Microorganism Temperature raI Psychrophiles -5°Cto20° 5°to30°c 20°cto45° Moderate thermophiles 40°Cto55° Extreme thermophiles up to 110.C Most of the organisms live in the mesophilic range of 20 to 45.C, which corresponds to the usual temperature on the surface of the earth. A. Corrosion of Specific Materials Microbially induced corrosion(MIC) may occur for metallic materials in many industrial applications. MIC has been reported in the following in- dustrial applications Industry Location of mic Chemical processing Pipelines, stainless steel tanks, flanged joints, natural river or well wate Nuclear power generating Copper-nickel, stainless steel, brass, and aluminum-bronze cooling water pipes carbon and stainless steel piping, and Water-saturated clay type soils of near nd a source of sulfate reducing bacteria Metalworking Increased wear from breakdown of machinery Onshore and offshore oil Mothballed and flooded systems, oil and gas and idling systems, particularly in environments soured by sulfate reducing bacteria-produced sulfides Water treatment Heat exchangers and piping Sewage ha Concrete and concrete reinforced structures Highway maintenance Culvert piping Aviation Aluminum integral wiring, tanks including fuel torage tanks MARCEL DEKKER. INC 270 Madison Avenue. New York. New York 10016
28 Chapter 2 Microorganism Temperature range Psychrophiles Psychrotrophes Mesophiles Moderate thermophiles Thermophiles Extreme thermophiles 5C to 20C 5C to 30C 20C to 45C 40C to 55C 55C to 85C up to 110C Most of the organisms live in the mesophilic range of 20 to 45C, which corresponds to the usual temperature on the surface of the earth. A. Corrosion of Specific Materials Microbially induced corrosion (MIC) may occur for metallic materials in many industrial applications. MIC has been reported in the following industrial applications: Industry Location of MIC Chemical processing Pipelines, stainless steel tanks, flanged joints, welded areas, after hydrotesting with natural river or well water Nuclear power generating Copper-nickel, stainless steel, brass, and aluminum-bronze cooling water pipes, carbon and stainless steel piping, and tanks Underground pipeline Water-saturated clay type soils of near neutral pH with decaying organic matter and a source of sulfate reducing bacteria Metalworking Increased wear from breakdown of machinery oils and emulsions Onshore and offshore oil and gas Mothballed and flooded systems, oil and gas handling systems, particularly in environments soured by sulfate reducing bacteria–produced sulfides Water treatment Heat exchangers and piping Sewage handling and treatment Concrete and concrete reinforced structures Highway maintenance Culvert piping Aviation Aluminum integral wiring, tanks including fuel storage tanks
Corrosion of metallic materials materials is hich microorganisms increase the rate of corrosion of metals and/or the susceptibility to localized corrosion in an aqueous envi 1. Production of metabolites. Bacteria may produce inorganic acids, organic acids, sulfides, and all of which may be 2. Destruction of protective layers. Organic coatings may be at tacked by various microorganisms leading to the corrosion of the nderlying metal 3. Hydrogen embrittlement. By acting as a source of hydrogen and/ or through the production of hydrogen sulfide, microorganisms may influence hydrogen embrittlement of metals 4. Formation of concentration cells at the metal surface and in par ticular oxygen concentration cells. A concentration cell may be formed when a biofilm or bacterial growth develops heterogene ously on the metal surface. Some bacteria may tend to trap heav metals such as copper and cadmium within their extracellular poly- meric substance, causing the formation of ionic concentration cells. These lead to localized corrosion Modification of corrosion inhibitors. Certain bacteria may con- vert nitrite corrosion inhibitors used to protect mild steel to nitrate while other bacteria may convert nitrate inhibitors used to protect aluminum and aluminum alloys to nitrite and ammonia 6. Stimulation of electrochemical reactors. An example of this is the evolution of cathodic hydrogen from microbially pro MIC can result from 1. Production of sulfuric acid by bacteria of the genus thiobacillus through the oxidation of various inorganic sulfur compounds. The concentration of the sulfuric acid may be as high as 10-12%0 2. Production of hydrogen sulfide by sulfate reducing bacteria n of 4. Production of nitric acid 5. Production of ammonia 6. Production of hydrogen sulfide Prevention There are many approaches that may be used to prevent or minimize MIC. among the choices are MARCEL DEKKER. INC 270 Madison Avenue. New York. New York 10016
Corrosion of Metallic Materials 29 MIC of metallic materials is not a new form of corrosion. The methods by which microorganisms increase the rate of corrosion of metals and/or the susceptibility to localized corrosion in an aqueous environment are 1. Production of metabolites. Bacteria may produce inorganic acids, organic acids, sulfides, and ammonia, all of which may be corrosive to metallic materials. 2. Destruction of protective layers. Organic coatings may be attacked by various microorganisms leading to the corrosion of the underlying metal. 3. Hydrogen embrittlement. By acting as a source of hydrogen and/ or through the production of hydrogen sulfide, microorganisms may influence hydrogen embrittlement of metals. 