Investigation Of Repassivation Behavior And The Stability Of Passive Film On 316L Stainless Steel

The excellent corrosion resistance of stainless steel (SS) is attributed to a thin passive film (～nm) formed on the surface. And they have been widely used in engineering fields, such as petrochemicals, marine engineering, nuclear power industry and so on. In addition, SS has a good performance to repassivation. It means that if the passive film is damaged or destroyed mechanically or chemically, SS could repair the passivity sothat improve the life of material. Therefore, a new type electrode abrading experimental system was desined to test the repassivation kinetics. At the same time, by using electrochemical measurements and advanced surface characterization techniques, repassivation kinetics and stability of passive film are investigated in this paper.In conventional film removing techniques, serious plastic deformation inducing the change of electrode electrochemical activity, uncertain depassive area and inconformity of repassivation are the three main deficits on the film-free surface, which restrict the acquirsition of accurate repassivation kinetics. The new type of electrode abrading apparatus can efficiently measure the electrochemical transients during repassivition of metals, so it provide us a new method to investigate the repassivation behavior of metal materials in various aqueous environment.The repassivation behavior of 316L SS in 0.05M H2SO4 solution containing various Cl- concentrations is investigated by using the new type electrode abrading technique. The decay of transient current of is recorded under different potential. According to the high electric field ions conduction model (HFM), the relationships of log i vs. log t and log i vs.1/q are analyzed, which reveals that the repassivation process including three stages:anodic dissolution stage, high-field stage and transition stage (transient state to steady state). The reasons and mechnisms are studied to interpret the formation of each stage. The effects of applied potential and Cl- concentration on repassviation behavior are invstigated on 316L SS. The increase of applied potential and the decreace of Cl-concentration accelerate the repassivation rate. The addition of Cl- in 0.05M H2SO4 solution improves the competitive adsorption between Cl- and OH-, which reduces the formation rate of oxide and hydroxide and retards the repassivation on 316L SS. At high potential, the increase of Cl" concentration will result in the dissolution and breakdown of passive film, even pitting corrosion.The kinetics of anodic dissolution and film formation of 316L SS in borate buffer solution is studied in this paper. The influence of ohmic potential drop on current decay in the initial stage of repassivation is analyzed by solution resistance compensation technique, and the results indicate that the anodic dissolution of 316L SS dominates the intial stage of repassivation. Based on HFM, a new repassvation kinetics model is established by stages:before monolayer oxide completely covering film-free surface, the anodic dissolution of 316L SS and oxide formation are occurred simultaneously; after monolayer oxide covering surface, the model only considers the oxide growth. The kinetics of anodic dissolution, oxide nucleation and growth could be analyzed independently depend on the new model. The film formation kinetics of Fe, Cr and Ni reveals that Cr oxide is prior fomed on 316L SS, and converts finally to a bilayer structure of Cr-rich inner layer and Fe-rich outter layer.Real-time spectroscopic ellipsometry, X-ray absorption near edge structure (XANES) with synchrotron radiation and X-ray photoelectron spectroscopy are employed to investigate the mechanisms of passive film formtiaon on 316L SS in borate buffer solution. The results indicate that the growth of passive film shows two-stage HFM behavior (high-current regime and low-current regime). The passive film mainly consists with oxide or hydroxide of Fe and Cr, and The increase of potential and film formation time induces the increase of valence of Fe and Cr in the passive film. The results demonstrate the oxide or hydroxide of Fe and Cr could dissolve selectively with the change of potential. A little of Ni and Mo are also detected in passive film and the content tends to be constant with potential changing.By using electrochemical measurements, like Chronoamperometry (CA), Electrochemical Impedance Spectroscopy (EIS), and Mott-Schottky (MS) and so on, the corrosion resistance and semiconductor property are studied under various film formation potential and time in borate buffer solution. The results point out that the charge transition resistance and film thickness increase with the increase of film formation potential and time, which also inducing the enhancement of corrosion resistance of passive film. The passive film of 316L SS behaves respectively p-type and n-type semiconductor property in different potential range. As E<0.2 VscE,the passive film is n-type, the donor concentration decreases with the increase of film formation potential and time. Accoding to point defect model (PDM) and high-field model (HFM), the diffusion coefficient Do of oxygen vacancies or interstitial metal ions are calculated in the passive film. Do decays exponentially with film formation time. The diffusion coefficient Do can be a parameter to measure the stability of passive film:the increase of Do results in the enhancement of stability and corrosion resistance of passive film. For 316L SS in borate buffer solution, when the Do reaches～10-18 cm2 s-1, the passive film is considered to be a quasi-steady-state.