There are many situations where painting or powder coating over hot-dip galvanized steel, known as a duplex system, is desirable. It is rapidly becoming more common as specifiers recognize the high life-cycle costs associated with providing no corrosion protection or simple painting of black steel.
Some of the reasons to paint/powder coat over HDG steel include:
- Aesthetics – typical applications may be light rail stations where the light poles, garbage cans, benches, and shelters are the same color/color scheme.
- Architects preference – first recognizing the need for corrosion protection, an architect may want to put a personal touch on a project or have his particular contribution stand out.
- Identification – paint schemes may be used to identify steam pipes v. gas v. water in a utility chase of a building. Safety orange is required for towers/poles greater than 200’ in height.
- Hostile Environments – chemical plants are harsh atmospheres for galvanizing alone and painting over is the best way to ensure little or no downtime for repairs of the facilities.
- Repair of Existing Galvanized Articles – when HDG steel exhibits substrate steel rust after many years, paint may be used to extend the life of the structure.
- Extending the Life of HDG – in the case of a bridge railing in place for 40 – 50 years, the zinc coating may be on average about 1 – 2 mils thick. To preserve the zinc coating and of course the structural integrity of the railing, paint may be applied to isolate the zinc from the atmosphere, effectively prolonging the service life of the railing.
Galvanized Surface Coating Conditions: Passivation Cycle
There are three states when hot-dip galvanized steel can be painted or powder coated as dictated by the formation of the zinc patina. The newly galvanized state is less than 48 hours and is a good time to paint if the zinc surface is profiled. The partially weathered state must be cleaned and then profiled (approximately 48 hrs to 6 months). The fully weathered state, zinc carbonate condition can be painted successfully after a simple cleaning (6 months to 2 years).
Let’s look at each condition in more detail. The passivation cycle (development of the zinc patina) for hot-dip galvanized steel is key to understanding the surface preparation required to topcoat hot-dip galvanized articles.
- In the first 48 hours after galvanizing, there are very few zinc corrosion products formed on the surface, so profiling is the only necessary preparation step.
- Within the first 6 months, the surface is covered with some combination of zinc oxide, zinc hydroxide, and zinc carbonate. In this state both cleaning and profiling must be don to obtain maximum paint adhesion.
- Within 6-24 months, the zinc oxide and zinc hydroxide combine with carbon dioxide in the air to form zinc carbonate. The zinc carbonate is tightly bound to the underlying zinc coating, non-soluble and fairly rough in surface. Only dirt and dust cleaning are necessary at this stage.
Because the galvanized surface exhibits different forms of oxidation during the first two years of service, the surface preparation methods must be geared to removing the specific contaminants.
Paint and galvanizing work in synergy. The paint prevents atmospheric attack on the zinc and the zinc prevents underfilm corrosion from cracking the paint. According to several studies conducted in Europe, the synergistic effect means the zinc-paint combination will last 1.5 to 2.5 times longer than the sum of the paint and galvanizing systems alone. So, for a 70 year galvanizing life and a 10 year paint life, the system will last from 120 – 220 years.
Practically speaking, no paint lasts that long without being repainted. But what the synergistic effect means is paint lasts longer, generally 1.5 to 2.0 times longer than if applied over black steel and the repaint cycle is thus extended. This represents a significant cost savings over the life of the project.