Study On Surface Activity And Localized Corrosion Of 304 Stainless Steel In Acidic Chloride Solution

Local corrosion, as a common type of corrosion, leads to tremendous harms to national economy. Traditional electrochemical methods can only describe the homogeneous kinetics between metal/solution interface and could not conduct research in situ. To solve the problems, electrochemical scanning probe techniques emerged later. As one of the electrochemical scanning probe techniques, the scanning electrochemical microscope (SECM) method can be used to study the dynamic process of the corrosive electrode, and deeply investigate the growth and local breakdown of passive film, as well as the mechanism of the corrosion initiation at sub-micrometer or nanometer spatial resolution.In this paper, the SECM tip, which is the first working electrode, is made of a platinum wire sealed in glass capillary, and a 304 stainless steel electrode is used as the second working electrode (i.e. substrate electrode). By using the characteristic mode of generation/collection in SECM technique, the concentration profiles of the corrosion products above the 304 SS surface have been investigated by approach curve and area scan imaging of SECM, in combination with traditional electrochemical methods, such as cyclic voltammogram (CV) and chronoamperometry, etc.The main work is as follows:1. The shape of tip's CV corresponds to the situation of the substrate. The CV of tip should be associated with solution components, distance between tip and substrate as well as property of substrate in order to decide the reactions happened on tip and substrate. Then the relation of potential and reactions on tip or substrate can be attained.2. Base on the reaction on tip with different potential, the Faraday current image of microprobe can be measured by area scanning in the acidic sodium chloride solution. At the same time, the surface morphology and the electrochemical activity distribution of substrate electrode have been characterized. Moreover, the influence of distance of tip and substrate can be investigated. 3. The passive film forms as a proper potential was set on substrate. When the keeping time changes, the thickness of the film changes also. Then there is a relation between the time and the current. The defects of the passive film can be observed directly from the area scan image.4. Pitting corrosion occurs on 304 SS at proper potential. Some images of Faraday current are obtained with continuous scanning on the substrate after pitting corrosion happens. The localized corrosion dynamic process is investigated relating with approach curves and character of substrate. The results indicate that 304 stainless steel shows self-repair effect with the extension of time.5. The diffusion coefficient constant of ferrous ion in acidic chloride solution is calculated with the chronoamperometry method. With steady-state limiting current formula, the concentration of ferrous ion can be obtained. So is the concentration distributing of ferrous in solution. Furthermore, the process of corrosion can be discussed. The mechanism of pitting corrosion in different solution is preliminary studied, from which the conclusion that SO₄^2- can block the formation of pitting obtained.