Corrosion Behavior Of 304 Stainless Steel In Cultured Microbial Medium

Microbiologically influenced corrosion (MIC) is the corrosion caused or being affected by microbes. MIC is a very dangerous process, which badly affects all kinds of structure constructions in marine environment such as platform, pipeline for petroleum, etc., and causes tremendous loss to the civil economy. So the MIC of steel in marine environment has been paid extensive attention. The effect of MIC on steel is very complex. The species and amount, the spatial relationship with surface of steel, the extent of influencing corrosion, and function mechanism of microbes vary with exposed time.In the paper, variation of the number of bacteria in medium at different time was measured by method of the most probable number (MPN). Growth curve of bacteria was described in order to gain the vegetal rule of bacteria (divided into four periods: logy periods, logarithmic growth periods, steady growth periods, contabescence periods). Two different colonies were gained by separating and incubating the bacteria in seawater. One is round, ridgy and has flexuous brim, the other is punctuate, compressed and has orderly brim. The shapes of bacteria were also observed by TEM and obtained five different kinds of bacteria. It is the bacteria of micrococcus and bacillus which play an important role in corrosion of 304ss in this method of incubation.Samples of 304ss and Q235 were exposed to sterile medium and medium with bacteria respectively to prepare rust layers with and without microbes. Pictures of 304ss and Q235 after exposing different time (30-270days) indicate that the anti-corrosion of 304ss is better than Q235 on the whole. But under the effect of microbes, localized corrosion easily occured which restrains the application of 304ss in marine environment; SEM of the appearance of biofilm and corrosion product as well as surface morphology show that, in medium with bacteria, loose biofilm which is rich in corrosion product is formed on the surface of 304ss and induces pitting corrosion. The corrosion products formed in medium without bacteria also accelerate the corrosion of 304ss, but it is more slowly than in medium with bacteria; Open-circuit potential of electrode shows that the variation of corrosion potential is different between sterile medium and have bacteria's medium at the very start. The corrosion potential of 304ss in sterile medium moves positive rapidly, while in have bacteria's medium, it first moves negative rapidly and then moves positive; The electrochemical impedance spectrometry shows that the impedance value in medium without bacteria first increases and then decreases, while in medium with bacteria decreases on a whole; The weak polarization curves and the cyclic potention dynamic polarization show that corrosion rate of 304ss which were exposed to sterile medium differ a little from those exposed to have bacteria's medium in initial phase. The corrosion rate of 304ss in have bacteria's medium is bigger than that in sterile medium after exposing about 30 days. The longer exposure time it is, the larger the difference between these two corrosion rates is.Preponderant bacteria were cultured in solid medium in laboratory. The electrodes of 304ss with biofilm were prepared. The effect of biofilm on 304ss has been studied by electrochemical impedance spectroscopy. In initial phase, compact biofilm is formed on the surface of 304ss, which restrains corrosion. As exposure time goes, biofilm becomes loose which accelerates the corrosion of 304ss. Bacteria stop growing and biofilm begins to fall off when nutriment in medium were used up. Later, passivation formed on the surface of 304ss rapidly. Actually, biofilm postpones the formation of passivation.