A new approach for estimating whether a present condition satisfies a critical one for initiating propagating corrosion cracks was developed. A change in potential fluctuation (potential noise) of a stressed specimen was measured in a chloride solution, with stepwise increase in temperature up to 80ºC. A fully sensitized UNS30400 (type-304) stainless steel was used for the specimen. Under the employed conditions, the potential fluctuations are regarded as resulting from the initiation and repassivation of non-propagating stress corrosion cracks. The charge in each event was calculated from the amplitude of the potential fluctuation. The frequency distribution of the occurrence rate of the potential fluctuation was examined as a function of the charge. Propagating cracks occurred at a temperature of 80ºC and stress levels of 341 or 440 MPa. At this temperature and these stress levels, the histograms of charge frequency had significantly higher amplitude in the range 0.25 to 2.5 µC, in comparison with conditions where propagating cracks did not appear. It is considered that this approach would be a promising method for monitoring the initiation of stress corrosion cracking at actual plants.
The electrochemical noise measurement (ENM) is establishing a place as a method 1 for measuring polarization resistance, following the linear polarization method and electrochemical impedance spectroscopy (EIS). In general, it might be said that the polarization resistance is not a tool for localized corrosion but for general corrosion monitoring. However, the electrochemical noise (EN) itself has a potential for assessing localized corrosion activity, because the primary source of the EN on a Copyright passivated electrode is a transient current resulting from the initiation and repassivation of metastable localized corrosion. Pioneer attempts 2-11 on the application of the ENM for localized corrosion activity were the use of spectrum analysis. The spectrum analyses of the EN have revealed some interesting probabilistic features in the initial process of localized corrosion. However, it may be said that comprehensive results that encourage the application of the spectrum analysis to a monitoring method at actual plants seem not to be shown. A method using localization index (LI)1,12 might be a representative approach for monitoring the localized corrosion activity. However, as it has been pointed out 13, the LI has the disadvantage that it is sensitive not only to the degree of the localization of corrosion activity but also to changes in the value of the mean current. Recently, a new method that might be immune to the change in the mean current was proposed 14; this method used the average charge from individual local dissolution for the analysis; the quantity was calculated with an equation delivered from the shot-noise theory. This method is unique but it seems not to be widely applied yet.
Stress corrosion cracking (SCC) is considered as the most serious corrosion damage in chemical plant, especially those for processing halide fluids. SCC is characterized by its high propagation rate in the cross-sectional direction relative to other localized corrosion phenomena, and it happens extensively on corrosion resistant materials, which are usually used in small thickness. Thus, the occurrence of the crack must be detected during the initiation stage or at a very initial stage of propagation, in order to monitor SCC successfully. Also, when we apply the ENM or other electrochemical method for localized corrosion monitoring, it must be kept in mind that we have to estimate a change happened on the huge area of structural material using information from the small area of working electr