A Preliminary Study About The Influence Of Typical Factors On The Passive Stability Of Stainless Steel

With the development of the industrialization process, the application of stainless steels have a rapid growth in both in the field of industrial and daily life due to its excellent corrosion resistance and other valuable properties. The naturally formed passive oxide layers on the surface of stainless steels is the key factor that influence the corrosion process of stainless steel in some aggressive environments. Generally, the stability of the passive film is influenced by many factors,the alloy chemical composition, corrosive environment and the growth conditions of the oxide layers play important roles in the properties of these oxide layer, Consequently, studying the passive film formed on stainless steels are expected to be crucially important in understanding the protective characters against corrosion. This paper mainly studied different factors including alloy chemical composition and corrosive environment on the influence of the stability the passive film formed on the stainless steels, it not only has the theoretical significance for extending the service life of the stainless steels, but also the practical value for the application of stainless steels in different area.This paper used the wirebeam electrode to study the concentration of chloride ions on the stability the passive film formed on austenite stainless steel in simulating concrete pore solutions. Different concentration of chloride ions leading in different performances of passive film. The localized corrosion intensity index(LCII)of the 304 SS and 301 SS in 5mol/L NaCl increased gradually with the increasing of the immersion time, suggesting the stability of the passive film gradually reduced under the attack of chloride ions. In the concrete pore solution containing 2mol/L NaCl, LCII decreased gradually and tended to be stable, suggesting the passive film stability is good. In addition, due to the high content of Cr element, the passive film formed on 304 SS surface is more stable than the passive film formed on 301 SS surface, thus, Cr element has an important influence on stability on the passivation film.Effects of pH on the stability of the passive film formed 2507 duplex stainless steel was using atomic force microscope and other methods in the simulating concrete pore solutions. Mott-Schottky analysis suggested that the defect concentration of passive film decreased with the pH. CSAFM tests showed that with the decreasing of pH the current of austenite and ferrite were decreased, the passive film turned to be more homogeneous, which is consistent with the results of a M-S test. EIS results found that polarization resistance of 2507 duplex stainless steel in alkaline solution increased greatly in a short time and tend to be stable with the extending of the immersion time, the polarization resistance of pH9.5 nearly an order of magnitude high than the other two solutions. The results show that the stability of the passive film formed on stainless steel increased with the decreasing of pH.The influence of alloying elements on the stability of passive film was investigated by electrochemical noise and other techniques. The addition of Al and Ce element made the SSET have a fine grain structure and a well mechanical properties to meet the requirements for the expandable tubulars material. The existence of aluminum influenced the distribution of chromium oxides causing the existence of Cr deplete area of passive, pits nucleated and propagated in these positions and developed into stable pitting.