Failure analysis of pin prick defects in galvannealed sheet – A case study

A peculiar type of pin prick spots/dents on the surface of some galvannealed sheets was observed, causing significant appearance problems in the finished product. In the present study, the surface defect was characterized by visual inspection, optical microscopy, microhardness, scanning electron microscopy and EDX to understand the source and mechanism of the defects. In the visual inspection, these peculiar defects were found to align in one direction. The SEM examination exhibited three distinct regions of the surface defect apart from the matrix: (1) particle, (2) a dense layer surrounding the particle, and (3) a crater region surrounding the particle. The EDX analysis of these regions indicated enrichment of Al and Fe and depletion of Zn in the particle. The microhardness measurements of the dense layer and the matrix indicated an increase in hardness in the dense layer compared to the matrix. Based on the preliminary results, the following hypothesis was made: Dross/zinc dust falling from snout wall or intermetallic phase or reaction product particles embed in the craters on the strip surface; during subsequent skin-pass rolling these particles peel out the zinc coating in the craters, thus densifying and flattening the region surrounding the particle. The paper presents the results of the investigation.