With the aim of enhancing coating quality control during the galvannealing process, an on-line optical image acquisition sensor has been developed at "Centre de Recherches Métallurgiques" , Belgium. In the galvannealing process, the ζ phase surface density is a coating quality characteristic, and the on-line microscope, placed at 20 mm from the surface, grabs images on which ζ crystals can be clearly identified. The resulting images are then processed, and the results of the analysis can be used to adjust the process parameters in order to reach the desired coating characteristics (iron content, powdering) .