With all the talking we do about corrosion protection, you may be tempted to think there are a million ways of doing it. But really, protective coatings achieve corrosion protection in one of only three ways.
Essentially, they either block the necessary elements from coming together to start the corrosion process, actively prevent the electrochemical reaction from occurring, or they steer the corrosion process in a direction that is ultimately not harmful to the asset.
Here’s how each of those three strategies work:
Barrier coatings
Barrier coatings aim to prohibit water, oxygen and other chemicals from making contact with the substrate. In reality, it’s taken for granted that some water and oxygen will reach the surface that barrier coatings protect. But since the water that does make it through a barrier coating isn’t significantly charged (meaning there is not a heavy concentration of ions in the water), the fundamental elements necessary for kicking off the corrosion process are not all present.
According to NACE, it is important that a barrier coating exhibit the following characteristics:
- Chemical resistance
- Vibration resistance
- Good wetting properties to ensure an even application
- Strong adhesion, even in the presence of moisture
Most coatings exhibit at least some of the properties of a barrier coating. Depending on the circumstances, some of the types of corrosion protection below may be combined with a product specifically designed with barrier qualities in mind, in order to achieve maximum protection.
Thermal barrier coatings are one of the most commonly used examples of this type of corrosion protection. These are used to protect against moisture on substrates that typically reach very high temperatures. Since a gap is often created between a layer of insulation and the substrate, any moisture that reaches the surface would be likely to begin the corrosion process in the absence of a barrier coating. Known as corrosion under insulation (CUI), this is a common problem thermal barrier coatings are called on to address.
Inhibitive coatings
Found in the primer portion of a coating system, inhibitive coatings are made up of pigments that actively impede chemical reactions. These coatings were designed to keep corrosion from ever starting. They do this by interfering with the electrolytes required to start the corrosion process.
Red lead is a well-known example of an inhibitive coating, which is why it was used so widely before the harmful effects of exposure to lead were fully understood. Since the lead pigments began to be closely regulated in the late 1970s, this strategy of corrosion resistance has been heavily regulated to the point of no longer being widely used in the United States.
Sacrificial coatings
Sacrificial coatings are a selfless breed of coating made up of a metal, usually zinc, which corrodes preferentially to steel. Essentially, this method hijacks the corrosion process and steers it in a direction that won’t be harmful to the asset the coating is meant to protect. To be effective, sacrificial coatings (also sometimes called “cathodic protection”) must be applied directly to a ferrous metal, such as steel.
Since sacrificial coatings like inorganic and organic zinc primers are meant to be applied directly to the substrate, they’re often combined with topcoats that exhibit other barrier properties for combined corrosion protection.
Zinc primers are great examples of cathodic protection for a substrate. Not only does zinc corrode preferentially to steel, but it generally corrodes more slowly than other sacrificial coatings, allowing for longer periods between coating applications.
Let’s talk
Which mode of corrosion protection is right for your project? That will depend on a number of factors. If you’re ready to discuss your project with experienced coatings professionals, get in touch with us today.
