Corrosion, deterioration, and abrasion of steel and concrete is avoidable when mills choose the proper surface preparation and coating system

Cost-effective Maintenance Painting Preserves Valuable Mill Assets

This is the first article in a two-part series dealing with cost-effective maintenance painting in pulp and paper mills. The first part in this series will address the surface preparation of carbon steel, galvanized steel, and concrete and will describe the characteristics, features, and benefits of protective coatings by generic class. In the October issue of Pulp & Paper, the second article will cover the selection of proper coatings, conducting a mill painting audit, specification writing, inspection procedures, inspection equipment, and coatings calculations and costs.

Carbon steel and concrete compose the vast majority of the buildings, structures, and equipment in a pulp and paper mill; and corrosion, deterioration, and abrasion of steel and concrete substrates in mills are pervasive. Both materials are susceptible to attack from the chemicals inherent in mill processes and require protection in order to preserve valuable assets.

The purpose of this month's article is to point out proper and cost-effective methods for selecting and applying protective coating systems to various substrates exposed to corrosive conditions. In addition, this article focuses on the coating materials, maintenance painting options, surface preparation, and costs associated with maintenance. It also includes a brief overview of new construction costs for coating steel and concrete.

NEW CONSTRUCTION COSTS FOR COATING STEEL AND CONCRETE. New structural steel is generally fabricated and painted in the fabrication shop. When compared to field conditions associated with weather, heights, contaminants, and mill operations, the shop environment is theoretically ideal.

Large fabrication shops are equipped with centrifugal shot and grit blast cabinets that significantly minimize the cost of surface preparation. Structural steel can be abrasive-blasted in accordance with the Steel Structures Painting Council's SSPC-SP 6 Commercial Blast Cleaning national standard for surface preparation at a cost of $0.25/ft2 to $0.30/ft2 in the fabrication shop. In addition, three coats of high-performance protective coatings can be applied for $1.20/ft2 to $1.50/ft2 (including labor and materials) for a blasting and coating total of approximately $1.75/ft2. The same procedure in the field costs about $2.35/ft2 under good conditions, but the cost can soar as high as $5.00/ft2 for some applications. Because of the elevated cost of field application, the best coating system possible should be applied from the beginning-in the shop.

As for concrete, new concrete that is properly prepared and coated costs a fraction of the replacement cost for failed concrete. A secondary containment protective coating system costs about $2.00/ft2 to $6.00/ft2, and repair of spalled, cracked, and contaminated concrete runs as much as $20/ft2. In addition, these estimates do not consider the fact that repairing and coating concrete as a maintenance procedure is always a battle with time constraints, mill operations, and the elements.

SURFACE PREPARATION METHODS. The longevity of a coating system for carbon steel and galvanized structures is dependent upon many factors. These include the quality of the coating itself, surface preparation, application, dry film thickness (D.F.T.), curing, and choosing the right coating for the atmosphere to which it is exposed.

Surface preparation is particularly important for two reasons. First, regardless of the quality of the coating, improper surface preparation reduces the performance and longevity of the coating. Second, the cost of proper surface preparation for maintenance painting may account for 50% or more of the entire painting project cost.

Corrosion, deterioration, and abrasion of steel and concrete substrates in mills are pervasive.

The use of abrasive blast media such as sand and grit is limited due to pollution restrictions, health concerns, and possible damage to equipment-particularly pumps and motors. Therefore, surface preparation has become more time consuming and expensive. Some typical surface preparation methods and general cost/ft2 of surface are shown in Table 1, and the following sections discuss these methods.

Water blasting or low-pressure water cleaning. Water blasting or low-pressure water cleaning uses high-velocity water at pressures ranging from 1,000 psi to 5,000 psi. Water is forced through a nozzle having an orifice that varies-0, 15, 25, and 40. The smallest opening concentrates the force of the water and gives better cleaning results, although the rate of cleaning is slower.

In addition, it is very important to hold the nozzle no farther than one foot from the surface being cleaned for optimum results. Water blasting is useful for removing loose coatings, chalk, dirt, and most chemical residues. In addition, water blasting units are available with an injection system that allows the use of chemical cleaning agents-such as detergents- that enhance the cleaning process.

Hand and power tool cleaning. According to standards from the Steel Structures Painting Council, hand (SSPC-SP 2) and power (SSPC-SP 3) tool cleaning utilize wire brushes, chipping hammers, chisels, power grinders, needle guns, and so forth to remove loose mill scale, rust, coatings, and other surface contaminants. Tightly adherent contaminants-those that cannot be removed with a dull putty knife-are not removed by these procedures.

