Correlations of electrochemical noise, acoustic emission and complementary monitoring techniques during intergranular stress-corrosion cracking of austenitic stainless steel Jaka Kovac a, * , Carole Alaux b , T. James Marrow b , Edvard Govekar c , Andraz Legat a a Slovenian National Building and Civil Engineering Institute, Dimiceva 12, SI-1000 Ljubljana, Slovenia b Materials Performance Centre, School of Materials, The University of Manchester, Manchester, M13 9PL, United Kingdom c Faculty of Mechanical Engineering, University of Ljubljana, Askerceva 6, SI-1000 Ljubljana, Slovenia article info Article history: Received 14 October 2009 Accepted 12 February 2010 Available online 19 February 2010 Keywords: A. Stainless steel A. Acoustic emission B. Electrochemical noise B. Digital image correlation C. Stress corrosion abstract Specimens of sensitized type AISI 304 stainless steel were subjected to constant load and exposed to an aqueous sodium thiosulphate solution. Intergranular stress-corrosion cracking was monitored simulta- neously for electrochemical noise, acoustic emission, and specimen elongation. A section of the gauge length was monitored optically with subsequent analysis by digital image correlation. Correlations between the results were observed and analysed. Electrochemical noise and elongation are associated with crack propagation from the early stages, whereas acoustic emission is associated with the final stages of fracture. Digital image correlation analysis is sensitive to crack development, and is used to measure crack length and crack openings. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Stress-corrosion cracking (SCC) is a well known degradation process of metals and alloys. The process is potentially dangerous and can lead to catastrophic failures of structural components [1–3]. Stress-corrosion cracking occurs in specific combinations of three essential conditions: tensile stress or strain of a sufficient level, an aggressive electrolyte, and a susceptible material. Syner- getic combination of mechanical and electrochemical processes could lead to two different modes of crack propagation: intergran- ular SCC (IGSCC) where cracks advance along crystal grain bound- aries, and transgranular SCC (TGSCC) where cracks advance through crystal grains. Several decades of intensive SCC research activities have passed, yet the exact mechanism of the process in some critical environment-material combination is still unknown [4,5]. However, there are several mechanistic models that describe the SCC process for the various environment-material combina- tions that are understood. Among them there are three most known mechanistic models: slip-dissolution [6,7], anodic reaction induced cleavage (several different models: i.e. film-induced cleav- age [8–10], etc.) and surface mobility [11–14]. These models agree well with specific experimental results, but there remain some issues and details that need to be explained. In order to obtain general knowledge and determine specific parameters in SCC processes various physical and electrochemical methods have been implemented since a couple of decades ago. As these dynamic processes can be relatively fast, the main emphasis has been focused on the ‘‘so-called” transient techniques. Electro- chemical noise (EN) consists of current and potential fluctuations spontaneously generated by corrosion reactions, so non-intrusive- ness is one of the main advantages of this technique [15,16]. Several studies have indicated that, on the basis of EN measure- ments, it is possible to detect and distinguish between different corrosion types, such as metastable and stable pitting [17], crevice corrosion [18], uniform corrosion [19], and stress-corrosion crack- ing [20]. The main properties of EN have been usually described by statistical parameters (e.g. noise resistance) [21], as well as by parameters obtained from spectral analysis (e.g. noise impedance) [22], and the theory of chaos (e.g. the correlation dimension and the largest Lyapunov exponent) [23]. All these parameters are, however, calculated by means of mathematical techniques that are based on the assumption of the stationarity of signals, so that the reliability of estimated parameters could be questionable in specific cases [24]. In cases where the non-stationarity of EN sig- nals is caused by rare and irregular fluctuations, the analysis of individual phenomena that signify corrosion processes is still a very useful approach [25,26] (the term ‘‘noise” might, however, in such cases be questionable). After some positive results in the detection of SCC by EN in the 1980’s [27–29], research efforts were oriented mainly into the 0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.02.035 * Corresponding author. Tel.: +386 1 280 44 22; fax: +386 1 436 74 49. E-mail address: jaka.kovac@zag.si (J. Kovac). Corrosion Science 52 (2010) 2015–2025 Contents lists available at ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci