Effect of Annealing Time on Coating Microstructure, Frictional and Electrochemical Behavior of Galvannealed Interstitial-Free Steel

Abstract

The present study deals with the effect of isothermal holding time (10, 20, and 30 s) at a fixed annealing temperature of 500 degrees C on the formation of various Fe-Zn intermetallic phases during galvannealing treatment of a galvanized (GI) interstitial-free steel, and its correlation with the friction behavior determined by scratch test and electrochemical behavior in a freely aerated 3.5 wt. % NaCl solution. Pure Zn coating in GI specimen transforms to the various Fe-Zn intermetallic phases consisting of overlay zeta (zeta) phase with elongated rod type morphology, compact delta (delta) phase as well as a very thin layer of gamma (Gamma) phase at the coating-substrate interface during isothermal annealing treatment of the GI sheet. The zeta phase has been noticed to get consumed by the more compact delta phase and increase in thickness of the thin Gamma phase layer at the coating-substrate interface as iron content in the coating increases with an increase in galvannealing holding time. The galvannealed (GA) coating prepared at a galvanneal time of 30 s has the lowest coefficient of friction (COF similar to 0.26) as compared to 20 s (COF similar to 0.31) and 10 s (COF similar to 0.36), and the lowest corrosion rate (CR similar to 0.184 mm/y) as compared to 20 s (CR similar to 0.214 mm/y) and 10 s (CR similar to 0.227 mm/y). The dominance of compact and crater-free coating surface morphology along with the lowest zeta to delta (zeta/delta) phase fraction results in the lowest COF as well as the lowest corrosion rate of the GA coating prepared at a galvanneal time of 30 s.