Inspection of Trivalent Chromium Conversion Coatings Using Laser Light: The Unexpected Role of Interference on Cold-Rolled Aluminium.

Joerg Rischmueller,Kai Brune,Mareike Schlag,Hauke Brüning,Yannic Toschke,Mirco Imlau

doi:10.3390/s20082164

Abstract

Laser-based inspection of trivalent chromium conversion coatings on rough, cold-rolled aluminium substrates is studied from a basic physics perspective by means of angle and wavelength dependent measurements. As a result, we show that the correlation between the scattered laser light and the coating weight of the conversion layer is dominated by the phenomenon of interference. The combined experimental and numerical approach of our study is based on an appropriate layer model which was developed from a set of reference measurements of confocal microscopy, electron microscopy and X-ray photoelectron spectroscopy. The aluminium alloy AA3003 with a trivalent chromium conversion coating serves as an example. Our derived model is capable to reconstruct the reflectance of a laser beam at grazing incidence even for a pronounced surface roughness of , for different coating thicknesses less than 70 corresponding to coating weights between zero and / and for laser wavelengths from 405 to 785 . In our discussion we outline the possibility to transfer the results to other aluminium alloys and/or other metallic substrates, as well as their potential for industrial applications such as 100% inline-capability, costs, velocity and ruggedness.

Highlights

It is impossible to imagine today’s everyday life without functional coatings modifying and enhancing the surface properties of different materials
While most publications focus on an improved understanding of the chemical formation process, the optimization of treatment conditions in terms of increased corrosion protection or use in situ spectroscopic ellipsometry (SE) as a tool for studying the growth kinetics of TCP coatings [15,16], this study primarily focuses on coating thickness measurements
In the first part of this chapter we present the results of the reference measurements, enabling us to perform an appropriate characterization of the samples

Summary

Introduction

It is impossible to imagine today’s everyday life without functional coatings modifying and enhancing the surface properties of different materials. Tailored for specific applications they serve a broad range of functionalities, for example the improvement of historic urban districts [1], drag reduction by means of riblets in the aircraft industry [2,3], increased biological compatibility in medical technologies [4,5] and corrosion protection in the light metal industry. One specific type of the latter used in this study are so called conversion coatings which can be applied as part of a pre-treatment process, providing excellent protection against environmental degradation and improving the adhesion of subsequently applied coatings at the same time. Sensors 2020, 20, 2164 coatings containing Cr(VI) in general. While those restrictions have led to further investigations of suitable alternatives, one of the most promising substitutes based on Cr(III) and Zr(IV)

Methods

Results

Discussion

Conclusion