For centuries, metals have been used extensively in our everyday lives. From aluminum and copper in electrical wiring to gold and silver in jewelry, silverware and electronics, metals have been used for a variety of purposes. One of the most widely used metals today is iron, or more specifically, its more popular alloy form steel. Steel has become one of the most versatile products, used in appliances, automotive panels, signs, buildings and bridges. However, like other metals, steel is prone to corrosion.
Understanding the Corrosion Process in Metals
To understand how metallic coatings protect metals, it is important to know what corrosion is and how it is caused. Corrosion is a natural electrochemical reaction that converts a refined metal to a more chemically stable state. For corrosion to occur, three components must be present:
- An anode (in this case, the iron found in steel)
- A cathode (oxygen)
- An electrolyte solution (such as the moisture found in air)
During the corrosion process, the iron in the steel undergoes an oxidation reaction due to the presence of oxygen to form hydrated iron (III) oxide, commonly known as rust. Unlike mild corrosion in aluminum, which is beneficial because the oxide provides a protective layer that prevents further corrosion, rust is brittle and easily flakes away, exposing more of the metal to the atmosphere and resulting in further corrosion and degradation. This continuous corrosion eventually leads to the loss of material thickness, decreased material strength, perforation and reduced service life.
Two Common Types of Metallic Coatings
Metallic coatings can be applied to steel to provide a layer of protection, thereby allowing steel to be used in various challenging environments. Metallic coatings offer two main types of protection: barrier protection and, in specific cases, galvanic protection.
When a metallic coating such as zinc is applied to steel, it dries and hardens, forming an impervious barrier that prevents moisture intrusion. This removes one of the essential components needed for corrosion to occur. Without the electrolyte (moisture/water), oxidation cannot occur and therefore rust cannot form.
Another important aspect of barrier protection is called corrosion film protection. As mentioned previously, some metals, such as aluminum, react with oxygen to form a protective oxide film on its surface. This oxide film is resilient and firmly adheres to the surface of the aluminum to prevent moisture intrusion and further corrosion. This makes aluminum an ideal material for sheet metal. (Learn more in The Corrosion Properties of Aluminum and Its Alloys.)
Zinc, most commonly used to coat structural steel, reacts with the oxygen and moisture in the atmosphere to form corrosion products that create a defensive layer that protects the underlying steel. Freshly exposed zinc reacts with oxygen to form zinc oxide, and with water to form zinc hydroxide. When zinc hydroxide reacts with carbon dioxide in the atmosphere, the resulting product is zinc carbonate. These corrosion film products, like the film that forms on aluminum, are resistant to water intrusion and tightly adhere to the steel's surface so that it does not easily flake off like the corrosion formed on iron.
Zinc, however, is a reactive metal and will slowly corrode and erode over time. The degradation rate of zinc is still several times less than that of steel and will therefore significantly prolong the service life of the steel it is meant to protect.
The second method by which metallic coatings protect steel is to provide galvanic protection of the underlying steel by allowing the metallic coating to corrode preferentially to the steel, thus acting as a sacrificial coating. For example, if bare steel is exposed due to a cut edge, a scratch or severe coating damage, the surrounding coating will corrode first, therefore sacrificing itself before the steel begins to corrode.
Galvanic protection (also known as galvanic cathodic protection) occurs because zinc, by nature, is more electronegative than iron/steel in the galvanic series. (For background reading see the article An Introduction to the Galvanic Series: Galvanic Compatibility and Corrosion.) When the zinc coating is applied it acts as a sacrificial anode and the underlying steel becomes the cathode; therefore the zinc will always tend to corrode first. This galvanic corrosion will continue until the anode material (zinc coating) is fully consumed.
Galvanic protection is important because coatings, especially in construction, can become scratched and damaged. (Discover the benefits of this protection method in Galvanization and its Efficacy in Corrosion Prevention.) It is for this reason that zinc metallic coatings are preferred over aluminum when protecting steel members. While zinc and aluminum are close in the galvanic series, the protective oxide film formed by aluminum is so impenetrable that it isn't consumed by corrosion and therefore does not offer galvanic protection to damaged steel. Zinc, on the other hand, while an effective barrier is not as protective as aluminum, so it will corrode preferentially and protect damaged or exposed steel.
The combination of barrier and galvanic protection that metallic coatings provide offer a form of corrosion protection that is unparalleled in terms of application, cost and versatility. They are an effective way to protect other metals such as steel (which is technically a metal alloy) from degradation due to corrosion. This protection allows vulnerable metals to be used in a number of demanding applications that would otherwise be deemed unsuitable.