Comparative study of the corrosion behavior of galvanized, galvannealed and Zn55Al coated interstitial free steels

• Hot dip coatings on steel galvanized (GI), galvanneal (GA) and Zn55Al were compared in 0.1 M NaCl.
• SVET shows that the protection on cut-edges is always by localized corrosion of the 3 coatings.
• Polarized Zn55Al and GI consume completely before perforation; protection due to FeAl3 on Zn55Al–
• Polarized GA perforates after consumption of ~ 70% of a equivalent GI coating
• At OCP dissolution of GA is slower than GI and Zn55Al and will provide longer protection.

In the present work, we comparatively studied the corrosion attack of Zn-based coatings on interstitial free steel. For this, the electrochemical behavior of industrially coated hot-dip galvanized (GI), galvanneal (GA) and galvalume (Zn55Al) interstitial free (IF) steels was analyzed by cyclic voltammetry in quiescent and stirred 0.1 mol/L NaCl solution and by scanning vibrating electrode technique (SVET). All three coatings protect the steel substrate by their localized corrosion leading to final coating perforation. However, Zn55Al corrodes more homogenously due to a well-distributed interdendritic η phase and the barrier effect of interfacial Al–Fe intermetallics. Due to its surface cracks, GA presents the opposite behavior, suffering a substrate attack with only 70% of the charge density expected for GI.