Quantitative estimation of the corrosion rate of metallic materials using confocal laser scanning microscopy

This study represents an alternative method for quantitative estimation of the corrosion rate of various metallic materials, in particular magnesium and its alloys, using confocal laser scanning microscopy (CLSM). This method makes it possible to obtain not only optical images of the specimen surface, but also accurate 3D height maps which allow a quantitative assessment of many characteristics such as surface profile, damage locality, number and maximum depth of corrosion pits, volume of metal loss and corrosion products. Specific methodological aspects and possibilities of using the CLSM method for the analysis of corrosion damage have been described. The proposed method is based on obtaining 3D images of the surface of specimens after corrosion tests with their subsequent processing and analysis, with the use of original software computer procedures for digital image processing and topographic data arrays. A technique for recording and subsequent analysis of specimens after corrosion tests using CLSM has been developed and tested. The optimal magnification, lens type, scanning step and noise filtering have been determined. The procedure prescribes mounting the test metallic sample into a non-corrosive material to create a stable reference plane surface. The dissolution of the metal at the specimen-mount interface is found to be notably more intensive than in the specimen interior when the conventional metallographic resins are used as the mount. This fact deteriorates the objectivity of the results of corrosion rate measurements. Polyethylene is proposed as the mounting material to minimize this effect. The geometry and the minimum size of the metal sample, providing a reduction in labor contribution for scanning and data processing without loss of accuracy and reliability of the data are established. The values of the uniform corrosion rate of commercially pure magnesium in Ringer's solution using the gravimetric method and the CLSM method showed satisfactory repeatability of measurements.