Surface Preparation for Duplex Systems
How does surface preparation of hot-dip galvanized steel differ when coating it with paint or powder coating?
Though many specifiers are familiar with one type of corrosion protection or another, few realize the intrinsic value of utilizing two corrosion protection systems together, known as a duplex system. A duplex system is formed by painting or powder coating over hot-dip galvanized steel. When used together, the two systems work synergistically to provide superior protection rather than when used independently.
The paint or powder coating provides barrier protection to the galvanized coating. Then, when the paint or powder coating has served its useful life, the galvanized coating offers barrier and cathodic protection to the base steel. In turn, the hot-dip galvanized coating serves as an incredibly durable primer that prevents the underfilm iron-oxide corrosion which causes paints and powder coatings to fail. The combined protection of the two protective coatings (hot-dip galvanizing covered by paint or powder coating) provides corrosion protection to the steel 1.5 to 2.3 times longer than the expected life of each protective coating alone. In addition to long-lasting, superior protection, duplex systems provide architects, engineers, and other specifiers design freedom. Utilizing a combination of paint/powder coating with galvanizing means you dont have to sacrifice corrosion protection for aesthetics, or vice versa. This advantage has led to more specification of duplex systems each year.
Hot-dip galvanized steel has been top-coated with paint for years. The specification ASTM D6386, Standard Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Painting, was created in 1999 to assist paint shops with the appropriate preparation of the galvanized coating prior to paint application. This specification has proven valuable at helping paint shops to achieve uniform paint coatings with high adhesion to the galvanized coating.
Powder coating has become popular in the last decade due to its good corrosion protection and having less environmental concerns than paint. When applied and maintained correctly, powder coating will not crack, peel or chip like paint can. In the last few years, powder coating over galvanizing has been specified with more frequency and development of another specification was appropriate. The specification ASTM D7803, Standard Practice for Preparation of Zinc (Hot-dip galvanized) Coated Iron and Steel Product and Hardware Surfaces for Powder Coating, was developed in 2012 to assist shops in preparing the galvanized coating for application of powder coating.
ASTM D7803 is similar to ASTM D6386 in many regards. However, there are two main differences between the specifications related to pretreatments. The first notable difference is D6386 allows for the application of wash primers (i.e., SSPC Paint Specification No. 27) and acrylic passivation/pretreatment to the galvanized coating prior to paint application. D7803 does not allow for these types of preparation of the galvanized coating prior to application of powder coating.
The second notable difference between the two specifications is D7803 gives guidance for thermal retreatment or baking of the galvanized steel prior to powder coating, whereas D6386 does not. Thermal pretreatment of galvanized steel is necessary prior to powder coating to prevent outgassing of the galvanized coating during the baking step in the powder coating curing process. Outgassing occurs when moisture trapped in the outer layer of the galvanized coating evaporates as the galvanized steel is heated when the powder coating is baked on. The result is pinholing or blistering in the powder coating. By preheating the galvanized steel or iron, the moisture evaporates prior to application of the powder coating thereby reducing the chance for pinholing or blistering. D7803 gives guidance for oven temperatures for preheating.
The American Galvanizers Association (AGA) is currently developing an instructional DVD and guidebook detailing necessary preparation of galvanized coatings prior to powder coating. A similar DVD and guidebook for preparing galvanized coatings prior to paint is currently available from the AGA. Please contact the AGA for a copy of this DVD. ASTM D6386 and D7803 can be obtained directly from ASTM International at www.astm.org.