ELECTROCHEMICAL NOISE AT CRACK INITIATION ON LOW-ALLOY STEEL IN HIGH-TEMPERATURE WATER

Taking as an example CERT-tests performed with a strain rate of 7.8 . 10(-6)/sec in oxygen containing (8 ppm) high temperature water on the low-alloy quenched and tempered steel 20 MnMoNi 5 5, the effect of mec hanical stress on the different corrosion processes such as crack init iation and crack growth could be proved and they could be distinguishe d from each other by means of electrochemical noise. This was realized by the development of an experimental measuring device which allows t o select the part which is important for the estimation of the corrosi on susceptibility of the metal surfaces from the large bandwith of the electrochemical noise. Thereby the transients which occur at crack in itiation and spalling of magnetite layers during the CERT-test are par ticularly obvious. Thus, new possibilities for the investigation and c ontrol of crack corrosion arise.