Effect Of Sulphate Reducing Bacteria And Shewanella Algae On Corrosion Of Zinc Sacrificial Anode Material

Equipment and materials used in the marine environment often caused biofoulingand corrosion due to attach of the microbial and organisms. This process significantlyreduced its performance and life, not only caused huge economic losses and serious,even imperil life safety. When a material or object immersed in sea water, the surface ofthe material was adhered by bacteria quickly. The bacteria could form a complexbiofilm between the bacteria and the substrate by way of secreting extracellularpolymeric substances (such as extracellular polysaccharide). Researchers have done alot of work about the microbial corrosion of metals in seawater, which of the moststudies are the microbial corrosion of carbon steel and stainless steel.Sacrificial anodes are widely used for protection of steel structures in marineenvironment due to their high theoretical current efficiency, low active potential andlow cost. Zinc anode outstanding advantage is its self-regulating features. Zinc anodecan protect better for metal facilities, so they are widely used on metal corrosionengineering. The influence of heat seawater and trace elements about the corrosioneffects of zinc anodes on the marine environment has some reported. While theinfluence of microbial film about the corrosion effects of zinc anodes on the marineenvironment has not reported.Hence, in this paper, the microorganism on corrosion behavior of zinc sacrificialanode has been studied using electrochemical impedance spectroscopy (EIS), weightloss, scanning electron microscopy (SEM) and fluorescence microscopy. This workmight be helpful on getting a better insight of evaluation of sacrificial anodes, and alsowell know about the influence of microbial film on the anode.At first, the sulphate reducing bacteria (SRB) and Shewanella were enriched andpurified cultivated from rusty scale. The Shewanella isolated from yellow rust layer wasShewanella algae identified by molecular biology techniques. The bacterial growthcurve was determined by the way of spectrophotometry. The results showed that the bacterial growth curve divided into exponential growth phase, steady phase and decayphase:1-4days for the exponential growth phase, the maximum number was reached infourth day;5-6days for the stabilize phase, the number of proliferation and the numberof deaths tended to balance, the number of bacteria was relatively stable;7-10days forthe decline phase, the reproduction of bacteria was getting slower and slower, thenumber of living bacteria declined rapidly because of depletion of limited nutrient.The EIS results showed that the Rctvalue in the sterile seawater was much smallerthan that of anode in natural seawater. This showed that the sample’s corrosion rate inseawater was less than the corrosion rate in sterilized seawater. Weight loss experimentshowed that the corrosion rate in sterilized seawater and in natural seawater were0.4998mm/a、0.4544mm respectively. This result was consistent with the EIS results.Pure zinc sacrificial anode had a similar law in natural seawater.The electrochemical results showed that corrosion potential for samples exposed tothe culture medium containing bacteria was higher than that for samples exposed to thesterile culture medium during the whole experiment. The EIS studies showed that Rctvalue in the culture medium containing bacteria was much greater than that of anode inthe sterile culture medium. This showed that the bacteria strains could inhibit thecorrosion of the specimen. The biofilm could prevent the sample from contacting withseawater directly and slow down the corrosion rate.The effects of Shewanella algae on corrosion of pure zinc were studied. Theelectrochemical results showed that corrosion potential for samples exposed to theculture medium containing bacteria was higher than that for samples exposed to thesterile culture medium during the whole experiment. The EIS studies showed that thebacteria strains could inhibit the corrosion of the specimen. The main reason was in theculture medium containing bacteria, the sample surface was formed a layer of biofilmand consumed the oxygen through the metabolic activities of bacteria.The corrosion potential for samples exposed to the culture medium containingmixed bacteria was higher than that for samples exposed to the sterile culture mediumduring the whole experiment period. The Rctvalue in the culture medium containingmixed bacteria was much smaller than that of anode in sterile culture medium. Thisshowed that the mixed bacteria could accelerate the corrosion rate of the specimen. Thereason was Shewanella algae consumed oxygen in the system to create good conditions for the growth of sulfate reducing bacteria that accelerated specimen corrosion. Purezinc sacrificial anode had a similar law in mixed bacteria system.SEM images revealed the presence of localized corrosion morphologies on thesample in the sterile seawater, while in natural seawater, the corrosion was muchuniform.SEM results showed that: at5d, in the culture medium containing bacteria, thesample surface was formed a layer of biofilm, while in bacteria-free system, the samplesurface had obvious corrosion pits and white corrosion products.In mixed bacteria system, at9d, in the culture medium containing mixed bacteria,the sample surface was formed a layer of biofilm, while in bacteria-free system, thesample surface had obvious corrosion pits and white corrosion products. The scanningelectron microscopy results verified and strengthened the electrochemical test results.Fluorescence microscopy results showed that uniformly attached to the sample first,with the growth and reproduction of bacteria, gradually reunited and thus form thebiofilms. At last, the biofilms detached from the sample because of the exhaustion ofnutrients and oxygen. All of them had similar growth rule though the formation time ofbiofilm was not the same.