Nondestructive evaluation of an environmentally friendly conversion coating for magnesium alloys using optical measurement techniques

Magnesium alloys have one of the highest specific strengths of all construction metals used. Specifically, magnesium alloy castings are used in the aerospace industry to reduce the weight of aerospace vehicles. Coating systems must be employed to prevent corrosion of these magnesium alloys as they are also the most corrosion prone construction metals. The use of chromium is employed for the conversion coating which forms the foundation of many of these coating systems. In an effort to phase these harmful chromates out of the coating system and continue to use magnesium alloys, an environmentally friendly conversion coating has been developed. This paper explores the best types of methods used to evaluate the thickness and coating coverage of the environmentally friendly conversion coating. Destructive and nondestructive techniques are developed to examine the thickness and surface coverage of this environmentally friendly coating. Specifically an eddy current measurement technique, light, confocal, scanning electron and transmission electron microscopy techniques are used to determine the coating thickness of the environmentally friendly coating through destructive evaluation. Three nondestructive evaluation techniques, including polarized light microscopy, infrared spectroscopy (Fourier Transform and Raman) and an infrared proximity sensor are used to determine surface coverage of the environmentally friendly coating.