Barrier, Inhibitive and Sacrificial: Understanding the Three Types of Anti-Corrosion Coatings
You know how in the healthcare industry there are three ways you could combat a disease? In the first place, we have vaccines, which work by providing immunity to a certain disease and are the reason why polio is nowhere to be seen in most of the civilized world. In the second place, we have the typical medicine, which work by fighting a disease once it's set in. Cough syrups and chemotherapy are examples of this kind of medication. In the third place, we have painkillers, which doesn't actually fight the disease and serve only to lessen the pain associated with the condition while it's being treated with other medication.
This isn't just a simple explanation, these three categories actually mirrors the types of anti-corrosion coatings available in the market, barrier coatings, inhibitive coatings and sacrificial coatings. All three of them try to achieve the same goal of preventing or at the very least, minimizing the amount of corrosion suffered by the substrate but they all do this in different ways. Understanding the difference between these three is important because they each have their own characteristics and way of application.
Protection against corrosion
There are various industries out there that have to deal with corrosion on a daily basis. It’s no exaggeration to say that any industry that deals with buildings and industrial tools have to protect their properties and equipment against corrosion, either for safety or economical purposes. The collapse of the Genoa Bridge in Italy last year for example was partly blamed on corrosion and almost any kind of machinery out there has to be protected against corrosion in order to avoid early mechanical failure.
The risk of corrosion is even more prominent when working in a highly corrosive environment, typical of tropical regions where there's a lot of humidity in the air. With critical structures such as the Genoa Bridge, lives are literally on the line, but even with simple machinery, there might be a lot of money involved depending on the cost of the machine in question. Corrosion is a problem even if you're not involved in the building of critical structures, hence the need for anti-corrosion coating. While there are dozens of such coatings available in the market, all of them belong in one of these three categories, barrier coatings, inhibitive coatings and sacrificial coatings.
The use of barrier coatings
In layman's terms, barrier coatings are akin to body armor for the substrate as they work by preventing the chemical elements that cause corrosion such as water, oxygen and other chemical elements from coming into direct contact with the substrate. In reality, it's impossible for a coating to completely repel those elements, with water capable of seeping through the coating; the water would lose most of their ion concentration, rendering them unable to kickstart the corrosion process.
Barrier coating typically comes with a level of vibration resistance to ensure that the barrier could last through a certain period of time without being easily damaged and strong adhesion properties to ensure that they still stick to the substrate through that same period. The lifespan of a barrier coating heavily depends on the film thickness, which is why the use of gauges such as Elcometer 224 might be necessary to ensure that you're meeting the intended specifications. Most coatings usually exhibit some characteristics of barrier coatings but the use of a dedicated barrier layer is still necessary in most cases.
The use of inhibitive coatings
In a multi-layer coating system, inhibitive coatings are usually positioned as a primer coat, directly on top of the substrate but underneath the base layer of coating. Inhibitive coatings works by preventing the actual corrosion process even if all of the necessary chemical elements are present, hence the name, and they do this by messing with the electrolytes that kick off the corrosion process. However, inhibitive coatings aren't used anymore because the most prominent example of this type, red lead, is dangerously harmful when exposed to humans for an extended period of time.
The use of sacrificial coatings
As with inhibitive coatings, sacrificial coatings are also typically used in a multi-layer coating system as a primer coat, usually underneath a barrier coating. Sacrificial coatings takes their name from the fact that this type of coating works by redirecting the corrosion process towards them instead of the substrate, taking the bullet for the substrate for a lack of a better word. Sacrificial coatings are usually made up of metal that corrodes better than steel, which in practice is typically zinc mixed with other additives, and are applied directly to the substrate.