Hand and power tool cleaning procedures are normally used to prepare small areas. In addition, since they are labor intensive, the rate of production is slow and fairly expensive.

Power tool cleaning to bare metal. Power tool cleaning to bare metal requires a more thorough cleaning than hand and power tool cleaning as defined by the SSPC-SP 11 standard. It also requires the production of a surface profile that is not less than 1 mil.

Power tool cleaning to bare metal provides a cleaner surface than does hand and power tool cleaning, produces an anchor profile, and costs more. This method is normally used in small areas that are difficult or impractical to abrasive blast.

TABLE 1. Cost/ft2 varies considerably according to the method required for a particular application.

Brush-off blast cleaning is particularly useful for preparing large areas that have not corroded beyond 5% of the entire surface area and will receive at least one additional coat of protective coating.

Commercial blast cleaning. Commercial blast cleaning (SSPC-SP 6 and NACE 3 standards) removes all mill scale, rust, coatings, and similar materials. However, some mill scale oxide staining-not to exceed 33% of a unit area (9 in2)-and tight rust in the bottom of rust pits may remain. This method is normally used when the existing coating system has failed and the structure is exposed to a corrosive environment that requires complete recoating of the steel.

Near-white blast cleaning. Near-white blast cleaning (SSPC-SP10 and NACE 2 standards) is identical to commercial blast cleaning, except that only 5% of a unit area may exhibit staining from mill scale oxides. This procedure is used most often for heavily pitted steel and immersion exposures.

White-metal blast cleaning. White-metal blast cleaning (SSPC-SP 5 and NACE 1 standards) is normally required only when protecting steel that is in a severe chemical environment or operating at a very high temperature exposure (more than 500F). It is also used for immersion exposures requiring high performance, high-build coating systems.

Water jetting. Water jetting (SSPC-SP 12 and NACE 5 standards) uses high- and ultra-high pressure (10,000 psi to 36,000 psi) water. This method requires very special equipment and skill. Water at 20,000 psi can completely remove all coatings, most corrosion by-products (rust and oxides), and most chemical contaminants. At pressures of 36,000 psi, most mill scale, chemical contaminants, and all coatings can be effectively removed. However, the cost of water jetting is relatively expensive.

Since it is around 25% cheaper to apply high-performance coatings in the fabrication shop than at the mill, it is advisable to apply the best coating system from the start.
Water jetting should be used in combination with a rust inhibitor to prevent flash rusting. It is also very important to verify that the inhibitor residues are compatible with the coating system that will be subsequently applied.

Baking soda and Epson salts. Baking soda and Epson salts blast cleaning are cleaning methods that use these substances under high pressure to remove grease, oil, dirt, corrosion by-products, and coatings. Both compounds are soluble in water, making cleanup easy.

Baking soda has the advantage of producing a surface that only requires cleaning by clean, oil-free air prior to painting. In contrast, Epson salts must be rinsed with a flash rust inhibitor to remove residues that may cause blistering of the coating system that will be applied. However, using current equipment and technology, the Epson salts will clean at a faster rate than baking soda.

Sponge blasting. Sponge blasting removes coatings and corrosion by-products. Abrasives of varying size and hardness are incorporated into sponge material that is used at pressures of 70 psi to 100 psi. Sponge blasting is advantageous whenever dust control and the weight of abrasive residue (overhead containment or in a pit) are factors.

PREPARING METAL STRUCTURES. Carbon steel structures that have minimal coating breakdown-less than 3% to 5%-may be effectively prepared by a combination of water blasting and hand and power tool cleaning. Water blasting can remove surface contaminants such as chalk, most chemical residues, and dirt. Spot rusted areas are then cleaned using hand and power tools.

Using the cost estimates in Table 1 and assuming the average ton of steel consists of about $250/ft2, the cost of this method will be approximately $45.00/ton, or $0.18/ft2. This is a cost-effective approach when compared with other preparation procedures. Clearly, maintaining a coating with this relatively minor degree of corrosion makes good financial sense, as compared with allowing the structure to deteriorate to the extent that complete removal and recoating is required.

