The Coatings Encyclopedic Dictionary (CED) defines corrosion as “the deterioration of metal or concrete by chemical or electrochemical reaction resulting from exposure to weathering, moisture, chemicals or other agents in the environment in which it is placed.” One volume of steel in the process of rusting yields 20 volumes of rust. To protect steel surfaces from attack by marine environments, industrial atmospheres, chemical fumes and moisture, we use barrier and galvanized coatings specifically formulated for corrosion resistance. But what characteristics separate a “good” corrosion-resistant coating from a “better” or “best” coating?
Coating BasicsAs their name indicates, barrier coatings form a film barrier between the environment and the coated material. The coating film must be continuous with no pinholes, skips or breaks.
A more recent innovation is sacrificial or galvanized coatings. These coatings are zinc-rich, containing about 85 to 95% zinc metal by weight. They employ the “galvanic-cell” principle, wherein water from rain, moisture or dew acts as an electrolyte, zinc at the anode and iron at the cathode completes all requirements of a functional galvanic cell. The resulting electric current will cathodically protect the surface from corrosion. Zinc is corroded and wasted away in process; hence the term “sacrificial-protection.” Since the protection is by electrical actions, a pinhole-free coating is not essential.
Flash RustingThe presence of small brown rust spots on white paint, known as flash rusting, often occurs during the drying phase when waterborne paint is applied to a ferrous substrate. Some light-colored coatings will become dark with flash rusting, although this effect is more prevalent with polyvinyl acetate than with acrylics.
Flash rusting is caused by the slow drying process of the paint film and the dissolved oxygen in the paint reacting with high humidity in the atmosphere. To avoid flash rusting problems, look for formulations that contain 0.5 to 1.0 lb/gal of a rust-inhibitive pigment, such as zinc hydroxy phosphite, molybdenum or zinc phosphate. Alternatively, 0.25% of any one of the following is also effective: ammonium benzoate, sodium benzoate, sodium nitrite, potassium nitrite, sodium phosphate, amino methyl propanol (AMP) or morpholine.
Other formulation considerations to keep in mind when choosing a corrosion-resistant coating include the following:
- Styrene acrylics offer good resistance to acids and corrosion.
- Polyvinyl acetate and vinylinine chloride are only marginally effective as corrosion inhibitors in latexes.
- Coatings containing aromatic hydrocarbons, ketones and esters result in a softer coating film.
- Latexes that crosslink on drying offer excellent corrosion protection.
- Corrosion inhibitive pigments with a slight amount of water solubility, such as barium metaborate (0.3 to 0.4% solubility in water) are preferred. Corrosion inhibitive pigments that are too soluble will shock a latex system.
- A good corrosion inhibitor has minimal water sensitivity.
- Surfactants should be used sparingly and should provide adequate stability.
- Lead silicocromate and strontium chromate are banned and should not be used as corrosion inhibitors.
Surface PreparationThe success of any coating depends on how well the surface is prepared. The preparation requirements vary depending on coating type and substrate conditions - whether the substrate is new or previously painted; if painted, the type of paint used (oil or waterborne); and the amount of rust or other imperfections on the surface (characterized as mild or severe). If mild, remove the loose rust with sanding and use a primer. If severe, sandblasting is recommended. Surface preparation is especially critical when waterborne coatings are used since the coating will not adhere to a surface that contains oils. A “fairly clean” surface is acceptable for most solvent-based paints.
Film ThicknessA good rule of thumb is to apply a minimum of 5 mils (dry) with a minimum of three coats. For primers, a 1.0- to 1.5-mil film thickness is adequate. Since film loss will occur over time due to chalking, weathering, service condition and mechanical abrasion, preventive maintenance should include periodically measuring the film thickness and repainting as needed to maintain a 5 mil dry film thickness.
By choosing the right coating and applying it correctly, you can ensure many years of corrosion resistance on a steel substrate.