Available options for corrosion-resistant applications

Available options for corrosion-resistant applications

Julius, William H

by William H. Julius, P.E. & Severino Bigante KTA-Tator Inc.

What are the coatings options available for corrosionresistant applications?

It is important to be aware of the various types of coatings that are commercially available for corrosion-resistant applications, as well as the characteristics of each, in order to determine which system is best for a specific application. Several different types of high-performance coatings exist. These include: alkyds, vinyl chloride/ vinyl acetate copolymers, catalyzed epoxies, coal tar epoxies, bituminous, zinc-rich, chlorinated rubber, and urethanes.

Alkyds: Alkyd resins are derived as the reaction product of polyhydric alcohols, polybasic acids, and drying oils. Generally, the greater oil content, the more flexible the resin and the more the film will resemble an oil-based paint. Alkyds cure by both solvent evaporation and by reacting with atmospheric oxygen. They also dry faster than oil-based paints, depending on the amount and type of oil incorporated during formulation. Alkyds are referred to as short-, medium-, or long-oil alkyds, determined by the number of gallons of oil added to 100 pounds of resin.

Alkyds have limited chemical and moisture resistance, but their low cost, ease of mixing and application, and excellent ability to penetrate and adhere to relatively poorly prepared, rough, dirty, or chalked surfaces make them the coating system of choice on steel exposed to non-chemical atmospheric service. They are also used on applications such as water tank exteriors, many highway bridges, boat and ship structures above the water line, and in baking formulations as coatings for containers, appliances, and machinery and equipment housing.

Vinyl chloride: Vinyl chloride is a tough, transparent, thermoplastic resin with high inherent strength, excellent chemical resistance, and is crystalline in nature. It is highly resistant to strong acids and alkalis, water, alcohols, aliphatic hydrocarbons, fats, and oils, but is swelled by aromatic hydrocarbons. It is soluble in only a few special types of solvent, mainly unsaturated ketones, alicyclic compounds, nitrobenzene, and dimethyl formamide. It is degraded by heat and by ultraviolet light unless properly stabilized. Because of its solubility and degradation properties, vinyl chloride by itself is not used in solvent-type coatings.

Vinyl chloride/vinyl acetate copolymers: Vinyl chloride / vinyl acetate copolymers retain the toughness and chemical resistance of vinyl chloride while gaining the greater flexibility and solubility of polyvinyl acetate. These copolymers must be stabilized against deteriorating effects of heat and ultraviolet light. They are soluble in ketones, esters, ethers, and chlorinated hydrocarbons. If the unmodified copolymers are incorporated with hydroxyl groups, the copolymer becomes compatible with various alkyd, phenolic, amino, and other resins. The coatings are tough and flexible, resistant to water and many inorganic chemicals, non-flammable, and abrasion and weather resistant, but have poor resistance to most solvents.

Epoxies: The resins in most epoxy coatings consist of a polymerization product of epichlorohydrin and bisphenol-A. These resins then undergo a cross-linking reaction at the time of application. Three types of catalyst are employed, resulting in epoxy coatings with specific properties. Polyamine catalysts produce hard, high-gloss, chemically resistant coating for exterior atmospheric exposure, and resistance to oil spillage. They are not recommended for immersion service.

Amine adduct catalysts produce coatings that are polyamines partially reacted with an insufficient amount of epoxy resin. Lastly, polyamide catalysts are polyamines which react with unsaturated fatty acids and produce coatings that are superior in resistance to chemicals and water, and are suitable for water immersion service. Epoxy coatings have excellent atmospheric exposure resistance, yet they chalk upon exposure to sunlight. They have good acid resistance and resistance to polar and non-polar solvents.

Coal tar epoxies: Coal tar epoxies make use of naturally occurring coal tar pitches which have been refined and are by-products of the coking industry. They are typically combined with polyamide resins and accelerators which polymerize to form a coal tar epoxy-polyamide paint. They are characterized by having a significant degree of chemical and moisture resistance. Coal tar pitches are inexpensive and serve as a filler. The resins also adhere well to steel and concrete substrates. Because of their hard, abrasion-resistant finish, coal tar epoxies provide heavy-duty protection. But, these coatings “blush” when they are applied or cured during periods of high humidity, or if a condensate forms on the coated steel during curing. They also lose gloss in prolonged sunlight exposure and are often difficult to recoat.

Bituminous: Bituminous coatings that are highly thixotropic and used as a cold-applied coating should be thinned only by mixing, but never with solvents. They are applied in a thick film and modified with various fillers. Bituminous coatings are highly impermeable to moisture and resistant to water deterioration. Although they are resistant to petroleum oils, weak acids, alkalis, and salts, they are not resistant to hydrocarbon solvents, vegetable oils, and sunlight. These coatings are primarily used for the protection of buried or submerged steel and/or concrete.

Zinc-rich: Zinc-rich coatings have 75 to 95 percent by weight zinc dust in the dried film. These materials are more difficult to apply than other coating materials, and proper agitation during application is critical in obtaining a homogeneous film. Organic zinc-rich coatings have an alkyl or alkali silicate as their binder. They cure to a hard film, which protects steel by sacrificially corroding in place of the steel. Upon exposure, insoluble zinc salts form a barrier to moisture, thus further reducing corrosion. Zinc-rich primers also have a property sometimes referred to as “throwing power,” which refers to the ability of the coating to protect damaged areas such as scratches, where the primer has actually been removed.

Organic zinc-rich: Organic zinc-rich primers are based on chlorinated rubber, vinyl, epoxy, and other organic substances. They are more flexible and tolerant of surface preparation than inorganics, and have heat resistance. They are also less reactive than inorganics and dissipate more slowly in severe environments to give much longer protection. Organic zinc-rich primers have excellent adhesion and resist undercutting.

Chlorinated rubber: The resin for chlorinated rubber paints is obtained by reacting natural rubber with chlorine. These films dry solely by solvent evaporation. They often have significant amounts of an alkyd resin blended with the chlorinated rubber or plasticizer. They exhibit very good adhesion to wood or concrete surfaces, have low moisture vapor and oxygen transmission properties, excellent chemical resistance, and are very resistant to water penetration and alkalis. They are used for painting interiors of potable water tanks and swimming pools.

Urethanes:

Urethanes have chemical- and moisture-resistant properties similar to epoxies, and can be formulated to maintain their gloss upon prolonged exposure. Curing consists of reaction of an isocyanate-containing material with a polyhydroxylated co-reactant (polyol). The properties of the urethane formed by this reaction depend to a large extent on the nature of the particular isocyanate and polyol. Although most urethanes have good chemical resistance, properties such as hardness and flexibility can vary with the particular formula.

by William H. Julius, P.E. & Severino Bigante KTA-Tator Inc.

William H. Julius, P.E. and Severino Bigante are materials consultant and project engineer respectively for protective coatings consulting firm KTA-Tator Inc. in Pittsburgh.

Send your questions to PWC Magazine, 8730 Big Bend Blvd., St. Louis, MO 63119. PWC stipulates that Coatings Clinic is merely general advice, and is not to be held legally responsible for actions in the field based on reading this column.

Copyright Finan Publishing Company, Inc. Mar/Apr 1998

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