Steel that exhibits more than 10% rusting over the entire area requires more extensive preparation and is more cost-effectively cleaned by abrasive blasting, water jetting, or using baking soda or Epson salts under high pressure (usually 5,000 psi to 15,000 psi). Steel that exhibits corrosion to this extent could be prepared by less expensive means; however, the extent of the corrosion present is indicative of a coating system that has failed and should be removed entirely. Attempting to spot prepare the substrate and apply a coating results in short-term corrosion prevention and only delays the inevitable.

Referring to the cost estimates mentioned previously, surface preparation can cost anywhere between $0.50/ft2 to $3.00/ft2 or more. So, the real goal is to properly maintain the protective coatings system to prevent the costly maintenance expense of completely removing a failed coating system.

Galvanized metal. Galvanized metal must be prepared and coated properly in order to protect its metallic zinc content. The outer skin of metallic zinc is readily reactive to strong alkaline compounds and virtually all acidic compounds. Left unprotected, the zinc will react with these compounds and show severe deterioration within a few years, depending on the corrosiveness of the environment.

New galvanized metal may or may not come with a surface layer of flux. Steel that is galvanized by the dry kettle method will not contain this surface layer. However, steel that is coated by the wet flux galvanizing process will have a surface layer of flux. This layer, when new, appears hard and glossy and must be removed before coating application by scrubbing with a strong detergent such as trisodium phosphate (TSP).

Next, it is critical to abrade the galvanizing to produce an anchor profile to which coatings can bond. The preferred method is to lightly, but uniformly, abrasive blast the surface until it is dull and has the appearance of 120 grit sandpaper. There are chemical treatments available that will etch galvanized zinc film; however, abrasive blasting is preferred.

The coatings manufacturer will recommend a coating system that is known to have good adhesion characteristics. Generally, most polyamide epoxies have excellent adhesion to properly prepared galvanized metal. The application of an additional coat of epoxy or urethane finish is recommended in order to obtain a protective coating system with at least 6 mil D.F.T. for optimum life.

Galvanized metal that has been exposed to mill environments without a protective coating system will develop a dull, grayish-white colored appearance. This is the natural oxidation (deterioration) of the metallic zinc that forms zinc oxides, zinc hydroxide, and, ultimately, zinc carbonate. Proper surface preparation at this stage is accomplished by removing the oxidation deposits and any rusting that has occurred with hand and power tools, followed by pressure washing using potable water.

Again, commercial chemical treatments are available and will usually do an adequate job. The galvanized metal can be coated as previously mentioned. After proper surface preparation, apply a rust inhibitive epoxy or organic zinc-rich spot primer to all bare or rusted areas.

EXAMINING EXISTING COATINGS. To choose the best method of surface preparation-not necessarily the method with the lowest cost/ft2-it is important to perform tests on the existing coating. These tests will identify the extent of corrosion, coating type, coating adhesion, and presence of chlorides and sulfates.

Determine the extent of corrosion. Determine the extent to which the existing coating has corroded. An excellent standard for quantifying the extent of corrosion is the American Society for Testing Methods (ASTM) D 610-95, Evaluating Degree of Rusting on Painted Steel Surfaces. A coating system that shows rusting of 10% or more (ASTM Rust-grade 4), or has extensive peeling or delamination should be completely removed. This is especially important if conditions are very corrosive.

Determine coating type. Determine the generic type of the existing coating and how long it has been in service. Coatings in service only a few years need to be carefully looked at to determine the cause of failure and to ensure that the same mistake is not made again. It is also important, and easy, to determine whether the coating is a thermoplastic or thermosetting coating.

To determine whether a coating is thermoplastic or thermosetting, perform the methyl ethyl ketone (MEK) double rub test. Thermosetting coatings (epoxies and catalyzed urethanes in general) are not affected by MEK, but thermoplastic coatings will leave a residue on a test rag.

Steel that exhibits more than 10% rusting over its entire area requires more extensive preparation and can be more cost-effectively cleaned by abrasive blasting, water jetting, or by using baking soda or Epson salts under high pressure.

To perform the double rub test, use the following procedure:

• Wearing a rubber glove and goggles,
• saturate the corner of a rag with a thimble full of MEK.
• Rub the MEK vigorously over a 2 in.2
• area for 100 double rubs (one pass up and down equals a double rub).

Check the coating adhesion. Check the adhesion of the coating to the substrate. An excellent standard to determine coating system adhesion in excess of 5 mil D.F.T. can be found in ASTM D 3359, Method A (Appendix B), Measuring Adhesion by Tape Test. Briefly, the procedure calls for using a razor or sharp utility knife to cut all the way through the coating to the substrate using the following steps:

• Make two cuts about 112 in. long to form an "X" pattern. Use a straight edge to keep the cuts straight and form an angle of 30 to 45.
• Place a piece of pressure sensitive tape, as recommended by the ASTM, over the X-pattern.
• Firmly rub the tape using an eraser and then rapidly pull it off-within 90 seconds-at a 180 angle. Remove the tape smoothly, without jerking.

