Corrosion of metals is a chemical and electrochemical phenomenon in which metals are converted into more chemically stable forms such as oxides, hydroxides, or sulfides.
This phenomenon leads to the destruction of man-made structures and causes extensive damage. Corrosion of metals can occur uniformly or locally and is usually caused by oxygen in the air or electrolytic solutions.
Corrosion of metals can have devastating consequences for industries, the environment, and humans. There are various methods to prevent corrosion, which can be mechanical, chemical, or electrical. In this article, we will specifically address the topic of anti-corrosion coatings.
Surface coatings
In many cases, the coating on a metal surface is the metal’s first line of defense against the corrosion process. Coatings can prevent metal from corroding through a variety of mechanisms.
Contact Armanifam to receive expert advice on stainless steel coatings.
The three main mechanisms in anti-corrosion coatings to prevent this process are:
Barrier coatings
The yellow color of the fuselage of American bombers in 1942, painted with a coating containing zinc chromate.
This type of coating uses a variety of pigments that, when exposed to oxygen in the air, form a resistant coating on the steel surface. As a result, these types of coatings are also called reactive coatings. When the pigment is exposed to oxygen in the air, it reacts with it and forms a penetration-resistant layer that covers the steel surface.
As a result, this type of anti-rust coating must be applied immediately to the steel surface. A wide range of pigments are used in this category of coatings. Pigments such as red lead, zinc chromate, iron oxide, calcium borosilicate and some molybdenum compounds are among the most important pigments with inhibitory properties. One of the most important advantages of this type of coating is its lower price compared to other anti-rust coating methods. Also, one of the most important disadvantages of this type of anti-rust coating is its inefficiency in submerged conditions such as the area below the waterline of ships.
Sacrificial coatings
A surface coating will inevitably chip and scratch over time. As a result, the inhibitory coating mechanism will no longer be effective in this situation. Another solution to this problem was to use pigments that can provide a type of cathodic protection for the metal. In galvanic corrosion, the more active metal corrodes faster and more. As a result, a more active metal can be protected by a more neutral metal in the galvanic series.
This principle is also used in anti-rust coatings, such as coatings containing zinc metal powder to protect steel structures. In these types of coatings, as long as the surface of the coating is not scratched or stained, the protective mechanism is to create a resistant barrier. However, as soon as a surface scratch occurs, the inhibitory substances in the paint come into action and create an electrochemical cell in that area, in which the metal surface becomes the cathode and the inhibitory substance becomes the anode, thus protecting the metal body.
Zinc-rich coatings, which use zinc metal powder, are widely used as anti-rust primers in severe corrosive conditions. In these types of coatings, zinc constitutes a high percentage of the total weight of the dried coating (even up to about 95% by weight of the final coating), so that in the final coating, the zinc metal particles are in direct contact with each other. Another feature of this type of coating is its high density. For example, the zinc-rich epoxy zinc coating has a density of more than 2.5 g/cc.
The magnitude of this number is determined when compared to the density of pure water, which is 1 g/cc.
Two resin bases, epoxy and ethyl silicate, are commonly used for zinc-rich paints. The epoxy type is known as organic sacrificial coatings, and the ethyl silicate type is known as inorganic sacrificial coatings. An important advantage of inorganic zinc-rich paints is their high resistance to heat.
For more information about Kansai heat-resistant coatings, visit this page.
Pure zinc metal is used in powder form in sacrificial coatings
These types of anti-rust coatings also have disadvantages, the most important of which are:
Relatively higher price compared to other anti-rust paints
- Due to the high percentage of metal, the adhesion to the surface and the continuity of the final coating are weaker compared to other coatings.
- Barrier cover
In this type of coating, the penetration of moisture and oxygen into the coating is prevented by using a resistant resin base such as epoxy and sheet-shaped pigments. The sheet-shaped shape of the coating makes the path of moisture penetration into the coating longer and it takes a long time for the moisture to reach the metal surface. The most important pigments with this property are: glass flake, aluminum flake, steel flake[10] and MIO.
This type of anti-rust coating is highly effective in the marine industry, especially in areas submerged in seawater. Usually, the parts below the waterline and the deck of the vessels are painted with barrier primers. These coatings can be used with a cathodic protection system under current or sacrificial anodes. Another feature of this type of coating is its very strong adhesion to the metal surface, and for the best adhesion of the paint to the metal, the surface must be clean and properly prepared.
Barrier coatings, while being effective and having unique properties, also have disadvantages, including:
The barrier property of the coating can be problematic during paint application and prevent the solvent from escaping from the coating, which is a factor in initiating many coating problems.
It is not possible to use coatings containing aluminum flake in acidic environments due to the reaction of aluminum with acid and the release of hydrogen.
Due to the difference in zinc and aluminum in the galvanic series, zinc-rich coatings and barrier coatings containing aluminum flake should not be used simultaneously.
For specialized advice and purchase of marine, coastal, offshore, industrial, and construction paints from Kansai, contact Armanifam.
