CORROSION OF MILD STEEL EXPOSED TO NATURAL TAPE-WATER BY PSEUDOMONAS DIMINUTA تآکل الحديد المطاوع المعرض لمياه الصنبور الطبيعية بواسطة بکتيريا السيدوموناس ديمنيوتا
Assiut University Bulletin for Environmental Researches
Pseudomonas diminuta was isolated from iron based deposits (rust layer) on nutrient agar medium. For corrosion's determination of mild steel (MS) two different techniques were used for Electrochemical Impedance Spectroscopy[EIS] expressed by (impedance [z, ohm], phase angle [degree]) and tafel polarization by using three test solutions (natural tape-water system [NTS], natural tape water containing iron-based deposits [NFS] and natural tape water containing Ps. diminuta and MS [NBS]). Parallel
... S [NBS]). Parallel to the above experiment, Colony Forming Units [CFUs/ml] in absence or presence of MS was determined (0-10 days). The results of impedance showed a decrease followed by fixed values and a vice versa results were obtained in case of phase angles ( o ) shift at high and low frequency respectively. Tafel polarization expressed by circuit potential (E) mV were reduced in the presence of bacteria compared to its absence) in anodic current density [uA/cm 2 ]. All data obtained were done after one and four days of incubation in the above three media, also CFU increased after one day in presence of MS. It can be concluded, that the above bacterial species contribute to the corrosion of MS. INTRODUCTION: The physical presence of microbial cells on the surface, in addition to their metabolic activities modifies electrochemical processes  . Adsorbed cells grow, reproduce and form colonies that are physical anomalies on a metal surface  . Under aerobic conditions, areas under respiring colonies become anodic and surrounding areas become cathodic  . Any geometrical factor that results in a high oxygen concentration in one area and a low concentration in another will create a differential cell  . A mature biofilm prevents the diffusion of oxygen to cathodic sites and the diffusion of aggressive anions, such as chloride, to anodic sites. Outward diffusion of metabolites and corrosion products is also impeded. If areas within the biofilm become anaerobic, i.e. if the aerobic respiration rate within the biofilm is greater than the oxygen diffusion rate, the cathodic mechanism changes [1, 2] . Microbes perform oxidation and reduction reactions that profoundly affect the stability of minerals in the environment, with consequences ranging from the promotion of acid mine drainage  to the bioremediation of organically polluted groundwater  .