Behavior Of Microbiologically Influenced Corrosion On 316 Stainless Steel In Marine Microbial Medium

Microbiologically influenced corrosion (MIC) of metal materials is a serious problem in the marine environment, with serious safety and economic concerns. MIC has been given severe attention in marine environment concerned of all kinds of structure constructions, such as pipeline, platform for petroleum, etc. However, the MIC of on metal materials is very complex, it still remains uncertain how large a role MIC plays in the corrosion of metal materials and by which mechanism it occurs in the marine environment. Accordingly, studying the formation rules, structural characteristics of biofilm on the oceanic materials surface and the correlation between the biofilm and electrochemical behavior of materials will be extremely important to explain mechanisms governing MIC and extend the applications of metal materials in the marine environment.In this paper, the corrosion behaviors of 316 stainless steel exposed to culture medium of marine microorganism and in sterile culture medium through different culture cycles were investigated by biology techniques, surface analytical techniques and electrochemical methods. Some conclusions of the formation rules of biofilm attached to 316SS surface and the variation of the adhesion and propagate of marine microorganism on the materials influence on electrochemistry property of 316SS have been achieved.Variation of the number of bacteria in medium with bacteria in different time was measured by method of most probable number (MPN) in Microbe experiment. Growth curve of bacteria was described in order to investigate the vegetal rule of marine mix bacteria (four periods: logy periods, logarithmic periods, steady growth periods, contabescence periods). TEM shows four different kinds of shapes of bacteria, and the proliferation of microbial cell by binary fission. The formation and distributing of biofilm on the 316 stainless steel surface exposed in medium with bacteria for different time was characterized by atomic force microscopy (AFM). with the increase of time,biofilm become thick and mature, the inhomogeneity of the biofilm induces the formation of corrosion micro-battery, the stainless steel occurs serious local corrosion owing to metal covered without biofilm was dissolved as anode and. SEM of the appearance of biofilm and corrosion product as well as surface morphology show that the formation of loose biofilm which is rich in corrosion product on the surface of coupon and the occurring of badly local corrosion damage of coupon surface beneath the biofilm after the 316SS was exposed in medium with bacteria for 90 days.Biofilm on metal surfaces produce an environment at the biofilm/metal interface that is radically different from that of the bulk medium in terms of pH, dissolved oxygen and so on. 316SS were exposed to sterile medium and medium with bacteria respectively to prepare the samples with and without microbe. Open circuit potential, polarization curves and impedance spectra curves of 316SS in culture medium with and without microbe were studied, and the variety of the corrosion behavior and mechanism of 316SS influenced by oceanic microbe were also investigated.Open circuit potential of electrode shows that the variation of corrosion potential is different between sterile medium and medium without bacteria. The corrosion potential of 316SS in medium with bacteria moves negative at first and then moves positive and level off at last. Polarization curves shows boifilm does not change the character of cathodic polarization, cathodic process is dominated by oxygen-diffusion. In culture medium with bacteria, the cathotic current density of 316SS electrode is increased and the corrosion potential moves towards negative severely as well as the effect of cathodic polarization related to bacteria accelerated the cathodic polarization process of 316SS electrode due to the presence and activities of microorganisms within biofilm. A distinct current peak of anodic polarization curve emerged due to the pitting corrosion and the cracking of the passivating film correlated to microbe. Passivation current is increscent and the passivation section moves toward negative. Charge transfer resistance is decreasing, EIS moves towards low frequency and appears two time constant in medium with bacteria. Biofilm promote charge transfer process of metal surface and accelerate local corrosion of 316SS. Equivalent circuit models of double deck film from EIS results bear out the presence of biofilm and passivating film on the metal Surface, Equivalent circuit models can effectively explain the extremely corrosion behavior and corrosion mechanism of 316SS MIC in marine microbial medium.