Scanning acoustic microscopy an application for evaulating varnish layer conditions non-destructively

For preventing corrosion and for surface protection metallic objects are commonly finished with layers of varnish. The integrity of the varnish and potential defect propagation influence the durability of the metal and hence are a measure of the quality of the finishing. Scanning acoustic microscopy provides high axial and lateral resolution, a sufficient penetration depth and is non-destructive. The goal of this work was the development of a method for detection and evaluation of delaminations of varnish layers on metallic surfaces. Investigated were samples containing one or two layers of varnish. One group contained priming only whereas the second group contained varnish on top of the undercoat. The surface integrity of the finishing was destroyed by a scratch through all finishing layers. Defect aging was then modeled by exposing the samples to a corrosion-friendly atmosphere. Scanning acoustic microscopy combined with signal analysis was performed for investigating the connectivity between the finishing layers and the substrate. A robust numerical deconvolution technique has been adapted and optimized to enable the separation of strongly overlapping pulses. Echoes originated at the substrate and the finishing layers have been localized and layer thicknesses/distances were estimated. Delaminated spots of the finishing were successfully evaluated using the proposed method.