KnowledgeBase » Duplex Systems

Reasons for Duplex Coating

There are seven good reasons to paint or powder coat hot-dip galvanized steel: Extended Durability – in what is called the synergistic effect between galvanized steel and paint, the zinc of the galvanized coating is a barrier protection to the substrate steel, cathodically protects it, and provides underfilm corro...
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Why paint over galvanizing may fail

Paint failure mechanisms are somewhat complex when paint is applied over hot-dip galvanized steel, otherwise known as a duplex system, and they may be manifested as peeling, flaking, or blisters.  And regardless of the age of the galvanized steel (new, partially weathered, fully weathered), the cause is either inadequate surface preparation or using a paint/powder coat system not compatible with zinc.  First, let’s examine inadequate...
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Painting or powder coating system types to be used over hot-dip galvanized steel

While all recommendations for paint types over a properly prepared galvanized surface should be made by an industrial paint supplier, there are a few emerging patterns that may help in the decision making process. Paints that have shown to be compatible with hot-dip galvanized steel are listed below. This list is only meant ...
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How does galvanized steel perform in seawater splash zones?

Splash zones are the most aggressive environment for hot-dip galvanized steel (or any other protective coating) because as the zinc coating reacts with the chlorides when wetted, zinc corrosion products are formed. Then those corrosion products dry and are later washed away when the steel is wetted again at which time more zinc corrosion products form. That process repeats its...
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Preparing the hot-dip galvanized steel surface for painting or powder coating

Similar to painting over black steel, surface preparation is critical when painting or powder coating over the zinc coating of hot-dip galvanized steel. However, before any surface preparation takes place, the galvanizer should be notified the steel will be painted. With this understanding, the galvanizer will forego any post-treatments that could ...
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What are the specific requirements for painting over galvanized steel?

The specifications for preparing hot-dip galvanized steel for painting or powder coating are ASTM D6386 and D7803, respectively. Successfully painting or powder coating hot-dip galvanized steel, also known as a duplex system, does not have to be difficult or confusing. Just like painting or powder coating over bare steel,
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