High alloyed corrosion resistant Cr-Ni stainless steels are an important group of structural materials which thanks to their unique corrosion resistance properties are increasingly applied in different fields. Their corrosion resistance properties are based on a spontaneous occurrence of passivity due to a thin oxide layer formation on the steel surface in oxygen rich environments. This passive film provides protection against corrosion processes in many media, among others also in different types of water. Therefore, stainless steels are more and more often used for construction of different structures and plants in water management. Quite as a rule these are welded structures. However, a number of original material properties may change during the welding process as a consequence of the heat input. Heat tints formed on the surface, which represent oxide layers of substantially different chemical structure and corrosion resistance properties compared to the original unspoiled passive film, have a particularly harmful effect on the corrosion resistance. Apart from chlorides, as the most influential factor that may lead to depassivation and activation of different local corrosion phenomena on these steels, in natural waters special initiators of these phenomena are different types of microorganisms, primarily bacteria which through their metabolism lead to significant changes of the environment and conditions to which the stainless steel structure is exposed. The overall corrosion process is in this case moving from a two component system metal – medium into an unforeseen three component system metal – medium – biofilm, i.e., a community of different bacteria. So apart from having changes on the interphase metal – medium, which has a negative influence on the availability of oxygen needed for passivity, through life processes of bacteria present in the biofilm, different types of harmful and aggressive chemical components develop too. In addition, in unprocessed natural waters where bacteria have the ability to oxidize manganese, following the formation of the biofilm created by them, a quite dangerous phenomenon of anodic shift of potential may occur. The consequences are extremely intense corrosion damages, most often in the form of pitting corrosion which predominantly appears in the welded joints areas. In this work, apart from a detailed description of the most frequent corrosion phenomena and passivity properties, also the case of a sudden and unexpected microbiologically influenced damage to a water treatment plant where, during the experimental part of the work, field corrosion testings of the influence of surface condition after welding, type of water and basic material, welding method and flow in relation to the resistance of the microbiologically influenced corrosion of stainless steels in the welded joints area have been conducted. The results of field corrosion testings along with the conducted bacteriological testings, whereby the types of bacteria present were determined, strongly confirm the importance of the surface condition of welded joints in respect of the resistance of microbiologically influenced damage. Pitting damages appeared exclusively in the areas of untreated welded joints, i.e., in the areas where heat tints have not been removed. As an area particularly inclined to depassivation and activation of corrosion due to the influence of bacteria has been identified as a zone of colours of interference of 1st order, what has been confirmed also by the conducted laboratory testings. On the basis of the obtained research results a model of microbiologically influenced damage of the welded joints area in natural water in the presence of bacteria which oxidize iron and manganese has been defined.