Materials Science and Engineering A 407 (2005) 188–195 Determination of corrosion types for AISI type 304L stainless steel using electrochemical noise method S. Girija, U. Kamachi Mudali ∗ , V.R. Raju, R.K. Dayal, H.S. Khatak, Baldev Raj Corrosion Science and Technology Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, Tamilnadu, India Received in revised form 29 June 2005; accepted 15 July 2005 Abstract Electrochemical noise measurements were made on AISI type 304L SS in 0.1% NaOH, 5% H 2 SO 4, and 0.1 M FeCl 3 to study passivation, uniform, and localised corrosion, respectively. Current noise and potential noise were monitored simultaneously, using a three-electrode configuration, under open circuit condition. The experiments were carried out in a closed cell at ambient temperature, without deaeration. The time records revealed salient features of the nature of corrosion process. The localization index and standard deviation of current noise as a function of time for AISI type 304L SS in 0.1 M FeCl 3 depict localised corrosion as the nature of attack and the electrode surface showed significant pits. However, the localization index for AISI type 304L SS in H 2 SO 4 tends towards mixed corrosion although the surface exhibited uniform corrosion. The power spectral density plots of the potential noise were analyzed to correlate to the nature of attack on the electrode surface. An attempt was made to study the corrosion behaviour of AISI type 304L SS in 0.1% NaOH, followed by addition of chloride ions. A pit initiation transient was observed in the potential time record. Noise resistance appeared to be a promising parameter to monitor changes in the corrosivity of the environment and the slopes of the potential power spectral density plots of the potential noise, at the higher frequency region, reflected the activities at the surface. © 2005 Elsevier B.V. All rights reserved. Keywords: Electrochemical noise; Corrosion mechanism; Localization index; Power spectral density; Noise resistance 1. Introduction Electrochemical noise technique for corrosion applica- tions was first described by Iversion in 1968 [1]. It has gained popularity in the recent years and has emerged as a promising technique for corrosion analysis. Electrochemi- cal noise describes the low level spontaneous fluctuations of potential and current that occurs during an electrochemical process. During a corrosion process, which is predominantly electrochemical in nature, the cathodic and the anodic reac- tions can cause minute transients in the electrical charges on the electrode. These transients manifest in the form of potential and current noise, which can be exploited to map a corrosion event. Electrochemical noise can be measured in potentiostatically-polarized conditions and in freely corrod- ∗ Corresponding author. Tel.: +91 4114 280121; fax: +91 4114 280081/280301. E-mail address: kamachi@igcar.ernet.in (U.K. Mudali). ing systems [3]. By measuring electrochemical noise at open- circuit, the corrosion system is not disturbed by an external voltage or current source and hence no additional corrosion effects are induced. The instantaneous response to change in the conditions of a surface as it begins to corrode enables this method to be a powerful online corrosion-monitoring tool. The measurement of electrochemical noise involves the monitoring of fluctuations of current and potential, indepen- dently, either using a two-electrode configuration, or simulta- neously with a three-electrode configuration [2]. A statistical analysis of the resulting noise time record provides mecha- nistic information of the corrosion processes and corrosion rate.The mostraditional way to analyze electrochemical noise data has been to transform time records to the frequency domain in order to obtain spectral power density plots, which is computed using mathematical algorithms like fast fourier transform and maximum entropy method. The application of electrochemical noise technique for studying corrosion pro- cesses has been described by many investigators [2,4–10]. 0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.07.022