Computed Tomography – a New Tool Studying Hidden Corrosion

Due to the progress in Computed Tomography (CT) applied for dimensional control new fields in materials characterization are opened. Especially volume properties can be non-destructively and quantitatively analysed which are non-accessible by other methods. These possibilities are used to study time dependent properties of corrosion. As an example concrete cylinders with embedded steel bars as working electrode are investigated. The mass loss of the working electrode is determined by CT and compared with results from electrochemical and gravimetric measurements. The limitations of the evaluation process of corrosion by CT are studied using a simulation program for the radiographic projections. Introduction The progress in Computed Tomography (CT) applied for dimensional control, which has been reached in the last years, has opened new fields in materials characterization [1]. Especially volume properties can be non-destructively and quantitatively analysed which are non-accessible by other methods. First experiments to characterize corrosion phenomena are published in [2]. This work describes time dependent properties of corrosion. As an example concrete cylinders with embedded steel bars are investigated. The used experimental set-up, the procedure to evaluate the volume of the working electrode before and after different periods of corrosion from the CT data are described and analysed. Along with the investigations by CT electrochemical methods were used to monitor corrosion of steel in concrete. Therefore the galvanostatic pulse measurement was applied to describe the continuous corrosion process on the surface of the embedded steel cylinders. Moreover the mass loss of the embedded steel cylinders was postulated from the electrochemical data using Faraday’s law. Finally the results of the electrochemical studies and the results from the CT data were correlated to give a prognosis of the corrosion progress. In addition the differentiation between total and pitting corrosion is essential for the corrosion progress and first shown in these investigations non-destructively by CT. Sample preparation For the electrochemical investigations, mortar cylinders with embedded electrodes were produced (Fig. 1). The mortar cylinders had a height of 120 mm and a diameter of 80 mm. The concrete cylinder simulates a constant concrete cover (35 mm). In the centre of the specimen a small steel cylinder (working electrode) welded to a stainless steel filler wire is fixed. The steel cylinder had a diameter of 9 mm, a height of 10 mm and a surface of 4.1 cm2. In addition to the working electrode a counter electrode (platinised titanium net) was embedded (Fig. 1). The concrete mixtures were made according to Table 1.