| 304 stainless steel is widely used in many fields for its excellent corrosion resistance. However, in the solution presence of aggressive ion(Cl-), it is subjected to the serious localized corrosion. Hence, in order to improve the corrosion resistance of substrate, Ti N and TiN/Ti coatings are deposited by magnetron sputtering. The electrochemical behavior of substrate and coatings were investigated in 3.5 wt. % NaCl solution by polarization curves and electrochemical impedance spectroscopy(EIS). The electrochemical noise(EN) had been also introduced to monitor the corrosion behavior during the different immersion period. The noise resistance Rn, EN time records, as well as power spectral density(PSD) and wavelet transform had been employed to analyze the EN datum. Combined with the corrosion morphologies the corrosion mechanisms are discussed. The results are shown as follows:(1) Uniform and compact coatings are obtained on 304 stainless steel by magnetron sputtering, with a thickness of 0.5 ?m Ti N coating and 1.6 ?m TiN/Ti coating, respectively. When a Ti interlayer is introduce, the XRD and GDOES results show that the way of the deposited TiN is changed and the TiN(111) plane appear.. And the N/Ti atom ratio results show a significant decrease in coating surface.(2) The results of polarization curves and electrochemical impedance spectroscopy(EIS) show that the TiN and TiN/Ti coatings can improve the corrosion resistance of substrate. Furthermore, the TiN/Ti coating possesses a better corrosion protection, which decreases the corrosion current density and increases the corrosion potential, charge transfer resistance Rct and coating resistance Rc significantly by introducing the Ti layer.(3) The electrochemical noise analysis including noise Rn, PSD plots and EDP maps reveal the corrosion behavior during immersion period. The TiN coating possesses better corrosion protection during beginning immersion. With prolonging the immersion time, the corrosion protection of TiN coating decreases and stable pit occurs on the surface of TiN coating electrode. Combined with the corrosion morphology analysis, the failure of TiN coating is mainly due to galvanic corrosion occurring between TiN coating and substrate caused the spallation of coating. However, all the results show that TiN/Ti coating has an excellent corrosion protection during immersion period. The results of EDP further show that the TiN/Ti coating can inhibit metastable pitting transient to the stable pitting.(4) During immersion period, the TiN coating acts as a physical barrier between 304 stainless and corrosion media. And the protective performance of TiN coating decreases is main due to the presence of pinholes and spalling damage on coating surface. However, when introduced Ti layer, the quality of the coating has been improved significantly, and Ti layer can effectively interrupt the diffusion of Cl-and inhibit metastable pitting transient to the stable pitting. |
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