Question & Answer
by Cliff Cracauer, Technical Service Engineer
When you look at a new car and see its shiny appearance, what is the first thing that comes to mind? Probably that you would have bought the car in a different color. After all, appearance is everything, right? Well, not quite. While the paint that covers the car gives it a pleasing appearance, it also provides an important barrier between the metal framework of the car and the aggressive environment outside. This barrier keeps the car protected from rust, which in turn allows it to continue looking great for years.
The concept of coating a metal surface is not only for cars, but also on almost all types of metal surfaces. Coatings are used everywhere, and in many different forms. In this edition of Questions and Answers, I will delve into the many aspects of coating systems to help give everyone a better understanding of how and why they are used.
Q: What actually causes metal to rust?
A: Metal rusts, or corrodes, due to an oxidation reaction between the surface of the metal and the surrounding environment. The rate of this reaction depends on the temperature of the environment and the composition of the air surrounding the metal. For example, a hot, salty environment is very aggressive towards metal, while a cool, dry environment will not be very aggressive to metals. By coating the surface of the metal, a protective barrier is formed between metal and the environment.
Q: How does coating a metal surface prevent it from corroding?
A: When a metal surface is coated, moisture and other aggressive elements are kept away from the surface of the metal. This prevents oxidation reactions from occurring, thereby preventing corrosion.
Q: Do all coatings provide the same protection to the metal?
A: No. Some coatings are meant to provide long-term protection, some are used for short-term applications, and some are used simply for appearance. Factors that affect how long a specific coating will protect the metal from corrosion include the thickness, chemical composition of the coating.
Q: Do all metals need the same type of coating?
A: No, not all metals require the same type of coating system. Each metal has a distinct molecular structure, and some metals are more conducive to oxidation. For example, some steel alloys will rust in a matter of minutes if they are sprayed with salt water, while gold can sit at the bottom of the ocean for years and show no sign of corrosion. For this reason, different coating systems are used for different applications.
Q: What does a "coating system" consist of?
A: There are a number of different coating systems. There are one-, two- and three-part coating systems for starters. A three-part system is made up of a primer, intermediate (coat), and a topcoat; while a two-part system eliminates the intermediate coat.
Q: What is the purpose of each component in a coating system?
A: The purpose of each component may vary slightly depending on the coating system used and the specific application. Generally speaking, a primer is used to provide adhesion to the metal surface and also to form a good bond with the overcoat (either intermediate or topcoat). The intermediate coat is meant to provide thickness and strength to the coating. It also provides resistance to the atmosphere, and at the same time provides adhesion to the topcoat. The topcoat is used for a number of reasons. It provides the first line of resistance to the atmosphere, helps to give strength and toughness to the surface, and forms a bond to the undercoating. In addition, the topcoat is what provides the appearance.
Q: What type of coating systems do we use?
A: At Cortec, one- and two-component coating systems are used. In fact, because of VCI technology, one-component coating systems can provide long-term protection to most metal surfaces. When two-component systems are used, many times the sole purpose of the topcoat is to provide an aesthetically pleasing appearance.