Steel corrosion stalled with rust-inhibitive coatings

Epoxy and polyurethane provide protection at a lower life cycle cost

By Peter Fisher, senior technical advisor for Rustoleum

Protecting steel against corrosion can be challenging, especially in industrial environments. Fortunately, rust inhibitive coatings make the job easier.

A rust inhibitive primer contains special pigments that react with moisture and alter the conditions at the substrate to make iron oxidation less likely. Though based on chemistry, pigment selection remains somewhat of an art form because pigments perform differently with each resin. The formulating chemist selects the pigment or pigments that have a synergistic effect with the selected resin.

Probably the best know rust inhibitive pigment is red lead. It is very effective for protecting ferrous metals and is compatible with many resin systems. However, environmental and health concerns have eliminated its use in the United  States. Zinc chromate also has been a popular rust inhibitive pigment, but its use is being constrained because of health concerns with chromates. 

Surface preparation

Regardless of coating system selected, its performance depends directly on surface preparation. Even though many coatings require only minimum surface preparation, it still must be done correctly. Minimal surface preparation does not mean no surface preparation. For example, a coating designed for direct application to a sound, rusted surface requires that loose material be removed. A rusted steel surface is considered sound when scrapping with a dull putty knife under moderate pressure removes no further particles.

Another concern is grease and oil contamination. This has become more of an issue because of greater use of water-based coatings. Solvent-based coatings could tolerate some grease or oil contaminants because they dissolve in the coating to some degree. This generally does not occur with water-based coatings; the contaminant interferes with the coating’s adhesion.

The presence of soluble chloride greatly enhances the corrosion process. When lead is present, it reacts with the chloride to form an insoluble salt. The elimination of lead from coating formulations also means that chloride contamination must be minimized. New surface preparation standards, such as SSPC-SP-12/NACE 5, specifically address the chloride issue.

Epoxy and polyurethane coatings

For routine industrial maintenance, epoxy and polyurethane coatings systems are becoming more popular. While their purchase price may be higher, they offer a longer service life, which reduces their overall life cycle cost.

Two-component epoxy coatings have excellent resistance to a variety of solvent and chemical exposures. They develop a tough, hard finish that resists abrasion and physical abuse. Epoxy resins also function as a corrosion inhibitor, which makes them a suitable high performance primers. Their downside is that they have poor outdoor color and gloss retention. Because they are viewed as protective coatings, their aesthetics are considered a secondary issue. Loss of gloss and color fade do not compromise the ability to protect and prevent corrosion.

Two-component water-based epoxies offer the same performance properties as their solvent-based counterparts, but provide the benefits of a water-based system. They are a good choice for areas where solvent odor is an issue. This includes facilities such as food processing plants.

When aesthetics are important, a two-component polyurethane finish is the best choice. It has the performance characteristics of an epoxy, but possesses good outdoor color and gloss retention. Just like an epoxy, it has excellent chemical and solvent resistance. The smooth hard finish is easy to keep clean.  Its downside is a rather strong solvent odor, which may limit its use for interior applications.

Water-based acrylic coating systems are becoming the coating of choice in mild to moderate industrial environments. They provide the same performance characteristics of traditional alkyd enamel systems, but have the benefits of a water-based system. They provide better outdoor color and gloss retention than alkyds. Water-based acrylics also meet current and proposed volatile organic compound regulations.

Key points

When making a coating selection, consider these points:    

Exposure conditions

  • What is the chemical environment?
  • Are there unusual chemical or corrosion conditions? 
  • What conditions occur during normal operations?   
  • What off-site factors may affect coating recommendations?

Surface preparation

  • How much and what kind of surface preparation is required?
  • Does the degree of surface preparation meet coating requirements?

Application method

  • Can the coating be applied according to desired application method? For example, some coatings can be applied only by spray.
  • Are you aware of the coating’s limitations and drawbacks? For example, windy conditions carry overspray long distances.

Drying or curing conditions

Is the coating being applied in the proper temperature range? Application outside of the range can affect curing, which may compromise the coating’s performance and service life.

Finally, make sure you understand the performance characteristics of the selected coating and its anticipated service life.

Peter Fisher is a senior technical advisor at Rustoleum. For more information, e-mail info@rustoleum.com..

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