How to Define the Chemical Composition Gradients Electrochemical and Optical Sensors

How to Define the Chemical Composition Gradients Electrochemical and Optical Sensors 

In corrosion domain, pH gradients are the most studied as local acidification or alcalinization are directly related to local electrochemical reactions as shown in Fig. 4. These pH changes control the nature of the species emitted, the homogeneous reactions at the vicinity of the surface. These reactions can induce precipitation, gaseous bubbling which can be modelled using a stationary approach (see part on modelling). 

Among amperometric sensors, the Scanning Electrochemical Microscope (SECM) is probably the most attractive but the less applied, even if SECM is a scarmed probe microscope (SPM) that has proven to be a powerful instrument for the quantitative investigation of a wide range of processes that occur at interfaces. The probe tip in SECM is an ultramicroelectrode (UME), which typically has a characteristic dimension in the 0.1-10 pm range. The amperometric, or potentiometric response of the tip UME is recorded as the probe is scarmed either normal to the interface of interest (tip approach measurements) or over the interface typically 

Current fluctuations at the substrate were observed within a few seconds, indicating pitting corrosion. In addition, the tip cm-rent also fluctuated, and correlated well with the sharp changes in current observed at the substrate. This behaviom was attributed to the release of iron species by the sample dttring the corrosion process, which was then reduced at the tip. 

On the other hand and in relation with the role of manganese sulphide in pitting of stainless steels already mentioned, R.C Al-kire, performed SECM experiments to demonstrate the role of adsorbed sulphur species near sttlphide inclusions like a driving force for pitting of Nickel. 

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