In any industrial plant, exposure conditions vary. Interior ceilings may be clean and dry in storage areas, yet subject to extreme humidity in manufacturing areas. Walls in laboratories may be subjected to an occasional splash or cleaning, while floors in processing areas may be frequently splashed with lubricants and scrubbed daily with hot water and harsh chemicals. Piping may be dry during one time of year while sweating profusely during another. As a result, choosing protective coatings for the wide range of structures and exposure conditions in a plant can be a challenge.
To properly choose coatings for specific plant areas, follow these steps:
- Establish plant zones.
- Classify corrosive environments.
- Analyze other factors, such as application considerations.
- Consider past performance of coating options using case histories, comparative studies and test results as predictive tools.
Establish plant zones.
Coating selection starts with a thorough analysis of the service conditions of coated structures. To make this task more manageable, the facility and structures being protected are sectioned into zones. Maps are created to delineate zone boundaries and identify structures within a zone.
Classify corrosive environments.
Once zones are established, observation, historical information and testing can determine the environment’s corrosiveness. Start by identifying the material to be coated, such as carbon steel, galvanized mental, concrete, plastic or wood. For steel substrates, visual inspection of the steel surface using ASTM D610, Test Method for Evaluating Degree of Rusting on Painted Steel Surfaces, is useful. The corrosive environment must be determined for each structure within each zone.
Environmental variables that contribute to corrosion include condensation, humidity, ultraviolet light (UV), wet/dry cycling, temperature cycling, abrasion, immersion and harsh chemicals and solvents. These variables frequently have a synergistic effect upon one another, multiplying corrosion potential. In general, corrosive environments are described as normal, light, moderate or severe.
To assist in classifying corrosive environments and choosing the protective coating system to use in a particular plant area, the Steel Structures Painting Council (SSPC) developed Environmental Zones Painting Systems, a program that identifies 12 environmental zones, ranging from 0 (interior exposure) to 3E (severe chemical exposure). Along with a description of each zone, it recommends coating system suitable for use in each area.
Analyze other factors such as application considerations.
Other considerations are involved in specifying coatings. Some questions to answer include: Can the surface be properly prepared to accept the recommended coating system? Will production continue while painting is in progress? Will plant workers be present in adjacent areas? Will plant safety be influenced? What physical limitations may impede coating application?
To help plant specifiers evaluate these variables, industrial painting contractors and coatings manufacturers may be called on to make recommendations. Many of the newer coatings require less labor to apply minimize disruption to the plant and shorten downtime.
Consider case histories, comparative studies and test results.
Predicting how a particular coating will perform under site-specific conditions is extremely important. Case histories and comparative studies aid in this process. Results from accelerated laboratory exposure tests are also useful, although not all accelerated tests provide meaningful information. The long-standing salt-fog method (ASTM B117) has been found to be particularly poor at forecasting coating performance. While no single exposure test can duplicate actual atmospheric conditions exactly, the newly adopted cyclic corrosion test (ASTM D5894) offers an improved technique for comparing the relative durability of protective coatings.
To aid the decision-making process, here is a sample of coating systems used in a broad range of plant environments.
Pulp and paper
At a large paper plant, mill engineers determined a complete repaint was needed of the interior and exterior of a steel-framed, 100 ft.-high exhaust stack and precipitator structure. Years of exposure to the elements had left the stack and precipitator rusted and bare. Because abrasive blasting was not allowed, a protective coating system, which could be applied over the existing surface, was needed. Other challenges included the structure’s heights, the 120ºF temperature of the stack’s skin and the fact spray paint couldn’t be used. As a result, products that could be easily brushed and rolled were required.
Workers applied a two-coat system of a high solids, high build, fast drying polyamide epoxy primer and a high solids polyurethane coating at 4 to 6 mils DFT and 3 to 4 mils DFT, respectively. The primer is a two-part epoxy that can be brushed and rolled over a marginally prepared steel surface. The two-part polyurethane topcoat provides a high-gloss, UV-resistant finish. The precipitator stack received two coats of a surface-tolerant epoxy mastic aluminum II. This two-part epoxy can be applied to surfaces as hot as 120F. The product was applied using a brush and roller to achieve 3 to 5 mils DFT per coat.
Another of the company’s plants, the exterior of a carbon steel caustic soda storage tank had significant rusting. A three-coat moisture-cured urethane coating system was chosen to allow for quick application and recoating ability. The intermediate coat was a single-component, moisture-curing urethane with micaceous iron oxide, which outperforms epoxy mastics when overcoating old red lead coatings. The topcoat was a single-component, moisture-curing urethane designed for low-temperature or high-humidity applications and provides excellent UV and chemical resistance.