Understanding the role of snout contamination in the formation of an oxide based defect in hot dip galvanised coating

Due to increased demand for defect free, quality critical outer panel material for the automotive sector, continued focus on zinc coating quality is required. The snout area of a continuous galvanizing line is often a major source of coating issues with various surface defects arising from poorly understood and uncontrolled snout practices. This paper investigates the formation of a snout defect termed 'the arrowhead defect', named after its characteristic arrowhead shape. Defective samples have been characterised with the use of SEM/EDX and XRD and compared with contaminated sources collected from within the continuous galvanizing line snout. It is common practice to inject wet HNx into the snout in order to inhibit the production of zinc vapour. The wet HNx promotes the formation of a ZnO layer on the surface of the liquid zinc bath, preventing vaporisation and thereby reduces zinc dust contamination. The presence of ZnO, deliberately formed through the injection of wet HNx into the snout was observed within the arrowhead defect and can be identified as the root cause of this defect. Discrete contamination particles were entrained within the tail of the defect. XRD patterns of both the defect & snout contamination have been presented to discern the nature of the contamination entrained within the zinc coating. Characteristic ZnO peaks were observed at θ = 32° for both surface contamination and at increasing penetration depths within the coating in the region of the arrowhead defect. The inclusion of the arrowhead defect in Full Finish material led to an increase in the rate of corrosion 2.5 × that of the corrosion rate on non-defective material, highlighting the need to produce defect free galvanized steel for both aesthetic and corrosion purposes. Whilst the injection of wet HNx as a method of suppressing the formation of zinc vapour is a long standing process for automotive Full Finish production, due to the nature of these oxide-based defects, this process is in fact a “double edged sword” in that it solves one problem but can create another. Alternative techniques to suppress zinc vapour formation should be investigated to further drive up the quality of zinc coatings for automotive applications.