Determination of corrosion types for AISI type 304L stainless steel using electrochemical noise method

Abstract

Electrochemical noise measurements were made on AISI type 304L SS in 0.1% NaOH, 5% H₂SO₄, and 0.1 M FeCl₃ 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 FeCl₃ 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 H₂SO₄ 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.