Electrochemical noise measurements have been performed on electrodes made from the same piece of stainless steel, of type AISI 304. Potential and current noise were measured under open circuit conditions in a three-electrode system, where one of the electrodes was kept under the load and acted as a common working electrode. Two types of electrode set- ups were used, differing in the method of load application. In the first case the common electrode was in the shape of a U- bend (under a static, undefined load), whereas in the second case the common electrode was subjected to a constant slowly increasing stress (the SSRT test). The first type of test was performed in a concentrated magnesium chloride solution at an elevated temperature. In order to detect the time when cracks initiated, the probes were dismantled daily, and examined visually using an optical microscope. SSRT tests were performed in diluted sodium thiocyanate solution with or without additions of chlorides. Electrochemical voltage and current noise and changes of stress and elongation were measured simultaneously. During the latter part of both types of tests, significant simultaneous spikes of voltage and current noise were observed. The spikes detected during the SSRT test were correlated with drops in stress and sudden increases in elongation. After the measurements had been performed, several characteristic time series (cracking non-active and cracking active) were subjected to spectral and chaos analysis. So far, analysis of such non-stationary signals, in particular of electrochemical systems (a U-bend exposed to a concentrated magnesium chloride solution) during uniform corrosion, cannot yet provide any satisfactory explanation of the complex mechanisms of stress-corrosion cracking processes
Stress-corrosion cracking (SCC) is widely known as an unpredictable and therefore dangerous type of localized corrosion. Since in many cases a very long time can elapse between the initiation of cracks and final failure, in recent years the detection of events concerned with the initiation and/or propagation of SCC has received considerable attention. A number of studies(?-?3)have proved that the electrochemical noise (ECN) technique is one of the most suitable methods for detecting the localized corrosion processes which can occur on passive metal surfaces. Measurement of electrochemical noise has been widely adopted as a satisfactory method for the detection of uniform, as well as pitting and crevice corrosionf3-~.On the other hand studies of the complex SCC processes by means of this method are still rare(713).Since several mechanisms have been proposed for SCC initiation and propagation M.Is) different electrochemical noise characteristics could be expected. In order to link noise fluctuations with the SCC process, suitable parallel techniques have to be employed during the ECN measurements. In a previous study(?o?)performed on flat electrodes under static load, optical microscopy in combination with a real-time computer visualization technique were used. Due to insufficient resolution of the measurement system, no direct correlation between the measured ECN and the digitized images of the corroded surfaces, was determined. Besides this type of test for SCC evaluation, two other commonly used tests for the rapid prediction of SCC resistance exist: the U-bend test and the slow strain rate test (SSRT)(?C).In first type of test, the largest amount of elastic and plastic strain is concentrated in the U-bend which, depending on the susceptibility of the metal and other factors, may or may not fail. The SSRT test, however, always leads to final failure ? brittle, ductile or mixed (the fractography depends on the sens
