Microbiologically influenced corrosion of carbon steel from secondary circuit of nuclear power plant

Microbiologically influenced corrosion (MIC) is the initiation or acceleration of the corrosion due to the interaction between the microbial activity and the corrosion processes. During 1980s , the electric power industry, particularly the nuclear industry , has devoted increased attention to MIC that affect reliability, operating and maintenance costs of plant systems. The economic impact of failures in such systems in large nuclear units can be extremely costly, about USD 1,000,000 per day. Costs for chemicals and delivery systems for water treatments to prevent MIC and biofouling can approach about USD 1,000,000 per year. Several plants have been forced to undertake replacement or to make extensive repairs of their service water system at a cost of about USD 30,000,000 per plant. Nuclear power plant can exhibit MIC as a result of their basic design philosophy. The large number of stand-by and redundant systems in nuclear plant design establishes stagnant or intermittent flow conditions, a situation in which a number of systems, some of which are safety-related, will be susceptible to MIC. The large size of nuclear generating facilities and the often prolonged licensing process has resulted in an extended construction phase, often with structural materials in contact with stagnant, untreated water used for hydrostat testing. This also produces a condition amenable to microbial growth. Carbon steels may experience random pitting, general corrosion, or severe degradation in flow as a result of MIC. Tubercles (comprising corrosion products, microbes, sticky exopolymer associated with both living and dead cells, and debris) often form on carbon steel pipes and other components. The tubercles create a hydraulic resistance to cooling water flow as well as sites for additional microbial activity. Tubercles can grow together, eventually becoming a severe impediment to cooling water flow. Pitting is also often observed beneath tubercles as mechanical and chemical conditions are established that encourage localized corrosion effects.Once tubercles have formed, microbial activity at the metal surface (i.e. , beneath the tubercle) is effectively insulated from the bulk fluid, its level of aeration, and most water treatments. A wide variety of bacteria, from the aerobic acid formers to the more well known anaerobic sulfate reducing bacteria, can contribute to tuberculation and pitting of carbon steels. The correlation of all results obtained confirmed that the iron-oxidizing bacteria initiate a corrosion of the surfaces of SA 106gr.B carbon steel while SRB accelerate the corrosion of the surfaces of SA 106gr.B carbon steel. In sterile media (Vinogradski, Postgate)a lot of bacteria (sulphate-reducing bacteria, iron-oxidizing bacteria,etc) when this medium was not maintained in sterile conditions developed. This situation proves that in practical conditions, the corrosion of the metallic surfaces is induced by consortium bacterial. (authors)