Why hot dip galvanizing?
Hot-dip galvanizing is highly effective in providing superior corrosion protection due to its multi-step process that creates a metallurgically bonded zinc-iron alloy layer. This acts as a barrier between the base metal and the environment. This method begins with preparing the raw metal through a five-stage process. Then we dip the project in a kettle of molten zinc at 835 degrees. The semi-liquid zinc coats not only the exterior, but also the interior walls of channels, posts, caprails. It also coats the intricate surfaces like scrollwork and collars on iron railings, property fences, and driveway gates.
As we pull the metal out of the molten zinc, it cools and the zinc solution drains, forming a “fudgy” texture with runs, sags, and icicles. This creates a distinct, textured finish called “spangle,” which gives a new fence or structure an authentic wrought iron look once painted or coated.
However, one challenge with hot-dip galvanizing is that welded areas can develop a “weld bulge.” Although we expertly smoothe the welds before galvanizing, the higher silicon content in the weld results in zinc buildup. This creates a slight protrusion of about 1/16” higher than the base metal. We must carefully sand and smooth these bulges, along with the icicles and sags, for quality control.
When hot dip galvanizing a tree statue, you need to make special provisions. It is important to vent all interior spaces so the air can escape as you lower the sculpture into the zinc. This allows the inside walls of the trunks and branches to be coated by the zinc. As you hoist the iron tree sculpture out, the liquid drains from the bottom while air enters from the top. You can then plug these drain holes with tapered zinc plugs and sand them smooth before powder-coating.
The duplex finish process—powder-coating applied on top of the hot-dip galvanized layer—eliminates corrosion concerns completely. The zinc provides cathodic protection, sacrificing itself to protect the base metal of the structure. However, ensuring proper adhesion between the zinc layer and the powder coating is vital. We achieve this by brush-blasting the galvanized surface before powder coating. Another issue to address is outgassing, caused by the rapid temperature change from room temperature to 835 degrees and back. This accelerated and decelerated temperature change causes moisture or gases to be trapped inside the base metal. When you apply the powder coating to this it sits on top and is held by electrostatic forces. During the curing process, the trapped gases can escape, causing eruptions in the finish.
To minimize these eruptions, we use a specific process:
- Place the raw custom iron project in the oven.
- Bake it at 400 degrees for 30 minutes.
- While the part is still above 200 degrees, spray the powder coating onto it, allowing it to melt and gel immediately.
When done correctly, this CIW-2135-I finish can be remarkable. The DOT rates this duplex finish system for up to 120 years with only a 20% failure rate. This finish guarantees both authenticity and longevity, letting your wrought iron designs—whether for exterior custom railings, forged fences, or driveway gates—be admired for generations.
