Online depth profiling of zinc coated sheet steel by laser-induced breakdown spectroscopy

During the production of galvanised sheet steel, key monitoring parameters are the thickness and chemical composition of the zinc coating. For this purpose a new method was developed for the online characterisation of galvanised sheet steel based on laser-induced breakdown spectroscopy (LIBS) and a transportable measuring unit was set up. Concepts for the determination of the coating thickness and the aluminium depth profile of hot-dip galvanised coatings were developed. To simulate the steel sheet velocity in the plant of up to 2 m/s LIBS measurements are performed on rotating sheet steel discs mounted on a rotary stage. The depth resolution is determined by using well characterised industrially coated sheet steels with different coating thicknesses (4.1 to 11.2 µm) and aluminium depth profiles as reference samples. A thickness resolution of better than 150 nm is achieved. For a measuring system designed for an online application the measurement time for an aluminium depth profile will be 10 seconds and for the determination of the coating thickness 200 ms. The impact of the LIBS craters on the sheet steel quality is minimised by using burst energies below 3 mJ. The influence of LIBS craters on the steel sheet quality was investigated by corrosion tests. WDX measurements, REM measurements and light microscopy and was found not to reduce the sheet steel quality.