Galvanized Coating vs Galvannealed Coatings - Composition and Forming

Galvanized coatings differ from galvannealed coatings.  Both are made by the hot-dip coating process, but the main difference is in the production process where the galvannealed coating is further heated by passing it through a furnace directly above the coating tank.  The heat is approximately 545 degrees C and the sheet is subjected to the intense heat for a specific amount of time.  The iron sheet and the molten zinc coating alloys with each other by diffusion.  The final product has a coating on it that is approximately 90% zinc and 10% iron.  

A galvanized sheet coating is almost 100% pure zinc, with less than one percent content of aluminum to improve the adhesion between the coating and the steel sheet.  The galvannealed sheet also contains the small percentage of aluminum for the same reason.  

Why would you choose galvannealed coated material over regular galvanized coated material? Three reasons:  

  • Improved spot welding results 
  • It's easier to paint
  • Improved coating adhesion

Galvanized coatings scratch easily because they are soft.  Galvannealed coatings are hard so with handling, your chances of scratching the surface is minimized.  Because of the improved adhesion and it's inherent softness, a galvanized coated sheet can be formed into many different shapes without an compromise to the coating.  Galvannealed coatings are hard and brittle, but the sheet can be bent, stretched and drawn if proper procedures are followed.  

The properties of the steel sheet display no significant differences when comparing galvanized or galannealed coatings.  The main difference can be noted when punching the sheet - die type, die clearances, lubrication type and hold-down forces.