New approach to online monitoring of the Al depth profile of the hot-dip galvanised sheet steel using LIBS

In this study a new approach to the online monitoring of the Al depth profile of hot-dip galvanised sheet steel is presented, based on laser-induced breakdown spectroscopy (LIBS). The coating composition is measured by irradiating the traversing sheet steel with a series of single laser bursts, each at a different sheet steel position. An ablation depth in the same range as the coating thickness (about 10 µm) is achieved by applying a Nd:YAG laser at 1064 nm in collinear double-pulse and triple-pulse mode. The ablation depth is controlled by adjusting the burst energy with an external electro-optical attenuator. A fingerprint of the depth profile is gained by measuring the LIBS signals from zinc, aluminium and iron as a function of the burst energy, and by post-processing the data obtained. Up to three depths can be sampled simultaneously with a single laser burst by measuring the LIBS signals after each pulse within the laser burst. A concept for continuously monitoring the Al depth profile during the galvanising process is presented and applied to different hot-dip galvanised coatings. The method was tested on rotating sheet steel disks moving at a speed of up to 1 m/s. The potential and limitations of the new method are discussed.