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An Intro to the Duplex System for Corrosion Protection

By Corrosionpedia Staff
Published: January 23, 2020
Key Takeaways

When it comes to increasing a substrate’s resistance to corrosion, two systems are better than one.

Protecting metals from the damaging effects of corrosion is usually achieved with the application of powder coating, paint or hot-dip galvanizing. However, due to the advancements in technology and corrosion protection systems, these two methods are increasingly being combined into what's called a duplex system. Here we'll take a look at duplex protection, how it works and when it doesn't work.


What Is a Duplex System?

A duplex system is a type of corrosion protection in which a powder coating or paint is applied to steel that has undergone hot-dip galvanizing. When these two methods are used, the level of corrosion protection is far better than using each system independently.

Powder coating or painting over hot-dip galvanized steel requires thorough preparation and clear knowledge of the two systems. For decades, a great number of products have used the duplex system very efficiently, including radio towers and automobiles. The duplex system can be very successful when the powder coating or paint achieves excellent bonding and there is correct substrate preparation of the galvanized surface. (Related reading: The 5 Advantages of Powder Coatings.)


The combined effect of two types of coatings can help extend the life span of steel. When a coating or paint is applied on top of a galvanized coating, the two systems become one called a duplex coating. This system offers a highly sophisticated protection against corrosion. Steels that are only coated or painted are highly prone to corrosion damage like rusting and peeling.

How Does Duplex Protection Work?

Prior to making a decision about how steel will be protected against corrosion, it's vital to learn the process of steel corrosion. Basically, rust is produced as a byproduct of iron. The formation of rust takes place when there is a dissimilarity in the electrical potential on the steel surfaces that involve cathodes, anodes and the medium that conducts ions, called the electrolyte. When electrolytes like water comes in contact with the surface of steel, this connects cathodes and anodes together, resulting in a corrosion cell. This generates flaky and loose iron oxide called rust.

To achieve steel protection against corrosion, something must impede the formation of a corrosion cell. This can be achieved through a certain type of electrolyte blockage. The two most typical methods used to prevent the formation of a corrosion cell are:


Hot-dip galvanizing can be used to achieve the two kinds of protection above. Then, a powder coating or painting on top of a structure that has been through hot-dip galvanizing offers an extra layer of barrier protection over the zinc coating.

Zinc is more anodic than iron. Thus, when the steel and zinc come into contact with electrolytes, zinc becomes the anode within the corrosion cell. Then, the zinc is consumed slowly while the steel becomes the cathode and is protected. Due to the galvanized coating and cathodic protection, the structure becomes capable of resisting corrosion as well as other forms of abrasion such as cut edges, scratches and drill holes. (Learn more in Galvanization and its Efficacy in Corrosion Prevention.)


The Advantages of Duplex Systems

Apart from the synergistic effect mentioned above, there are many other reasons why a duplex system is advantageous and different from other types of protection systems. Here are some additional benefits of duplex protection:

On its own, steel that has been hot-dip galvanized is already attractive. However, when the galvanizing finish does not fit a certain requirement, powder coating or paint can provide a more aesthetic substitute. These are perfect in projects that require more vibrant shades and perfect blending with the environment. Plus, coating or painting galvanized steel can result in superb protection against corrosion. (Be sure to download our Guide to Galvanized Steel.) The use of a duplex system has become a popular trend as a top coat in structures like stadiums.

Extra Corrosion Protection
Due to the synergy between galvanizing and powder coating or paint, there will be an extended corrosion protection, which is one of the most important reasons for using duplex systems. This system can be extremely useful in harsh settings, where a single system may fail to provide adequate protection.

Less Maintenance

Because a duplex system can extend the service life of its protected structures, there will be a greater chance that the steel will outlast its expected life span. This also means less maintenance, making this system a more viable option.

Safety and Color Coding
A duplex system can also be used to meet safety regulations. This system can enhance safety in various environments through the determination of dangerous walkways and work areas, color coding of chemical pipes, gas and steam, as well as creating marks on electrical lines and high-voltage equipment. For instance, using a duplex system on high aviation structures with alternating orange and white patterns guarantees that there is correct color coding without putting corrosion protection at risk.

Easy Repainting
As mentioned earlier, one great thing about painting on top of galvanizing is the maintenance cycle extension. Apart from this, duplex systems can also make repainting more convenient. When the paint wears out, the zinc present in the galvanized paint or coating acts as a barrier and provides cathodic protection until repainting is finished. The exposed zinc can be repainted with minimal surface preparation.

The Drawbacks of Duplex Protection Systems

Similar to other types of corrosion protection systems, duplex also has a few disadvantages. One is that meticulous surface preparation and a thorough understanding of the characteristics of galvanized coatings is necessary. There are actually three kinds of galvanized steel:

  • Newly galvanized
  • Partially weathered
  • Fully weathered

Each kind possesses unique properties at every weathering stage. The rule is that the material must not be chromated or water quenched regardless of the galvanizing type. Also, standard procedures and precautions should be observed to achieve correct galvanizing of steel for coating or painting.

Apart from the careful surface preparation required for this system to work correctly, the paint should be matched with the coating being galvanized to produce a highly efficient duplex system. A lot of paint systems and paint have worked well with materials such as galvanized steel. There are also certain types of paint that will not bond to galvanized steel. Thus, a paint that is totally compatible with a zinc substrate is highly recommended.

The Bottom Line on Duplex Protection

Using a duplex protection system can drastically improve the life span of various materials and structures. The powder coating or paint provides the first level of defense, while the hot-dip galvanized steel beneath will generate a zinc patina, which acts as a barrier and provides cathodic protection, keeping structures protected from corrosion for decades. However, in order to ensure the success of a duplex system, logical and simple preparation steps should be observed.

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Corrosionpedia Staff

Corrosionpedia aims to provide the first steps in the research journey for asset integrity professionals the world over.

Working with our team of internal writers, contractors and third-party experts, we source world leading educational content on the subject of preserving the long-term integrity of the world's infrastructure and assets. We designed our proprietary platform to fit the needs of the industry, and build the in-demand tools to help connect industry professionals to the solutions and solutions providers they need.

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