On-line laser-induced breakdown spectroscopy determination of magnesium coating thickness on electrolytically galvanized steel in motion.

The application of laser-induced breakdown spectroscopy (LIBS) for online analysis of novel Zn based alloy coatings during continuous production of galvannealed steel has been demonstrated. Field trials were carried out at the ThyssenKrupp Steel (TKS) pilot plant in Dortmund, Germany. For this purpose, a portable LIBS demonstrator was constructed and evaluated, based on a dual-pulse Q-switched Nd:YAG laser, operated at 1064 nm. This system was used to generate plasmas on the moving sample surface after the annealing process, in order to control on-line the thickness of Mg on electrolytically galvanized steel. For variable Mg thicknesses (depending on strip speed of the pilot line, 100-1200 nm), and for steel sheets with a predetermined and constant Zn thickness (of 2 or 9 μm), a satisfactory agreement between plant LIBS measurements and data from laboratory chemical analysis (dissolution of the metallic coating and subsequent inductively coupled plasma-optical emission spectroscopy (ICP-OES) analysis) of Mg coating thicknesses has been obtained. The effects of environmental conditions on field measurements (strip temperature, mechanical vibrations, moisture on surface, etc.) have been demonstrated to be negligible, whereas minimal damage (crater diameters less than 150 μm) to the sample surface was caused.