resistance of a zinc-coated surface after plating. This is typically performed by a process known as salt spray testing. With salt spray testing, the relative protective value of a finish can be determined fairly easily. The results are measured in terms of the number of hours it takes for the formation of white rust to occur after spraying.
Types of Zinc Plating
The various types of zinc plating can be identified by the color produced by the different formulations. Each color offers different levels of corrosion protection:
- Yellow Zinc — This type of zinc coating is commonly used in the automotive industry and offers an average level of protection against corrosion.
- Black Zinc — Black zinc coatings are available as a black oxide that can actually be olive-drab in color, or as a pure black coating. The former will normally provide slightly better corrosion resistance.
- Clear or Blue Zinc — This coating formulation tends to offer fewer amounts of corrosion protection, although it’s regarded as being more environmentally friendly.
- Acid Zinc — Acid zinc plating differs from other types of zinc plating, as it does not introduce cyanide into the plating bath. A key advantage of acid zinc is that it can be effective for use on hard-to-plate substrates.
Classifying the Types of Zinc Plating Finishes
The American Society for Testing Methods has instituted a classification system based on the level of corrosion protection a zinc coating will provide. This can be helpful when determining the most appropriate coating type. There are four different levels, ranging from coatings that are ideal for mild indoor applications to coatings that are more suited for extreme outdoor environments. In general, the greater the thickness of the coating, the more protection it will offer.
Plating With Zinc Alloys
When it comes to maximizing corrosion protection, plating zinc alloys is typically more effective than plating zinc by itself. Perhaps the most widely used alloy in zinc plating is zinc-cobalt, which can increase corrosion resistance by as much as five times. A zinc-cobalt alloy is also extremely ductile — it won’t lose its toughness even when deformed or bent into different shapes.
Other commonly used zinc alloys include zinc-iron, tin-zinc and zinc-nickel. Regardless of which alloy is used, it will provide significantly better corrosion protection than when plating with zinc alone.
Other Important Zinc Plating Benefits
When considering zinc plating, corrosion protection isn’t the only potential advantage. Other important benefits include:
- Low cost — Zinc is much more abundant and affordable than many other metals used for plating. When it comes to economical plating, zinc makes for an excellent alternative.
- Bright finish — Plating is sometimes used to enhance the appearance of a metal surface, often to make the object more attractive to prospective buyers. Zinc plating can provide a level of brightness that is the equivalent of chromium. Brightness levels can be controlled with the use of various additives. The wide range of available colors also offers a great deal of flexibility for choosing the most appropriate finish.
- Various surface appearances — Zinc coatings can be produced in a wide range of textures and patterns. A smooth, plain surface or a rough texture with a spangled pattern can both be achieved with ease.
- Easy application — Zinc coatings are easy to apply and the zinc plating process results in a low-stress deposit that won’t damage the substrate. In addition, zinc can be used with a wide range of plating bath chemistries.
- Compatible with different plating processes — Zinc can be used with both rack and barrel plating processes. This means zinc plating can accommodate parts of various sizes and workload volumes.
- Works well as an undercoat — If your applications involve painting, zinc can be an extremely effective undercoat due to its excellent adhesive properties. Of course, it will also enhance the corrosion resistance of the painted or coated surfaces.
As with any metal finishing process, zinc plating does have certain limitations. For instance, zinc typically does not perform well at high temperatures. Its corrosion protection capabilities begin to decrease significantly when temperatures reach and exceed 212° F. Unless the parts are properly baked after plating, embrittlement can occur when the temperature exceeds 500° F.
It can also be a challenge to distribute a zinc coating evenly over a metal surface. Objects that are confined in enclosed areas that are prone to moisture may promote rapid corrosion of zinc-plated surfaces. Zinc doesn’t hold up well against alkalis or acids.
A wide range of factors can impact the lifespan of a zinc coating, including the composition of the atmosphere and the type and thickness of the coating.
Sharretts Plating Company Offers a Wide Range of Zinc Plating Services
SPC is a trusted source for a variety of cost-effective zinc plating services used by many different companies and industries. Many of our customers choose zinc plating as an affordable alternative to plating with precious metals such as gold or platinum.
We offer both rack and barrel zinc plating processes that provide numerous flexible options for our customer base. In addition to zinc, we also provide zinc-nickel plating solutions for enhanced protection against corrosion.
Our zinc plating services are backed by nearly 90 years of experience in providing customized metal finishing solutions. We can even provide effective zinc plating options for those hard-to-plate substrates that are beyond the capabilities of many of our competitors. It’s all part of our ongoing quest to achieve total satisfaction for every customer we serve.
Don’t let corrosion take its toll on your metal parts and components. Contact SPC today to learn more about the many benefits our zinc plating services can bring to your business. We’ll also be happy to schedule a no-obligation plating consultation at your convenience and provide a free quote for our services.