According to the most recent estimates available, corrosion carries a $276 billion cost for the U.S. economy. This cost includes critical failures, property losses, repair and restoration costs, downtime losses, and even loss of life. The loss estimates are indicative of losses suffered worldwide, particularly by advanced economies.
The good news: A big chunk of these losses can be prevented by planning and implementing proactive and systematic corrosion prevention practices and programs. Because the process of corrosion begins due to the electrochemical reaction of a surface with corrosive environments, one of the most effective strategies is to effectively isolate a substrate's surfaces from electrochemical (corrosive) attack by covering it with a protective barrier or coating. Different types of coatings are used to create and maintain this type of protective barrier on the surface, and a large number of coatings are continuously being developed and offered in the market. In this article, we'll look at some of the most widely used coatings, the criteria used to select a particular coating, and the surface preparation required for the application of the coatings.
What is an Anti-Corrosion Coating?Anti-corrosion coatings are primarily applied for the protection of solid metal surfaces and, occasionally, some non-metals. When selecting a suitable corrosion protection system, we must consider the nature of the corrosive environment faced by the assets to be protected, such as pipelines, structures, vehicles, ships, bridges, plant equipment, and buildings. That environment could be urban or rural, buried under soil, underground tunnels; and open atmospheres, marine, mining, or industrial—all are subject to corrosion. However, coatings must be selected to be compatible with the substrate material, its composition and the conditions the material will face in the field. So let's first examine the major factors that corrosion engineers must consider.
Service ExposureIn order to select a suitable corrosion protection system, corrosion engineers must consider the impact of environmental factors (service exposures) such as:
- Soil chemistry (the presence of reactive salts, acids, and alkalis), electrical resistance, and moisture
- Corrosive airborne contaminants (indoor or outdoor)
- Temperature range and peak levels, fog, storm and snowfall, radiation (UV exposure)
- Humidity, condensation, splash exposure
- Exposure to salt water or natural water in case of partial, complete, or fluctuating immersion
- The reactivity of the type of product to be transported, processed or handled, and the hazard associated with leaks
- Proximity to any electrochemical activity such as cathodic or anodic protection, high-voltage lines, grounding stations, or a railway system causing flow of unintended induced stray currents, and availability of mitigation techniques
- Exposure to UV radiation, sunlight, and chemicals such as solvents
- Expected lifespan of operation of the project, if available
- Likely exposures to reactive acids and alkaline substances, smog from industry, acid rain, and sewage
- Site considerations and limitations applicable to surface preparation, priming, coating, and curing process requirements
MaterialsCathodic protection and surface coatings techniques are concurrently used in most critical protective applications. The main function of the surface coatings is to act as an insulating barrier that eliminates the likelihood of contact between a substrate and electrolytes in the working environment, thus preventing the likely electrochemical reaction that can cause corrosion.
Thin coatings made up of metals and certain nonmetallic substances can form an effective barrier between the solid substrate to be protected and its working environment. In some cases, a metallic coating may work as a sacrificial coating, as in the case of zinc coatings.
Metallic CoatingsMetal coatings have the advantage of an inherent formability and provide a durable barrier against any attack of corrosive substances. However, any surface defect such as porosity can lead to localized corrosive failure. Different metallic coatings can be used for various substrates to prevent corrosion. Sometimes they are used for aesthetic as well as decorative purposes. Metal coatings are applied using the following techniques:
- Hot dipping
- Electrodeposition or electroplating
- Thermal spray
- Mechanical plating
Zinc coating by electrodeposition (as per ASTM A591) provides a coating that's less than 0.2 mil thick and is suitable for indoor appliance panels (limited to light duty indoor usage). Procedure ASTM B 633 covers electroplating of zinc on steel for similar applications with a coat thickness of less than 1.0 mil.
The hot dip process of zinc coating on steel is covered by ASTM A653, ASTM A123, ASTM A153, and provides a coat thickness up to 5.0 mi. It's used for outdoor and indoor structural items, roofing, auto body parts, and nut bolts.
Peening or mechanical plating (ASTM B695) provides a coating thickness of 0.2 to 4 mil, which is useful for hardware items used indoor or outdoor.
