Similarities of the Two:
Both galvanized and galvannealed steel are coated at the mill or plant and are designed for resistance against rust and corrosion. They fall under the ASTM A 653/A 653M category and they both have the same weight of coating. Finally, the beginning of the production process is the same with both types of steel going through the hot-dip process.
Differences in Appearance and Usage:
Galvanized steel is familiar to anyone who has looked closely at HVAC ducting or highway guardrails. It has a characteristic marbled pattern of light and dark gray patches called spangling.
If you’re looking for an industrial appearance on an unpainted surface, shiny galvanized steel certainly fits the bill. One thing that’s guaranteed is that no two pieces of galvanized steel ever look the same—each has its own unique, spangled pattern.
Galvannealed steel has a flat gray, slightly-matted finish. The surface is attractive. That’s a result of the annealing process, and it’s easy to paint because the zinc coating is more absorbent. It is often chosen for architectural applications and also in the auto industry.
Galvanized steel has a softer coating, making it easy to machine by manufacturing processes such as CNC machining. Galvannealed steel, on the other hand, is strong and highly ductile due to the annealing, and they have higher formability.
After going through the hot-dip process, the carbon steel goes through a secondary process to create a galvannealed coating. The zinc-coated steel is heated to a higher temperature, pulling the iron from the steel and causing it to mix with the zinc coating and create a zinc-iron alloy coating. The added iron is what makes the galvannealed metal have different properties than galvanized metal. It also contributes to a different appearance; galvannealed metal has a more consistent, matte finish made of 90% zinc and 10% iron. However, the presence of iron in the coating can cause it to have a red tint, but it does not mean that the steel is rusting. Galvannealed coatings are designated with an “A”.
Galvannealed steel is harder and stronger than galvanized steel – it is not easily scratched. This type of coating is also more easily painted without the need for a primer, although a pretreatment or primer will enhance the performance. Galvannealed coatings are usually painted because it increases their durability and hides the red tint that people associate with rust. Galvannealed metal can also be welded more easily; its high electrical resistance, melting point, and hardness allow for easy welding at lower currents. For these reasons, galvannealed steel is the most recommended for architectural applications, including steel door and frame construction.
