Bacterial Phosphating of Mild (Unalloyed) Steel

Abstract

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...

The dual role of microbes in corrosion

“…Also, Pseudomonas and Rhodococcus sp. biofilms were shown to produce corrosion-inhibiting layers of phosphate minerals on metal surfaces (Volkland et al, 2000(Volkland et al, , 2001. The EPS layer of biofilms has an important role in the mineral precipitation process, for example, the organo-metal (iron-EPS) complex in which the EPS interacts with the dissolved metal to form a complex (Chongdar et al, 2005;Decho, 2010).…”

Investigation of corrosion inhibitory process of marine Vibrio neocaledonicus sp. bacterium for carbon steel

“…Different mechanisms have been proposed to explain the corrosion inhibitory efficiency of aerobic bacteria. The main mechanisms are the oxygen depletion on a metal surface because of biofilm formation [9], formation of inorganic materials including vivianite [10], generation of antimicrobials by biofilms [11] to control SRB bacteria, production of a biofilm-secreted biosurfactant [12] and corrosion inhibition by bacteriophages [13].…”

Microbial Resource Management: The Road To Go for Environmental Biotechnology

“…are underway [91][92][93][94]. Of interest is the capacity to use ureolytic bacteria to generate a high pH micro-environment around their cells and thus enhance the precipitation of calcium carbonate and even magnesium ammonium phosphate [97][98]. Numerous reports deal with the recovery of P from wastewater, e.g., by means of the bacteria accumulating poly-P.…”