Food Engineering
TECH FLASH

Tank coating: an underrated aspect of selection

Tank surface preparation methods diverge, and a tank’s coating is its only defense against corrosion.

May 28, 2013

Tank coating: an underrated aspect of selection

Most commonly, considerations for storage tank purchasing include factors affecting maintenance requirements and expected lifecycle including size, capacity, cost, turnaround time and ease of construction. However, according to Mark Eklund of CST Storage, manufacturers often overlook a fundamental point of selection: the quality of a tank’s coating.

A tank’s coating is its only defense against corrosion, which could contaminate your liquid. Coating application processes also help protect against contamination and aid in differentiating tank quality. All tank manufacturers should remove the thin layer of corrosion that forms on uncoated steel as soon as raw sheets are formed, but tank surface preparation methods diverge.

Some manufacturers prepare the tank surface with chemical treatments, which is cost effective but doesn’t help the tank coating adhere to the surface. Blasting the tank surface with grit does help the tank coating adhere to the tank’s steel in addition to effectively removing surface corrosion. To ensure coating adheres properly, sandblasting procedures should meet standards such as NACE No. 2/SSPC-SP 10 Near White Cleanliness.

The coating process must begin quickly following preparation, as the surface is vulnerable to environmental contamination, and flash rust can occur in as little as 30 minutes. The amount of time elapsed between surface preparation and coating with paint can greatly affect the expected useful lifespan of a tank. Some customers even specify that a first coating application take place no more than 15 minutes after the completion of surface preparation.

Because particles from environmental contamination also affect the coating’s surface adhesiveness, quality control measures including cleaning and filtering equipment to remove particulates from the air should be part of the manufacturing process.

While factory-coated tanks receive surface treatment at the factory, tanks set to be coated in the field should undergo close inspection to ensure adequate preparation of the tank surface, especially in hard to reach areas. The underside of the tank floor may be difficult to reach, and the underside of roofs requires a blasting shot straight upward.

Factory-coated tanks are tested incrementally for wash water temperature and pH, the proper grit blast profile, paint booth temperature, humidity and air quality. Coatings are applied individually to tank panels under closely monitored and controlled conditions.

Common factory coatings include high-quality glass-fused-to-steel (porcelain enamel) and powdered epoxy. Glass-fused-to-steel coatings are usually applied in layers of two to three coats including a top coat, while powdered epoxy is applied electrostatically for uniform thickness and good adhesion to corners and edges. The coating application process is controlled by high-tech lines that measure the coating’s quality and thickness, capture overspray, monitor particulates in the air and test for uniformity.

When tank coatings are applied in the field, they are typically sand-blasted on and coated with epoxy using a handheld sprayer. Two coats are often followed by a primer such as acrylic polyurethane. In some areas, environmental regulations require that blasting materials and paint overspray be captured to prevent the release of silica and VOCs into the atmosphere. As glass frit cannot be fused to substrate in the field, glass-fused-to-steel coatings are only used in factory applications.

Glass-fused-to-steel coatings can only be cured thermally, while epoxy coatings may also be cured with ambient air—although thermal curing via passing through a 400°F oven is preferred.

To maintain quality, tanks must be checked regularly for rust and corrosion spots, which often cannot be seen by the human eye. To test for corrosion spots, called “holidays,” a wet sponge with an electrical charge is moved across the tank surface. The metal behind the coating also receives a charge, and when the sponge touches bare metal, a circuit is completed, revealing the corrosion spot. Glass-fused-to-steel coatings do not exhibit corrosion creep, but still must be tested for possible spots missed prior to the first use of the tank.

A high-voltage spark detector is a more stringent test that uses a material’s dielectric strength to determine minimal coating thickness. High-voltage testing helps ensure uniformity of coating thickness, though its use is not common among tank manufacturers.

Food companies should audit or visit the factories of potential tank manufacturers, or visit a site where a tank received its coating in the field. Most tank manufacturers claim to utilize best practices and materials, but verifying quality will ensure long-lasting, durable storage.

For more information on tank selection and coating, visit CST Storage’s site.