Study On The Pitting Process Of 304 Stainless Steel In 3.5% NaCl Solution By Electrochemical Methods

304 stainless steel was widely used in petrochemical, aerospace and many other fields for its good mechanical properties, processing properties and corrosion resistance, but it was prone to pitting corrosion in the corrosive environment. In this paper, the effects of temperature and dissolved oxygen(DO) on the initiation and electrochemical behavior of 304 stainless steel were studied by using the method of potentiostat polarization, potentiodynamic polarization and electrochemical impedance spectroscopy. Meanwhile, the impacts of temperature and dissolved oxygen on the geometric characteristics about the transition from metastable to steady state of 304 stainless steel were investigated by means of dynamic potential electrochemical impedance spectroscopy and three-dimensional video microscopy. The molar concentration of iron ion in pit hole was measured by ultraviolet spectrophotometry, the main research results are as follows:In the stage of metastable pitting initiation, with the temperature increased from 20 ℃ to 40 ℃, the initiation period of metastable pits was shorter, the nucleation number per unit time increased, the average peak current and the lifetime of metastable pits increased. The metastable pitting behavior was studied by potentiostat polarization, the parameters were 0.45 V and 1000 s. The nucleation number of metastable pits for 304 stainless steel were 8, 12, 34 and 72, the average peak current were 0.323 μA、0.336 μA、0.574 μA and 0.886 μA, the average life time were 5.27 s、4.98 s、3.08 s and 2.64 s in the solution with the DO concentration of 5.05 mg/L、9.78 mg/L、14.96 mg/L and 20.16 mg/L respectively. Under different DO content, the development speed(k) of the metastable pitting first increased slowly and when it reached a certain time(1~2 s before the passivation of metastable pitting) increased rapidly, and with the elevated of DO content the value of k increased gradually, that was k20.16>k14.96>k9.78>k5.05. Potentiodynamic polarization results showed that with the DO content increased, the Ecorr of 304 stainless steel decreased, icorr increased, the ability to repair passivation film increased, Eb decreased and the pitting corrosion resistance decreased. And Rct decreased and Rf gradually increased with the increased of DO content, which suggested that the increase of O2 both accelerated the corrosion process and conducive to the restoration of the passivation film.In the early stages of the process that metastable pit transfer to steady pit, the growth rate of pitting current, pit volume, pit mouth diameter and pit depth of single pit increased with the increasing temperature by applied potential of 0.15 VvsSCE. The growth rate of pit mouth diameter and pit depth gradully decreased with the extension of time under the same temperature. The surface of pit had incomplete lacy cover, and the integrity of lacy cover decreased with the increase of temperature. In 3.5% NaCl solution of different DO content with anodic overvoltage 0.45 V, the relationship between the growth current and time of single steady pit for 304 stainless steel was satisfied with the linear equation I=kt+b, and the value of k increased with the increase of the DO content. The growth rate of the diameter for stable pitting would be reduced after experiencing rapid growth for a period of time; At the first 30 s for pitting began to grow, the depth changed little; after that the growth rate of pit depth increased, pitting depth increased with the increase of DO content after same time.The molar concentration of iron ion in pit hole was measured by ultraviolet spectrophotometry and potentiostat polarization, and the value was 3.29 mol/L. By the method of deoxidized the simulated solution, determination the pH value change before and after the experiment and hydrogen collecting, it had proved that 304 stainless steel in simulated solution had the hydrogen reduction reaction. The electrochemical test results of 304 stainless steel in simulated solution showed that Ecorr and Icorr increased, Rct reduced, that indicated the corrosion rate in simulated solution was greater and showed a kind of uniform corrosion characteristics.