A rating of 3B or better, according to the ASTM D 3359, Method A, indicates good adhesion and, therefore, a coating system that can be overcoated.

Identify the presence of chlorides and sulfates. Identify the presence of chlorides and sulfates prior to surface preparation. These compounds can cause premature coating failure through a complex failure mode known as osmotic blistering. Commercially available test kits and neutralizing chemicals are readily available. Consult a coatings specialist or painting contractor for details.

CHOOSING THE BEST METHOD OF SURFACE PREPARATION. After performing the four previously-mentioned tests, you must choose either complete or partial removal of the coating. In addition, it is important to test the new coating for adhesive properties before painting.

Complete removal. If the coating shows more than 10% surface area failure according to ASTM D 610-95 or fails the adhesion test, complete removal is in order. This necessitates the use of abrasive blasting, water jetting, or baking soda or Epson salts sponge blasting.

If the coating is simply softened or partially dissolved when using the MEK test, an overcoat system is a possible answer. Using the cost guidelines previously listed, consult a competent painting contractor and coatings specialist and determine if the use of abrasives is feasible. This will give you a good idea of the preparation methods from which you have to choose.

Partial removal. Partial removal (power wash or hand and power tool clean) of a coating system consists of three operations:

• Remove grease and oil with TSP solvent, or detergent.
• Water blast to remove chalk, water soluble residues, and peeling or loose coatings.
• Hand and power tool clean to remove anything that is not tightly attached.

Comparing the costs of field maintenance painting vs. new construction painting in a shop is also worth examining. Table 2 shows the cost comparison per square foot of surface.

SURFACE PREPARATION FOR CONCRETE/MASONRY. Poured concrete and precast concrete have three inherent features that make proper surface preparation essential to their preservation:

• Concrete is alkaline, having a pH of 12 to 13. This alkalinity makes it readily reactive with acids.
  
• Concrete is porous, especially when compared with steel. This porosity allows it to absorb virtually any liquid. Additionally, particulate matter such as chlorides and sulfates lie on concrete until dissolved and absorbed. These and other like compounds cause concrete to deteriorate and spall by chemical reaction and expansive forces.
  
• Poured concrete and precast concrete, including centrifugally spun concrete pipe, always have a surface layer called laitance. Laitance is a mixture of concrete fines and cement that has much weaker strength than the base concrete. Concrete laitance disbonds in time-the length of which depends on the elements and the corrosiveness of the environment. Laitance must be removed prior to applying a protective coating system.
  
• In addition, poured and precast concrete may have been treated with surface hardeners, form release agents, and curing compounds during its construction and placement phases. These materials almost always require removal prior to the application of a coating system. Always consult with the manufacturer of the compound and with the manufacturer of the coating to determine if a compatibility issue exists.

Additional caution must be exercised to ensure that the concrete has properly cured, or hydrated. Generally this process takes 28 days. However, certain admixtures can be used to expedite the curing process. Coatings applied before the concrete reaches the appropriate cure almost always disbond, usually from the release of residual moisture in the "green" concrete.

PROPER SURFACE PREPARATION FOR CONCRETE. The International Concrete Repair Institute has published an excellent technical guide for concrete surface preparation. It is titled Guideline No. 03732, Selecting and Specifying Concrete Surface Preparation for Sealers, Coatings, and Polymer Overlays. This and other documents should be referenced for the complete details of concrete surface preparation.

The basis of proper surface preparation for concrete consists of four objectives. The concrete must be properly hydrated or cured, the surface laitance must be removed, surface compounds and chemical contaminants must be removed and neutralized, and the resulting voids and bugholes must be filled, or surfaced, prior to the application of the protective coatings system. These objectives are usually accomplished by removal of petroleum-based contaminants, acid etching, and abrasive blasting. In addition, care must be taken to ensure that the surface is moisture-free and clean before filling imperfections.

Removal of petroleum-based contaminants. Petroleum-based contaminants can be removed with a hot TSP solution by dissolving 2 lb TSP/gal of hot water. Other detergents and surfactants are also effective. The solution may require reapplication, depending upon the extent of the contamination. After applying, always rinse well with potable water and allow thorough drying.

