Mild (unalloyed) steel electrodes were incubated in phosphate-buffered cultures of aerobic, biofilm-forming Rhodococcus sp. strain C125 and Pseudomonas putida mt2. A resulting surface reaction leading to the formation of a corrosion-inhibiting vivianite layer was accompanied by a characteristic electrochemical potential (E) curve. First, E increased slightly due to the interaction of phosphate with the iron oxides covering the steel surface. Subsequently, E decreased rapidly and after 1 day reached ؊510 mV, the potential of free iron, indicating the removal of the iron oxides. At this point, only scattered patches of bacteria covered the surface. A surface reaction, in which iron was released and vivianite precipitated, started. E remained at ؊510 mV for about 2 days, during which the vivianite layer grew steadily. Thereafter, E increased markedly to the initial value, and the release of iron stopped. Changes in E and formation of vivianite were results of bacterial activity, with oxygen consumption by the biofilm being the driving force. These findings indicate that biofilms may protect steel surfaces and might be used as an alternative method to combat corrosion.Due to the poor corrosion resistance of mild (unalloyed) steel, virtually all items made of this material have to be protected against corrosion. The most common protection method is phosphating, i.e., coating the steel surface with the phosphates of zinc, iron, or manganese (34). This procedure is carried out at temperatures up to 95°C and pH values between 2 and 3.5 (21). Media used for phosphating normally contain high concentrations of zinc (in the range of several grams per liter) or manganese and also contain accelerators like nitrate, nitrite, chlorate, peroxides, and organic nitrocompounds (31). During phosphating, a considerable amount of heavy metal sludge is formed and must be removed. Several attempts have been made to develop alternative methods that are less toxic to the environment.Pedersen et al. (17) showed that Pseudomonas sp. strain S9 and Serratia marcescens sp. strain EF 190 can decrease the corrosion rate of mild steel when applied as dense suspensions (10 9 ml Ϫ1 ) or as living biofilms (17)(18)(19). A protective effect of Pseudomonas fragi and Escherichia coli DH5 was found by Jayaraman et al. (13). Here, the formation of a biofilm was crucial, as oxygen depletion under the biofilm was responsible for the corrosion protection (12). However, the mechanical instability of biofilms was seen as a drawback for their technical application.In a recent study (32), we showed that growing the aerobic biofilm-forming bacteria Rhodococcus sp. strain C125 and Pseudomonas putida mt2 in mineral medium containing more than 2 mM phosphate induced a surface reaction on mild steel coupons, resulting in the formation of vivianite. Vivianite, a barely insoluble iron(II) phosphate, is one of the compounds formed in technical acidic phosphating and is known for its corrosion protective effect. The biologically vivianite-coated steel coupons showed goo...

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Mild steel coupons were incubated in cultures of three different aerobic bacteria, viz. Rhodococcus sp. C125, Pseudomonas pulida mt2 and Streptomyces pilosus DSM 40714 and then exposed to a corrosive aqueous medium. A significant reduction in the corrosion rate was observed in a corrosive medium when the steel had been incubated in mineral media containing more than 2 mM phosphate with growing, biofilm-forming bacteria which had direct access to the steel surface, but not with the non biofilm-forming S. pilosus DSM 40714. Rhodococcus sp. C125 and P. putida mt2 induced a surface reaction, resulting in the formation of vivianite. This barely insoluble iron(II)-phosphate was found to be the cause of the corrosion inhibition. The surface reaction was always accompanied by an increase in the iron concentration in the medium. In contrast to biocorrosion processes known so far, iron release stopped after some days. The results suggest that bacterial activity may induce the inhibition of corrosion of mild steel.

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