Diffusion coating involves alloy formation by the diffusion of an alloying element on the substrate. For this reason, the process is also called surface alloying. Solid parts to be coated are packed in a chamber and exposed to the vapor of the alloying elements. Examples are the deposition of zinc, chromium, and aluminum on steel.
This process ensures high strength with good corrosion resistance to steel, and is used for gas pipes handling gases containing corrosive sulfur compounds. Aluminum-coated steels are used in petroleum and chemical industries that handle corrosive acids, while glass-lined steel is used for corrosive processes in the food industry, breweries, and the drug industry.
In the electroplating process, the coating layer is formed on the substrate by interposing an electrical voltage between the substrate metal (as the cathode) and an anode of the substance to be deposited, immersed in an electrolyte. The electrolyte is normally an aqueous solution consisting of a salt of the metal to be formed on the substrate, and other additional chemicals to facilitate the electrochemical process.
While chrome plating and nickel plating processes provide the necessary corrosion resistance to the parent metal, the resulting residual tensile stresses can cause a loss of fatigue strength, which needs to be kept in mind for design calculations. This loss of fatigue strength can be minimized by nitriding the steel components prior to the plating process.
Chrome-plated steel is extensively used in process industries. It is applied for a very wide range of geometries ranging from superfine bores and cylindrical rollers, to holes and close-tolerance curvatures. It provides ultra-high wear resistance compared to the best heat-treated metals. It has a bright, decorative finish, responds to polishing, and is ideally suited for the textile and paper industry. Chrome-plated steel is suitable for the exposure of organic acids and gases (with the exception of chlorine), hot oxidizing or reducing environments, and in processes that produce and handle brine, gas fuel, molten glass, oils, and liquid fuels.
In electroless plating, the coat formation occurs due to a pure chemical reaction, aided by a catalyst without the application of voltage. Nickel plating or cobalt plating is performed by this method as well.
In this process, a superfine metal powder is cold-welded onto the metallic substrate through tumbling the component part, the powder solution (aqueous), and a medium (glass beads, for example) with chemical agents to facilitate the process. This process is employed to form a zinc or cadmium coating on fasteners and other auto components.
In a hot dipping process, a thin coating of metal is formed on a component surface by dipping the component in a hot, molten bath of the coat substance. This is a popular manufacturing process for corrosion-resistant underbody sheet metal structural parts.
The steel plants supply hot rolled as well as the cold rolled sheets with necessary anti-corrosion coating such as zinc, zinc-nickel, aluminum-zinc, lead-tin, or tin. Hot dipping and electroplating are the common facilities used. Many steel plants also provide customized coatings.
Thermal spray facilitates the development of a huge variety of coatings for different severe corrosion and wear-resistant applications. Some examples of thermal spray coatings are found in components of jet engines, orthopedic implants, surgical applications, and electrical-electronic devices.
The coat thickness here will be around 0.003 to 0.010 inch, which is advantageous for severe chemical corrosion conditions. This type of coating also tends to provide a surface that's free of voids and pin holes. After the coat is completed, surface finishing may be needed for a dynamic application. However, coverage is limited to simple geometries and outer surfaces only. This method is suitable for ferrous metals used in hostile chemical environments with oxidation and extreme temperatures.
Non-Metallic CoatingsMost non-metallic coatings provide protection by making an insulating barrier that is completely impermeable to the moisture and electrolytes that are necessary for the process of corrosion. These liquid, non-metallic coatings are made up of a solvent, a resin, and pigment. Each of these has a role to play in its effectiveness.
The solvent dissolves and disperses the resin material, which is the backbone of the coating. It helps provide ease of application, adequate adhesion, and overall effectiveness.
The resin of the coating is the basic film-forming component and provides corrosion protection and other vital properties. Thus, coatings are generally known by the name of the resins used.
The pigment constitutes the second solid part of the coating. The main function of this component is to provide opacity to prevent the organic substance from being exposed to sunlight. Titanium is frequently used for opacity. Pigments can also improve adhesion, color, weather resistance, or reduce moisture ingress.