One method of checking for residual petroleum-based contamination is to hold a propane torch an inch from the surface for a minute. Any residue will bubble to the surface and leave an oily contaminant.

Acid etching. Acid etching works well for horizontal surfaces and surfaces that are not contaminated with petroleum-based compounds. It is important to note that acid etching will not be effective if petroleum-based contaminants, curing compounds, and surface hardeners are present.

Acid etching can be achieved using 10% muriatic acid (hydrochloric acid). Phosphoric and citric acids are sometimes used, but are generally less effective.

When acid etching concrete, wear the proper safety equipment and use the following procedure:

• Apply the acid as recommended.
  
• Allow the acid to react for 5 to 10 minutes while scrubbing it into the surface with a stiff bristle brush or broom. Do not allow acid solution to dry.
  
• Rinse well with potable water.
  
• If the surface does not resemble 100 grit sandpaper, repeat steps 1 through 3.
  
• Check the substrate with pH paper to make sure the acid has been properly removed. The reading should be between 6 and 8.
  If the pH reading is below 6, the surface is too acidic and must be rinsed again or neutralized with a 2% ammonia solution. Apply the ammonia, allow it to remain on the surface for five minutes, and rinse well with potable water.

Abrasive blasting. Abrasive blasting using compressed air or centrifugal shot blasting equipment effectively removes many coatings. The latter is commonly used on horizontal surfaces and is very effective. The former can be used instead of centrifugal shot blasting and to prepare vertical surfaces. Both methods remove laitance, contaminants, and make voids and bugholes easily visible for repair.

Moisture content. After the concrete has been etched or abrasive cleaned, but before filling voids, bugholes, and imperfections, be sure to check for moisture content-even if a known moisture vapor barrier is present-since damage to the barrier may have occurred.

To check for excessive moisture, use the following procedure:

• Obtain a piece of plastic film at least 4 mil thick
• Cut several pieces of the plastic film that are approximately 2 ft x 4 ft. You will need one of these for every 1,000ft2 of surface area that you are testing.
• Double these pieces over to make a 2 ft x 2 ft squares.
• Tape the squares to the concrete using duct tape.Tape all four sides completely to make sure no moisture vapor can escape.
• Place these 2 x 2 squares in the areas to be tested and allow them to remain undisturbed for 24 hours.
• Remove test squares and visually check for condensation on the plastic or darkening of the concrete beneath the plastic. Evidence of either indicates a moisture problem that requires correction before coating. The ASTM D 4263-83 method describes this procedure fully.

Filling voids and imperfections. Once the laitance and contaminants are removed, all loose debris should be vacuumed or blown off using compressed, oil-free air. Mortar fins, splatter, cracks, and protrusions should be removed.

Voids, bugholes, and imperfections can be filled with a suitable patching or surfacing compound that is compatible with the coating system. Often, a 100% solids epoxy mortar or grout is troweled or "squeegee-ed" to fill the imperfections and to prepare for the coating system.

DETERMINING COATING PERFORMANCE. Coatings are made up of a resin (vehicle), pigments, additives, and solvent. Each of these four basic ingredients is available in various types and qualities.

It is impossible to start with a low-quality resin and develop an excellent high-performance coating. However, you can select a high-quality resin and add appropriate pigments and additives to develop a product with excellent performance qualities. There are at least two means to determine the relative performance of a coating-performance tests and the use of test patches.

Performance tests. All reputable coatings manufacturers test the performance of their coatings by using nationally recognized test methods. Many tests are conducted in accordance with procedures published by the ASTM. Performance tests such as salt fog, adhesion, abrasion, chemical resistance, immersion, and color or gloss retention are common. The test results should be acquired, examined, and compared.

Test patches. When possible, apply candidate coating systems directly to steel, galvanized metal, or concrete that is located in the area to be painted and that has been prepared according to specifications. Allow the systems to remain as long as possible and then examine them to determine their performance.

Carefully observe the coatings in the test area, noting their adhesion, the presence of any rusting, film thickness, film formation, and appearance. Rusting along edges is the first sign of failure. The test patch application also provides an opportunity to see firsthand the application and drying characteristics of the coatings.

GENERIC COATINGS DESCRIPTIONS. The following generic coatings descriptions serve as a good example of the overall characteristics of a given class of coatings. However, R&D modifications are numerous and can improve the characteristics of a coating or, unfortunately, can cheapen it and lower performance. It is always a good idea to compare performance data of coatings and coating systems.

