Analysis of an uncommon coating defect on industrial galvannealed high strength interstitial free steel

The surface quality of galvannealed coated steels has improved substantially with the use of integrated process control. However, some defects appear on the surface of these products. In this paper a hitherto unknown and uncommon defect found on the galvannealed coating of a high strength interstitial free steel has been subjected to in-depth study and analysis to determine the root cause of the defect. Scanning electron microscopy along with energy dispersive spectroscopy, quantitative depth profiling using glow discharge optical emission spectroscopy, auger electron spectroscopy and crystallographic texture analysis of the coated layer was employed for characterization purpose. It appears that formation of a sticky oxide layer of Fe2O3 during upstream processing is the root cause of this defect. The defect manifests itself as a mixture of galvanized layer coupled with an under-alloyed delta phase that forms over and above the pickling-resistant oxide layer during the galvannealing process.