The resin with pigment remains on the surface after the solvent has volatilized, and hence determines the film thickness of the coat.
Classification of CoatingsBased on the mechanism of the protection, non-metal coatings are classified as:
- Barrier coatings
- Inhibitor coatings
- Galvanic coatings
These form an insulating and physical barrier, thus stopping the contact of corrosive elements, such as the electrolyte, with the substrate. Coal tar epoxy coatings are an example of this mechanism.
Inhibitor coatings cause a chemical to be released by the pigment. This interferes with the electrolyte and stops electrochemical reactions. Chromate-type pigments are an example of this mechanism.
Galvanic coatings are primer coatings rich in zinc content, which provide a sacrificial type of coat and the cathodic type of protection to a ferrous substrate. These primers must be directly coated on the substrate.
Generic Polymer Coatings
Some of the generic polymer coatings widely used in corrosion protection include:
- Acrylic Coating: Acrylic coatings are water-based, eco-friendly, and suitable for outdoor use with significant exposure to sunlight. They can be used as a primer or a top coat, and are suitable for moderate to high temperatures.
- Alkyd Coatings: These are based on modified natural oil, and are used as primer for the atmospheric environment. They're not suited for an alkaline environment such as concrete.
- Bitumen-Based Coating: Bitumen coatings possess very good moisture resistance but are not resistant to solvents. These are used for the protection of aluminum and ferrous surfaces.
- Amine Epoxy: This has good resistance to acids, solvents, and alkalis, but is sensitive to temperature, moisture, and sunlight, and is therefore used underground.
- Polyamide Epoxy: Has resistance to salt solution and water, but is not suitable for other chemicals. It is suitable for immersion and buried service.
- Epoxy Coal Tars and Urethanes: These are widely used for large-diameter pipe-coating systems in the petroleum industry.
- Multi-Layer Epoxy or Extruded Polyolefin Systems: These multi-layer systems provide strong adhesion under polyethylene. With the addition of an epoxy primer, these multi-layer systems are the most popular in the U.S. and Europe for pipelines for outdoor and buried applications.
- Mill-Applied Taping System: Mill-applied tape systems are provided with a primer, an inner layer of tape, and some outer layers for abrasion protection. Concern about a disbonded coating due to cathodic protection has aided in the development of fused layers of tapes. The availability, ease of on-site construction, and low cost make this tape system very popular.
- Coal Tar Epoxy: This has moisture resistance as well as chemical resistance, but it may become brittle with age. It is used for immersion service and is also suitable for tank lining and industrial maintenance coating. Coal tar epoxy with glass filament reinforcement also works at high temperatures.
- Fusion-Bonded Epoxy: Widely used for buried and immersion service. Now sometimes replaced by fusion-bonded nylon. Fusion-bonded epoxy is used for moderately high temperatures.
- Urethane and Polyurethane: Polyurethane is a type of urethane. It is cured by chemical reaction as well as solvent evaporation. These are suitable for atmospheric exposure and fluctuating immersion operations.
- Moisture-Cured Urethane: Suitable for damp environments, this can be customized with specific pigment formulations for a variety of service exposures such as burial, immersion, chemicals, and gases.
- Extruded Polyolefin Systems: The system is used for large-diameter pipes (for pipe up to 24 in diameter). Recent improvements in the adhesive properties and availability of polypropylene for use at wide and high temperatures (up to 190±F [88±C]) has made the system effective and popular. It is able to provide holiday-free coatings.
General Tips for Selecting a Coating
- For acid, alkali, or salt resistance, use urethane, coal tar epoxy, and chlorinated rubber coatings
- For solvent resistance, use zinc inorganic, urethane, or chlorinate rubber
- For an oxidizing environment, use vinyl and urethane, or zinc inorganic coatings
Reasons for Coating FailureApart from improper application, other factors responsible for the failure of coatings are:
- Improper surface preparation
- Poor adhesion of priming coat
- Improper selection of top coats
- An improperly cleaned and prepared substrate surface
- Primer that lacks good adhesiveness, or is incompatible with the top coat