4. Formation of concentration cells at the metal surface and in particular oxygen concentration cells. A concentration cell may be formed when a biofilm or bacterial growth develops heterogeneously on the metal surface. Some bacteria may tend to trap heavy metals such as copper and cadmium within their extracellular polymeric substance, causing the formation of ionic concentration cells. These lead to localized corrosion. 5. Modification of corrosion inhibitors. Certain bacteria may convert nitrite corrosion inhibitors used to protect mild steel to nitrate, while other bacteria may convert nitrate inhibitors used to protect aluminum and aluminum alloys to nitrite and ammonia. 6. Stimulation of electrochemical reactors. An example of this type is the evolution of cathodic hydrogen from microbially produced hydrogen sulfide. MIC can result from 1. Production of sulfuric acid by bacteria of the genus thiobacillus through the oxidation of various inorganic sulfur compounds. The concentration of the sulfuric acid may be as high as 10–12%. 2. Production of hydrogen sulfide by sulfate reducing bacteria. 3. Production of organic acids. 4. Production of nitric acid. 5. Production of ammonia. 6. Production of hydrogen sulfide. Prevention There are many approaches that may be used to prevent or minimize MIC. Among the choices are
1. Material change or modification 2. Environment or process parameter modification 3. Use of organic coatin 4. Cathodic protection 5. Use of biocides 6. Microbiological methods 7. Physical methods The approach to follow depends upon the type of bacteria present. A tech nique that has gained importance in addition to the preventative methods is that of"simulation of biogenic attack. By simulation of the biogenic attack, quick-motion effect can be produced that will allow materials to be tested for their compatibility for a specific application. In order to conduct the simulation properly it is necessary that a thorough knowledge of all the processes and participating microorganisms be known. The situation may be modeled under conditions that will be optimal for the microorganisms re sulting in a reduced time span for the corrosion to become detectable IX. SELECTIVE LEACHING When one element in a solid alloy is removed by corrosion, the process is known as selective leaching, dealloying, or dezincification. The most com- non example is the removal of zinc from brass alloys. When the zinc cor- rodes preferentially, a porous residue of copper and corrosion products re- main. The corroded part often retains its original shape and may appear damaged except for surface tarnish. However, its tensile strength and pa ticularly its ductility have been seriously reduced. Dezincification of brasses takes place in either localized areas on the metal surface, called plug type, or uniformly over the surface, called layer type a plug of dezincified brass may blow out leaving a hole, while a water pipe having layer type dezincification may split opel a hole Conditions which favor dezincification are 1. High temperature 2. Stagnant solutions, especially if acidic 3. Porous inorganic scale formation Brasses which contain 15%o or less of zinc are usually immune. Dezincifi- cation can also be suppressed by alloying additions of tin, aluminum, ar- enic,or phosphorus. Other alloy systems are also susceptible to this form of corrosion, including the selective loss of aluminum in aluminum-copper alloys and the loss of iron in cast iron -carbon steels MARCEL DEKKER. INC 270 Madison Avenue. New York. New York 10016
30 Chapter 2 1. Material change or modification 2. Environment or process parameter modification 3. Use of organic coatings 4. Cathodic protection 5. Use of biocides 6. Microbiological methods 7. Physical methods The approach to follow depends upon the type of bacteria present. A technique that has gained importance in addition to the preventative methods is that of ‘‘simulation of biogenic attack.’’ By simulation of the biogenic attack, a quick-motion effect can be produced that will allow materials to be tested for their compatibility for a specific application. In order to conduct the simulation properly it is necessary that a thorough knowledge of all the processes and participating microorganisms be known. The situation may be modeled under conditions that will be optimal for the microorganisms resulting in a reduced time span for the corrosion to become detectable. IX. SELECTIVE LEACHING When one element in a solid alloy is removed by corrosion, the process is known as selective leaching, dealloying, or dezincification. The most common example is the removal of zinc from brass alloys. When the zinc corrodes preferentially, a porous residue of copper and corrosion products remain. The corroded part often retains its original shape and may appear undamaged except for surface tarnish. However, its tensile strength and particularly its ductility have been seriously reduced. Dezincification of brasses takes place in either localized areas on the metal surface, called plug type, or uniformly over the surface, called layer type. A plug of dezincified brass may blow out leaving a hole, while a water pipe having layer type dezincification may split open. Conditions which favor dezincification are 1. High temperature 2. Stagnant solutions, especially if acidic 3. Porous inorganic scale formation Brasses which contain 15% or less of zinc are usually immune. Dezincifi- cation can also be suppressed by alloying additions of tin, aluminum, arsenic, or phosphorus. Other alloy systems are also susceptible to this form of corrosion, including the selective loss of aluminum in aluminum-copper alloys and the loss of iron in cast iron–carbon steels
