CORROSION And PROTECTIVE COATINGS McNeil Coatings Consultants, Inc. 1132 Kay Drive Greenwood, IN 46142

Corrosion is defined as the deterioration of a material, usually a metal, because of a reaction with its environment.

A natural phenomenon that occurs over time. Corrosion is: A natural phenomenon that occurs over time. An electrochemical reaction (on metals) Happens at different rates with different metals and in different environments

Depending on the environment the rust may develop in minutes. If we expose iron or steel to air and water we can expect to see rust form in a short time, showing the familiar color of red-brown iron oxide. Depending on the environment the rust may develop in minutes.

With other metals such as copper, brass, zinc, aluminum, and stainless steel we can expect corrosion to take place, but it might take longer to develop.

One reason for the reduction of the corrosion rate with these metals is the potential formation of metallic oxides of copper, zinc, aluminum, and chromium.

Unfortunately ordinary iron or steel does not form this protective layer, so must be separated from the environment by some other means. Generally protective coatings are utilized for this purpose.

It is generally accepted that the usual cause of corrosion of metals involves electrochemistry. A flow of electricity occurs from one area of the metal to another area through an electrolyte, i.e. any solution capable of conducting electricity such as seawater, hard water, or other moisture on the substrate.

We refer to the corrosion process as an electrochemical reaction, sometimes referred to as a galvanic action.

For galvanic corrosion to occur we must have four things: Anode Cathode Metallic Pathway Electrolyte

The steel itself contains anodes and cathodes (areas on the surface that have different electrical potentials) and it is its own metallic pathway.

Therefore, all we need for corrosion cells to form is the electrolyte. The electrolyte is provided in the form of rain, dew, humidity, or chemicals.

Factors that affect the rate of corrosion are: Temperature Oxygen Humidity Chemical Salts Chemicals and airborne gases

Control of the Environment Dehumidification :There are several ways that we use to try to control corrosion: Design Inhibitors Material Selection Protective Coatings Cathodic Protection Control of the Environment Dehumidification

Design Generally speaking we want a design that: Avoids entrapment of water. Allows access for surface preparation and coating operations.

Inhibitors are materials that may be injected into the system Inhibitors are materials that may be injected into the system . They plate out on the surface and inhibit the formation of corrosion cells. They are commonly used in pipelines and other vessels that will contain materials that are corrosive.

They are sometimes injected into the water stream that may be used for the surface preparation of steel, as in the case of water jetting.

In the formulation of some primers inhibitive pigments are used In the formulation of some primers inhibitive pigments are used . These inhibitive pigments inhibit, or interfere, with the corrosion process. Examples of inhibitive pigments are red lead and barium meta borate.

Some metals, such as gold and platinum, corrode very slowly or not at all. Choosing a corrosion resistant material can reduce the rate of corrosion.

Material Selection - Galvanic Series [Seawater at 77⁰ F.] Magnesium Zinc Aluminum Mild Steel Cast Iron Copper Stainless Steel Gold Platinum

Combining dissimilar metals can result in corrosion Combining dissimilar metals can result in corrosion. It may be very rapid or it may be relatively slow, depending on the metals combined, the environment, and the ratio of one to the other.

We can also use this concept to protect a surface, such as when we hot dip galvanize steel. The zinc we apply to the steel is the more active metal and will sacrifice itself to protect the steel. When we combine zinc and steel the zinc becomes the anode and the steel becomes the cathode.

One of the most common ways of preventing corrosion to steel surfaces is to apply protective coatings. The coatings we apply may be inhibitive , barrier, or sacrificial.

The key to a successful coating project is surface preparation The key to a successful coating project is surface preparation. We want a surface that is clean and suitably roughened. Adhesion is the most important attribute of a coating. A clean surface with the proper profile provides a surface to which a coating can adhere.