TABLE 2: Comparison of cost/ft2 of surface for shop and field painting.

Acrylic (water-based). Acrylic products are single component materials that are normally used because of their exterior gloss and color retention. Technological innovations have led to the development of direct-to-metal (DTM) acrylic products that may be applied to carbon steel without flash rusting.

Recent state-of-the-art developments define the very best DTM acrylics as hydrophobic (water repelling) polymers. Properly formulated, these DTMs may be spray applied at more than 4 mil dry film D.F.T.. in a single coat. One or two coats perform well in mild-to-moderate conditions such as woodyard cranes, conveyors, barkers, and chippers. Warehouse areas, some galvanized metal, and other benign environments may be protected with a single coat.

Some of the DTMs have the advantage of being "dry fall," or safety spray, by nature. In other words, their overspray will not adhere to other structures such as cars. This feature saves substantial dollars because of spray application rates vs brush and roller application rates.

Styrenated acrylic. High film build, flexibility, and the ability to bridge hairline cracks in masonry are the predominant features of styrenated acrylics. These single component, water-based, elastomeric acrylic products are normally used on above grade masonry substrates to provide a waterproof and aesthetic finish. They are ideal for the protection and aesthetic enhancement of exterior precast or tilt-up concrete buildings.

Alkyd. Alkyd coatings-sometimes called enamel or oil-based-have been used for decades. Alkyds are manufactured as primers, intermediates, and gloss or semi-gloss finish coats. They are easy-to-apply, single-component materials, but have poor resistance to alkaline environments and certain substrates such as galvanized metal and masonry. As a rule, their use is limited in pulp and paper mills to warehouse areas, offices, and other benign environments.

Epoxy ester. Epoxy ester coatings are very similar to alkyd coatings in that they are single-component, oil-based materials. Their characteristics, limitations, and performance are also similar. Generally, the dried film is somewhat more abrasion resistant than alkyds. However, their resistance to ultraviolet (UV) light is poor. Epoxy esters are used on interior doors, frames, and equipment in benign environments.

Epoxy. There are hundreds, if not thousands, of epoxy coatings. These materials are normally two-component materials having a resin and a hardener-also known as catalyst, Part B, or curing agent-that must be mixed together.

Once the epoxy is mixed, it has a pot life limitation at a specific temperature before it becomes unusable due to a viscosity increase in the container. Epoxies are modified in many ways, especially by the addition of rust inhibitive pigments, aluminum, zinc dust, and extender pigments like glass fiber and flake.

As a class of coatings, epoxies will "chalk" when exposed to UV light. Chalking is a surface effect whereby the pigment-to-resin bond is broken down, or oxidized, when exposed to sunlight. Depending on the intensity of the UV light and the corrosiveness of the atmosphere, chalking will usually occur at a rate of 0.2 mil/yr to 1.0 mil/yr of exposure. Therefore, epoxy coatings will begin to lose their color and gloss within a year of exposure to sunlight.

EPOXY COATINGS AND THEIR APPLICATIONS. Epoxy coatings are commonly referred to by the hardener or curing agent that is used. These agents include polyamide, polyamine, amido-amine, amine, and cycloaliphatic amine. Other epoxy coatings include water-based, coal tar, and surface tolerant epoxies.

Polyamide epoxies. Polyamide epoxies are the most widely used and versatile. They normally have a 1:1 mixing ratio and a pot life of 8 to16 hours. They have good resistance to alkaline, mildly acidic environments, and many solvents. Abrasion resistance is very good and some are used in immersion service. Polyamide epoxies generally have slightly more flexibility and better adhesion than amine or amine-modified epoxies.

Polyamine and amido-amine epoxies. Polyamine and amido-amine epoxies generally have better chemical and abrasion resistance than polyamides. Their mixing ratio may range from 1:1 up to 10:1 and their pot life is commonly between 1 and 8 hours. They are frequently used as a primer, intermediate, or finish for carbon steel. They are often formulated for various immersion conditions such as concrete floor and wall coatings and those within secondary containment.

Amines and cycloaliphatic amine epoxies. Amines and cycloaliphatic amine epoxies have the best chemical and abrasion resistance. Their mixing ratios and pot life are similar to the polyamines, though they have a somewhat shorter pot life. Their high cross-link density makes these products useful for aggressive service in immersion, secondary containment, and direct contact with many chemicals.

