What Is Corrosion?

Corrosion is a deterioration of a material caused by environmental interactions. It is a natural phenomenon, requiring three conditions: moisture, a metallic surface, and an oxidizing agent known as an electron acceptor. The process of corrosion converts the reactive metal surface into a more stable form, namely its oxide, hydroxide, or sulfide. A common form of corrosion is rust.

Corrosion can have a variety of negative effects on metal. When metal structures suffer from corrosion, they become unsafe which can lead to accidents, such as collapses. Even minor corrosion requires repairs and maintenance. In fact, the annual direct cost of metallic corrosion is approximately $2.2 Trillion USD worldwide!

While all metals corrode, it is estimated that 25-30% of corrosion could be prevented using suitable protection methods.

How to Prevent Corrosion

You can prevent corrosion by selecting the right:

Metal Type

Protective Coating

Environmental Measures

Sacrificial Coatings

Corrosion Inhibitors

Design Modification

Metal Type

One simple way to prevent corrosion is to use a corrosion resistant metal such as aluminum or stainless steel. Depending on the application, these metals can be used to reduce the need for additional corrosion protection.

Protective Coatings

The application of a paint coating is a cost-effective way of preventing corrosion. Paint coatings act as a barrier to prevent the transfer of electrochemical charge from the corrosive solution to the metal underneath.

Another possibility is applying a powder coating. In this process, a dry powder is applied to the clean metal surface. The metal is then heated which fuses the powder into a smooth unbroken film. A number of different powder compositions can be used, including acrylic, polyester, epoxy, nylon, and urethane.

Environmental Measures

Corrosion is caused by a chemical reaction between the metal and gases in the surrounding environment. By taking measures to control the environment, these unwanted reactions can be minimized. This can be as simple as reducing exposure to rain or seawater, or more complex measures, such as controlling the amounts of sulfur, chlorine, or oxygen in the surrounding environment. An example of this would be would be treating the water in water boilers with softeners to adjust hardness, alkalinity, or oxygen content.

Sacrificial Coatings

Sacrificial coating involves coating the metal with an additional metal type that is more likely to oxidize; hence the term “sacrificial coating.”

There are two main techniques for achieving sacrificial coating: cathodic protection and anodic protection.

Cathodic Protection
The most common example of cathodic protection is the coating of iron alloy steel with zinc, a process known as galvanizing. Zinc is a more active metal than steel, and when it starts to corrode it oxides which inhibits the corrosion of the steel. This method is known as cathodic protection because it works by making the steel the cathode of an electrochemical cell. Cathodic protection is used for steel pipelines carrying water or fuel, water heater tanks, ship hulls, and offshore oil platforms.

Anodic Protection
Anodic protection involves coating the iron alloy steel with a less active metal, such as tin. Tin will not corrode, so the steel will be protected as long as the tin coating is in place. This method is known as anodic protection because it makes the steel the anode of an electrochemical cell.

Anodic protection is often applied to carbon steel storage tanks used to store sulfuric acid and 50% caustic soda. In these environments cathodic protection is not suitable due to extremely high current requirements.

Corrosion Inhibitors

Corrosion inhibitors are chemicals that react with the surface of the metal or the surrounding gases to suppress the electrochemical reactions leading to corrosion. They work by being applied to the surface of a metal where they form a protective film. Inhibitors can be applied as a solution or as a protective coating using dispersion techniques. Corrosion inhibitors are commonly applied via a process known as passivation.

Passivation
In passivation, a light coat of a protective material, such as metal oxide, creates a protective layer over the metal which acts as a barrier against corrosion. The formation of this layer is affected by environmental pH, temperature, and surrounding chemical composition. A notable example of passivation is the Statue of Liberty, where a blue-green patina has formed which actually protects the copper underneath. Corrosion inhibitors are used in petroleum refining, chemical production, and water treatment works.

Design Modification

Design modifications can help reduce corrosion and improve the durability of any existing protective anti-corrosive coatings. Ideally, designs should avoid trapping dust and water, encourage movement of air, and avoid open crevices. Ensuring the metal is accessible for regular maintenance will also increase longevity.

《》Turn to non-corrosive metals such as aluminum and stainless steel.
•Keep the area around the metal surface dry.
•Use drying agents and moisture barrier products......