关键词: 电化学噪声, 不锈钢, 点蚀, 动电位极化

Abstract:

A measurement system for detecting the atmospheric corrosion of stainless steels was built based on electrochemical noise technology. The electrochemical noise data have been collected for 304 stainless steels which were exposed to the simulated marine atmospheric environment. The pitting behaviors of 304 stainless steels were studied by analyzing the electrochemical noise results with the time domain spectrogram, statistic, frequency domain spectrogram and shot noise theory. Meanwhile, potentiodynamic polarization and morphological analysis were also used as assistant measurements. It has shown that the 304 stainless steel in the whole wet-dry cycle could be divided into three stages, including passivation stage, metastable pitting stage and stable pitting stage. During the passivation stage, the electrochemical current noise and electrochemical potential noise exhibited minor synchronous-perikinetie fluctuations. The corrosion events occurred at high frequencies with the low average electric quantity. During the metastable pitting stage, the electrochemical current noise and electrochemical potential noise increased exponentially and at the same time decayed abruptly, and the frequencies for corrosion events decreased with the increased average electric quantity. However, no pitting spots were observed from the SEM micrographs. During the stable pitting stage, the electrochemical current noise remained an exponential increase and abrupt decay, corresponding to the pulse in electrochemical potential noise which is characterized by an exponential decay following by an abrupt recovery. Meanwhile, the lifetimes of fluctuations in electrochemical current noise and electrochemical potential noise were longer. The frequencies for corrosion events became lowered with the significant rise of average electric quantity. In addition, some small and flat pitting spots could be found from the SEM micrographs. The potentiodynamic polarization technique can be applied to verify the nucleation of stable pitting. The pitting potential decreased sharply when the stable pitting was formed at the 304 stainless steel surface. The results obtained by the two methods were consistent with each others, proving that the measuring system can continuously monitor the pitting behaviors of 304 stainless steels in the simulated marine environments, and judges the occurrence of stable pitting.

Key words: electrochemical noise, stainless steel, pitting corrosion, potentiodynamic polarization