Water-based epoxies. Water-based epoxies are resins that have been emulsified and are sometimes modified with acrylic resin. This gives them somewhat better resistance to UV light, but slightly inferior resistance to chemicals as compared with their solvent-based counterparts. Water-based epoxies have a milder odor, dry faster, and are somewhat more expensive.

Coal tar epoxies. Coal tar epoxies are true two-component epoxies that have been made with a blend of coal tar pitch, epoxy resin, and curing agent. These coatings have excellent resistance to water and are most often used in wastewater and effluent immersion environments to protect both steel and concrete. They may be formulated as a polyamide or polyamine. The hardener, or curing agent, and quality of pitch greatly affects their performance and price. Coal tar epoxies are applied in one or two coats to achieve 16 mil to 24 mil D.F.T.

Surface-tolerant epoxies. Surface-tolerant epoxies-also called mastic epoxies-are generally polyamide or polyamine cured. These coatings are formulated with special solvents, low viscosity resin additives, and wetting agents, and are sometimes modified with a resin for better flexibility.

Surface-tolerant epoxies are used extensively as steel primers wherever surface preparation is likely to leave tightly adherent rust and not likely to produce a good mechanical anchor pattern for coating adhesion. These coatings are also used as a tie coat or sandwich coat over an existing primer that has marginal-to-good adhesion.

URETHANE COATINGS AND THEIR APPLICATIONS. Urethane coatings originated in Germany and are a relatively new technology. They offer enhanced flexibility and weather, abrasion, and chemical resistance. Urethanes, or polyurethanes, are either aliphatic or aromatic.

Aromatic urethanes. The aromatic urethanes are usually single component, moisture-cured coatings that have excellent abrasion resistance. Some formulations have good immersion and chemical resistance. However, they will readily lose gloss and color when exposed to sunlight.

Aliphatic isocyanates. Two-component, aliphatic isocyanates that are crosslinked with acrylic or polyester resin are the workhorse of the urethane family. These materials have excellent abrasion, chemical, and UV resistance.

Polyester-modified urethanes generally have better chemical resistance and hardness than the acrylic-modified urethanes. They are more expensive than typical epoxies and are therefore primarily used as a 2 mil to 5 mil D.F.T. finish coat over organic zinc-rich primers, inorganic zinc-rich primers, epoxy primers, and epoxy intermediates. The inorganic zinc-rich primer almost always has a sandwich coat of epoxy between it and the urethane to prevent pinholes from forming in the urethane.

Aliphatic urethanes make excellent finish coats for exposed steel in corrosive environments. Some versions-especially the polyester-modified urethanes-are excellent as concrete floor finishes and even in some secondary containment applications.

If the coating shows more than 10% surface area failure and/or fails the adhesion test, complete removal is in order.

Elastomeric urethanes. Elastomeric urethanes are two-component, very high-build (more than 50 mil), high-volume solids (90% to 100%) coatings that often require special equipment for application. Their elasticity makes them ideal for concrete coatings and lining materials. They have excellent immersion resistance to water, wastewater, many caustics, and some weak acids. Material cost is rather expensive, ranging from $1.00/ft2 to $2.00/ft2.

ZINC-RICH COATINGS AND THEIR APPLICATIONS. Zinc-rich coatings are primers that provide galvanic corrosion protection for steel substrates. They come in inorganic and organic forms.

Inorganic zinc-rich primers. Inorganic zinc-rich primers are silicate resins combined with metallic zinc dust. The ethyl, or alkyl, silicate resin is mixed with zinc dust at a rate of 9 lb/mixed gal to 18 lb/mixed gal of coating, depending on the manufacturer's formulation. As a matter of reference, the price of zinc dust varies from $0.65/lb to $1.00/lb. This cost translates into as much as $18.00/gal for the zinc dust component alone.

Often compared with galvanizing for steel, these primers contain from 65% to 90% zinc-by-weight in the dried film. The zinc provides galvanic protection to steel by sacrificing itself preferentially to steel.

It is very important to topcoat zinc-rich primers so that service life is extended by preventing direct contact with corrosive elements. The silicate resin bound with zinc dust yields a dried film that, when highly magnified, appears very porous. These pores are voids that are filled with air, and they make it difficult to apply a topcoat without the occurrence of outgassing.

As a topcoat is applied, it displaces the air in these pores. The air passes up through the topcoat and, upon escaping, may leave a small crater. The center of the crater will likely contain a pinhole that prevents proper film formation. A thin mist coat is often applied to correct this problem, but is not always a satisfactory solution.