A clean steel surface should be free of all: Oil Oxides Grease Corrosion Products Dust Other foreign matter Dirt Mill scale Rust Coating

Mill scale is the blue/black oxide layer that forms on hot rolled steel during the cooling process. Mill scale is cathodic to the steel. If mill scale is left on the surface, and painted over, the steel will rust and sacrifice itself trying to protect the mill scale.

So let’s look at the common coating systems that are used to protect and beautify steel and concrete structures.

First of all let’s look at what’s in the can. Coatings consist of pigment, vehicle, solvent, and additives. Pigment is a discrete particulate solid. Vehicle is the binder (resin) and other liquid components.

Most coatings are named after the binder (resin). Examples: Alkyds Epoxies Urethanes Acrylics Other coatings may be named after the pigment such as zinc rich primer or red lead primer.

There are hundreds of pigments available to the formulators There are hundreds of pigments available to the formulators. They are selected based on what we want the coating to do for us. Pigments come in three basic shapes. These are: Nodular Acicular Lamellar

Nodular pigments are lump shaped Nodular pigments are lump shaped. Examples are titanium and zinc phosphate. Acicular pigments are needle shaped. Examples are asbestine and magnesium silicate. Lamellar pigments are plate like. Examples are glass flake and MIO.

Nodular pigments generally add color. Most pigments are nodular. Acicular pigments give the coating mechanical or cohesive strength. Lamellar pigments improve the films impermeability to moisture.

Some general classifications of coatings are: Organic Inorganic Non-Convertible Convertible

Organic resins are made of something that was once alive such as vegetable oil or fish oil. A lot of our coatings are made from petroleum products. Most resins in use in today’s industrial coatings are synthetic or man made.

Non-convertible coatings are those coatings that dry and cure by solvent evaporation. When they are dry they are cured. Examples are: Vinyl Chlorinated Rubber Asphalt Bitumen

Convertible coatings are those that cure principally by some type of polymerization. The solvent must evaporate before the polymerization takes place. After polymerization it is a different chemical compound. Examples are: Epoxies 2-Pack Urethanes Polyesters Vinyl Esters

Coatings are generally applied as systems Coatings are generally applied as systems. These systems consist of one or more coating layers. Coating systems may be: Single layer Multiple layers of the same generic type. Multiple layers of different generic types.

A typical coating system consists of a primer, an intermediate coat, and one, or more, topcoats. An example of a coating system for a bridge might be: Zinc Rich Primer Epoxy Intermediate Coat Urethane Topcoat

There are three basic types of primers. They are: Sacrificial Inhibitive Barrier

Sacrificial primers protect the steel substrate by containing metallic pigments that are anodic to the steel. The most common sacrificial primer is a zinc rich primer. Zinc is a more active metal than steel and will become an anode when in direct contact with the less active steel substrate.

Inhibitive primers contain pigments that passivate the steel and therefore mitigate corrosion. Red lead primer is one of the best of the inhibitive primers. Unfortunately we do not see much red lead used any more because of health concerns.

There are many bridges out there that have red lead primer on them and they have been protected from corrosion for years. Pigments that have replaced red lead for inhibitive qualities are barium meta borate, iron oxide, and zinc oxide to name a few.

Barrier type primers create a barrier to the passage of moisture through the film to the substrate, thus preventing the electrolyte from getting to the steel. Without electrolyte we cannot have corrosion. Barrier type primers often contain lamellar type pigments.

Intermediate coats also serve as barriers in the coating system, as well as adding film thickness. Intermediate coats must be compatible with the primer as well as the topcoat.

Topcoats in the system must protect the substrate, and previous coats, from the environment in which the coating will be exposed. This may be simply atmospheric exposure, or a more severe environment such as immersion or chemical.

Protective coatings have been used to mitigate corrosion for years and new technology is making coatings better and better every year. This has been a very basic and simple explanation of how coatings help to protect steel and concrete from corrosion. The subject is much more complex that what I have presented here.

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