Microbial corrosion resistance

The finish on metallic materials such as stainless steel, whether it is a refined mill finish, polished to specific grit, or an electro-polished treatment, should complement system design and provide satisfactory corrosion and microbial activity resistance. 

The heart of corrosion science has been identified as electrochemical science coupled with the thermodynamic and kinetic values. Other limbs are oxidation and high-temperature oxidation of metals, protective coatings, passivity, inhibitors, microbial-induced corrosion, corrosion fatigue, hydrogen embrittlement and corrosion-resistant alloys. Having identified the limbs of corrosion science, it is instructive to examine how the various aspects came into existence over a period of time. 

Hilbert LR, Bagge-Ravn D, Kold J, et al. Influence of surface roughness of stainless steel on microbial adhesion and corrosion resistance. Int Biodeterior Biodegrad 2003 52(3) 175-85. 

XPS has also been applied recently in this field to copper films for the study of microbial effects [55], the behavior of UO2 in mineral water [56], corrosion-resistant amorphous alloys [57]-[59], and the hydrolysis of fluorozirconate glasses [60]. 

Alloying elements are added to improve mechanical and electrochemical properties of the metal. For exanple, it is a well-known practice to add chromium to steel to increase its corrosion resistance. However, alloying elements can sometimes have other inpacts as well they may affect the way the metal responds to the environment from a microbial corrosion point of view. 

According to Kurissery et al. [19], the first study reporting weldments as preferred spots for microbial colonisation dates back to 1950. Those researchers also quote some references where most of the corrosion failures in cooling water systems made up of corrosion-resistant alloys are around or within weldments. 

Almost no engineering material is safe from or immune to microbial corrosion. In Chapter 8 the vulnerability and susceptibility of copper and cupronickels, duplex stainless steels and concrete will be discussed in a brief and informative manner. I had my reasons for picking these materials copper and its alloys have the reputation of being poisonous to micro-organisms, duplex stainless steels are known for their high resistance to corrosion thanks to their duplex microstructures of ferrite and austenite, and concrete is widely used in both the marine and water industries because of its good performance and cost effectiveness. 

Soil resistivity The role of soil in the electrical circuitry of corrosion is now apparent. Thus the conductivity of the soil represents an important parameter. Soil resistivity has probably been more widely used than any other test procedure. Opinions of experts vary somewhat as to the actual values in terms of ohm centimetres which relate to metal-loss rates. The extended study of the US Bureau of Standards presents a mass of data with soil-resistivity values given. A weakness of the resistivity procedure is that it neither indicates variations in aeration and pH of the soil, nor microbial activity in terms of coating deterioration or corrosion under anaerobic conditions. Furthermore, as shown by Costanzo rainfall fluctuations markedly affect readings. Despite its short comings, however, this procedure represents a valuable survey method. Scott points out the value of multiple data and the statistical nature of the resistivity readings as related to corrosion rates (see also Chapter 10). 

MIC depends on the complex structure of corrosion products and passive films on metal surfaces as well as on the structure of the biofilm. Unfortunately, electrochemical methods have sometimes been used in complex electrolytes, such as microbiological culture media, where the characteristics and properties of passive films and MIC deposits are quite active and not fully understood. It must be kept in mind that microbial colonization of passive metals can drastically change their resistance to film breakdown by causing localized changes in the type, concentration, and thickness of anions, pH, oxygen gradients, and inhibitor levels at the metal surface during the course of a... 

Thus, we have a very limited number of case histories of structural applications with which to prove the durability of fibrous composites beyond dispute, although the evidence we have is very encouraging. This promising start is not surprising because, except in a very few special cases, they resist microbial organisms and are unaffected by electrolytic corrosion. 

The remote crevice assembly technique (see Chapter 19) is a research tool that allows one to separate the anode and cathode areas of a crevice corrosion test sample so that the current flowing between them can be measured with a zero-resistance ammeter. This technique is similar to the dual cell method, and it lends itself well to studies of microbial effects on crevice corrosion [7]. It allows direct measurement of microbial effects on both the initiation time and propagation rate for crevice attack, provided again that a suitable control experiment without the microbial influence can be done concurrently. The scime technique of separating the anode and cathode can be used to study the influence of microbes in biofilms on galvanic corrosion [li]. 

The choice of the appropriate material is decisive for resistance against microbially influenced corrosion. This means that before the choice of material can be made, what kind of impacts is has to resist needs to be considered. Microbial influencing factors must also be considered. Accordingly, in the presence of volatile sulfur compounds, e.g., in sewage pipelines, it is recommended not to use materials like unprotected concrete which may be destroyed by the end product of the microbial degradation process (in this case, sulfuric acid formed by Thiobacilli). Another example would be the choice of a stainless steel or of an alloy that cannot be attacked under the conditions of a biofllm and the complex metabolic processes occurring underneath it. If, for instance, a material has to be chosen for static reasons, this material has to be protected by a coating or a liner made of an inert material. All these examples are based on the consideration that all attack factors have been identified by a complete inventory. 

In brackish and wastewater systems, microbially influenced corrosion (MIC) can occur, especially in systems where equipment has been idle for extended periods. A 6% molybdenum alloy offers protection from manganese-bearing, sulfur-bearing, and generally reducing types of bacteria. Because of its resistance to MIC, Alloy 25-6Mo is being used in the wastewater piping systems of power plants. 

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