Inorganic, zinc-rich primers require atmospheric humidity to properly cure. In fact, research indicates that hot or dry conditions need to be avoided during application and drying if these primers are to achieve their performance potential. Furthermore, if sufficient atmospheric moisture is not present during the first several hours after application, these primers will virtually never achieve full performance potential.1 Additionally, this situation is inherently dangerous because, without proper cure, there is a risk of the topcoat splitting the primer. Subsequently, the coating system delaminates and leaves only a fraction of the original primer, which is then exposed to direct attack from corrosive electrolytes.

Inorganic zinc-rich primers must be applied by spray application over steel that has been prepared by abrasive blasting to SSPC SP-6 Commercial Blast Cleaning standards or better. These primers have excellent resistance to salts, most solvents, and dry temperatures as high as 750F. However, they have poor resistance to acidic contact. When properly applied and topcoated, inorganic zinc-rich primers are excellent primers for carbon steel.

Organic zinc-rich primers. Organic zinc-rich primers consist of two or three components and are usually epoxy or urethane based. Like their inorganic counterpart, they normally contain 12 lb to 20 lb of zinc dust and 75% to 85% zinc dust-by-weight in the dried film.

Organic zinc-rich primers protect steel in two ways-as a barrier to corrosive electrolytes and by galvanic action. They are generally spray applied to abrasive-blasted steel. However, some formulations can be brushed and rolled onto small areas. Organic zinc-rich primers are more easily applied and topcoated than the inorganics and are more tolerant to less-than-perfectly prepared steel.

Organic zinc rich primers are more resistant to mudcracking-a phenomenon caused by excessive film build-than are inorganic zinc-rich primers. Their dry temperature resistance is 225 to 300F. In addition, they have slightly inferior resistance to salts, but better resistance to mild acids and alkalis, as compared with inorganic zinc-rich primers.

Organic zinc-rich primers are generally topcoated faster and cure more rapidly than inorganics. When properly topcoated, organic zinc-rich primers are excellent primers for carbon steel.

Recent research provides significant performance data on the immersion characteristics and chemical resistance of an organic (urethane-based) zinc-rich primer topcoated with a polyamide epoxy. The research indicates that this system performed for seven years in potable water immersion, two years in seawater immersion, and 10,900 hours in ASTM B 117 salt spray exposure with no signs of system failure.2

CHEMICAL AND HIGH TEMPERATURE RESISTANT COATINGS. Vinyl ester and phenolic epoxy coatings are used for the protection of steel and concrete in aggressive chemical atmospheres. Silicone-based coatings are used to protect steel exposed to temperatures as high as 1,200F.

Vinyl esters. Vinyl esters have excellent resistance to many acids, caustics, and solvents and are extensively used for secondary containment and tank linings. They are high-build coatings, often reinforced with fiberglass, and usually applied at a system thickness of 25 mil D.F.T. or more.

Vinyl esters form very dense films and have excellent abrasion resistance. These coatings have a very short pot life, may require special application equipment as dictated by the specific formulation, and are more expensive than conventional epoxy and urethane coatings.

Phenolic epoxy coatings. Phenolic epoxy coatings have similar chemical resistance to vinyl esters. They are brittle coatings and, to obtain the best chemical resistance characteristics, should be force-cured by baking at temperatures of 300 to 400F for several hours. They are specifically known to have excellent solvent resistance.

Silicones. Very high temperature resistance of up to 1,200F may be obtained with silicone coatings. These coatings are usually modified with aluminum, and some products contain ceramic or zinc pigmentation.

Silicone coatings can be modified with alkyd or acrylic resins to lower cost, while still remaining effective for temperatures as high as 600F. Silicone coatings do not normally have very good chemical resistance. Therefore, when used for coating carbon steel where the temperature may range from ambient to very hot, a zinc-rich primer is applied first. Silicone coatings that are unmodified will require a heat cure cycle around 400F to achieve full cure.

Linc A. York is national sales manager for Tnemec Co. Inc., Kansas City, Mo.

REFERENCES

1. Gerald Eccleston, "The Effect of Cure Temperature and Humidity on the Properties of Solvent-Borne Zinc Silicate Coatings," Journal of Protective Coatings and Linings, January 1998.

2. Mike Bauer, "Organic Zinc-Rich Primer for the Interior and Exterior of Potable Water Tanks," SSPC 1997 Seminars, SSPC #97-09, January 1998.