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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...

In scanning tunneling microscopy operation tip and object are routinely covered by adsorbates, hydroxides, or oxides. These undesirable layers are difficult to identify and to control in ultrahigh vacuum, high vacuum, or air. In an electrolytic environment the potentials of electrolyte–tip or electrolyte–substrate change the interfaces and, hence, the intermediate tunneling states, in a well-controlled way. Intermediate states nl(x) yield tunnel channels parallel to the direct one: I∝ exp(−2d√m2φ/ℏ) with d as the gap distance and φ≂φvac as the tunnel barrier height. With a uniform density nl the tunnel current decays exponentially with deff=d/(n+1) by n≥1 intermediate states thus simulating a reduced tunnel barrier height φeff=φ0(n+1)−2≪φvac. Exchanging nl(x) with x yields nonexponential I(d) dependences. Using the dependences of I on distance and voltage, the charge transfer processes, tunnel channels, and intermediate states in the oxide, hydroxide, and solid part of the electrolytic double layer can be identified. Atomic resolution has been found for n≤4 intermediate states. The localized states are proposed to be related to dipole resonances in the polarized and ordered interface HOH–HO− layers. Beside charge transfer between tip and substrate, dipole resonance also results in charge transfer along the polarized layer. So a floating electrode and, hence, imaging of organic molecules or Coulomb barriers, are possible.

Dimensional changes in electroactive films of nickel hydroxide, iridium oxide and polyaniline were investigated in situ in the electrochemical environment by scanning tunneling microscopy (STM), atomic force microscopy (AFM) and intermittent contact atomic force microscopy (ICAFM) during electrochemical oxidation and reduction. The advantages and disadvantages of the three measuring techniques are demonstrated with specific examples.While STM measurements are ambiguous due to the changing conductance of the sample during the redox process, AFM measurements on soft films such as anodic iridium oxide (AIROF) may result in surface modifications. Intermittent contact AFM (ICAFM) appears to be best suited for most samples.Shrinking of up to 40% could be observed on nickel oxide films during oxidation, which depended upon the location on the sample investigated and on the measuring technique employed. For a polyaniline film with a reversible charge capacity of 4.3 mC/cm 2 corresponding to 75 ± 7 nm film thickness, a thickness increase of 10 ± 3 nm was observed during oxidation.Electroactive materials like conducting polymers and metal oxides are of interest to be used as electrode materials in charge storage devices, electrochromic displays, sensors and actuators [1][2][3][4][5]. During electrochemical oxidation and reduction, electroactive materials reveal pronounced changes in their chemical and physical properties. Dimensional and morphological changes of electroactive films may cause mechanical stress and pressure within a device, and may affect the adhesion of an electroactive film to its substrate as well as its long-term stability during repetitive electrochemical cycling.Electrochemical processes are conventionally investigated by a variety of techniques that involve measurement of the current response following a perturbation of the electrode potential (e.g. cyclic voltammetry, linear sweep voltammetry, potential or current step techniques). However, these approaches generally provide a macroscopic picture of behavior which may be interpreted in terms of a statistical ensemble of atomic-or molecular-level events. While spectroscopic methods and scanning electron microscopy, as well as scanning tunneling microscopy (STM) and atomic force microscopy (AFM), can be used for ex situ characterization of electrodes, scanning probe microscopy (SPM) is uniquely capable of direct in situ characterization of the electrode-electrolyte interface. STM allows simultaneous measurement of the electrode topography and the electric current between a sharp tip and the electrode, while AFM permits the simultaneous measurement of the electrode topography and the interaction force between a small tip integrated in a cantilever and the electrode. Therefore, SPM techniques provide an opportunity to characterize the surface properties and their changes in situ at the nanometer scale [6][7][8]. Furthermore, it offers the advantage of studying material changes (crack formation, swelling, shrinking) directly in the electrolytic environmen...

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