2005 Volume 46 Issue 7 Pages 1636-1645
Microbiologically Influenced Corrosion (MIC), otherwise coined as biocorrosion, is the influence of microorganisms on the kinetics of corrosion processes of metals, minerals and synthetic materials caused by their adhesion and growth. A closer observation of the MIC failure case analyses of engineering components showed that MIC occurs at or near welds. Preferential bacterial attack on the austenite phase leaving a skeleton like appearance is commonly reported on welds. The initial step of this phenomenon is the attachment of bacteria, which, over a period of time develop into a biofilm on the material surface. However, the intriguing question that remains to be answered is “Why welds are prone to preferential MIC attack?” Experiments on bacterial attachment on stainless steel surfaces revealed that surface roughness plays a pivotal role. Naturally, welds are with rough surfaces and that explains the preferential attachment of bacteria. Also, it was noticed that shape of the weld beads, determines the extent of bacterial attachment on to their surfaces. Leaving apart the surface roughness as the major contributing factor for the preferential bacterial attachment, studies were carried out to determine whether the underlying microstructure has any influence on it. Results suggested that there is. There was significant difference in percentage area of bacterial attachment between base metal and weld even after nullifying the surface roughness by polishing. Weld metal was preferred for attachment by bacteria more than base metal. Subsequent studies using an image superimposing technique revealed that grain boundary or the austenite/ferrite interface was preferred by bacteria as their initial attachment site. Welds have more such interfaces/or grain boundaries than base metal and thus resulting in more bacterial attachment, quickly leading to the initiation of MIC. The influence of inclusions, energy gradient and alloying elements are also discussed as factors associated with this phenomenon. In order to test the effect of elemental segregation occurring at grain boundaries on bacterial attachment, experiments were carried out using sulfur enriched stainless steel welds as well as high nitrogen stainless steels. Presence of sulfur and nitrogen as alloying elements enhanced bacterial attachment. Sulfur and nitrogen are essential elements for bacterial growth and their presence must have enhanced bacterial attachment. This result led us to the development of antibacterial metals. The hypothesis that worked out was if essential elements enhance attachment, toxic elements should deter it. Controlling bacterial attachment would be beneficial, both for deterring MIC and preventing infection and maintaining hygiene, especially in hospitals and food industries. Silver and copper, the well-known toxic elements were tried as alloying elements in stainless steel. While doing so, the material properties were kept largely in par with the silver/copper free stainless steel. The efficiencies of these antibacterial stainless steels were tested in the laboratory and were found to resist bacterial attachment and MIC. Silver containing stainless steels were tested in a freshwater environment as well. They were found to resist microfouling build up until a period of one month (the maximum duration of the study). Coupon exposure studies for longer duration are being carried out in our laboratory. In order to help in the development of high quality antibacterial metals, a base line data on the antibacterial efficacies of different candidate alloying elements are necessary. For this purpose, various metals were tested for their antibacterial efficacy using a standard method, known as film contact method as well as conventional coupon exposure tests. This chapter also discusses the merits and de-merits of antibacterial metals and their applications.