Corrosion of metallic materials X CORROSION MECHANISMS Most of the commonly used metals are unstable in the atmosphere. These unstable metals are produced by reducing ores artificially; therefore they tend to return to their original state or to similar metallic compounds when exposed to the atmosphere. Exceptions to this are gold and platinum, which re already in their metal state Corrosion by its simplest definition is the process of a metal returning to the materials thermodynamic state. For most materials this means the formation of the oxides or sulfides from which they originally started when they were taken from the earth before being refined into useful engineerin naterials. Most corrosion processes are electrochemical in nature, consisting of two or more electrode reactions: the oxidation of a metal(anodic partial reaction) and the reduction of an oxidizing agent(cathodic partial reaction The study of electrochemical thermodynamics and electrochemical kinetics is necessary in order to understand corrosion reactions. For example, the corrosion of zinc in an acidic medium proceeds according to the overall Zn+2H+→Zn2++H, This breaks down into the anodic partial reaction Zn→Zn2+ and the cathodic partial reaction 2H++2e→H The corrosion rate depends on the electrode kinetics of both partial reactions. If all of the electrochemical parameters of the anodic and cathodic parti eactions are known, in principle the rate may be predicted. According to Faraday's law a linear relationship exists between the metal dissolution rate at any potential VM and the partial anodic current density for metal disso where n is the charge number (dimensionless) which indicates the number of electrons exchanged in the dissolution reaction and f is Faraday constant(F=96,485 C/mol). In the absence of an external polarization, a metal in contact with an oxidizing electrolytic environment acquires spon taneously a certain potential, called the corrosion potential, Ecorr. The partial anodic current density at the corrosion potential is equal to the corrosion current density icor. Equation(4) then becomes MARCEL DEKKER. INC 270 Madison Avenue. New York. New York 10016
Corrosion of Metallic Materials 31 X. CORROSION MECHANISMS Most of the commonly used metals are unstable in the atmosphere. These unstable metals are produced by reducing ores artificially; therefore they tend to return to their original state or to similar metallic compounds when exposed to the atmosphere. Exceptions to this are gold and platinum, which are already in their metal state. Corrosion by its simplest definition is the process of a metal returning to the material’s thermodynamic state. For most materials this means the formation of the oxides or sulfides from which they originally started when they were taken from the earth before being refined into useful engineering materials. Most corrosion processes are electrochemical in nature, consisting of two or more electrode reactions: the oxidation of a metal (anodic partial reaction) and the reduction of an oxidizing agent (cathodic partial reaction). The study of electrochemical thermodynamics and electrochemical kinetics is necessary in order to understand corrosion reactions. For example, the corrosion of zinc in an acidic medium proceeds according to the overall reaction 2 Zn 2H → Zn H (1) 2 This breaks down into the anodic partial reaction 2 Zn → Zn 2e (2) and the cathodic partial reaction 2H 2e → H (3) 2 The corrosion rate depends on the electrode kinetics of both partial reactions. If all of the electrochemical parameters of the anodic and cathodic partial reactions are known, in principle the rate may be predicted. According to Faraday’s law a linear relationship exists between the metal dissolution rate at any potential VM and the partial anodic current density for metal dissolution iaM: iaM V = (4) M nF where n is the charge number (dimensionless) which indicates the number of electrons exchanged in the dissolution reaction and F is the Faraday constant (F = 96,485 C/mol). In the absence of an external polarization, a metal in contact with an oxidizing electrolytic environment acquires spontaneously a certain potential, called the corrosion potential, Ecorr. The partial anodic current density at the corrosion potential is equal to the corrosion current density icorr. Equation (4) then becomes
Chapter 2 The corrosion potential lies between the equilibrium potentials of the anodic and cathodic partial reactions. The equilibrium potential of the partial reactions is predicted by elec- trochemical thermodynamics. The overall stoichiometry of any chemical re- 0=Ev;B where B designates the reactants and the products. The stoichiometric co- efficients v; of the products are positive and of the reactants negative. The △Gis where Ai is the chemical potential of the participating species. If Reaction (6) is conducted in an electrochemical cell, the corresponding equilibrium potential Erey is given by AG=-nFE Under standard conditions(all activities equal to one), where AG represents the standard free enthalpy and E represents the stan- dard potential of the reaction. Electrode reactions are commonly written in the form (10) where vox i represents the stoichiometric coefficient of the"oxidizedspe- Box i appearing on the left side of the equality sign together with the ree electrons, and Vred i indicates the stoichiometric coefficients of the re- ducing species, Bred,i appearing on the right side of the equality sign, opposite to the electrons. Equation (10) corresponds to a partial reduction reaction and the stoichiometric coefficients vox i and Vredi are both positive By setting the standard chemical potential of the solvated proton and of the molecular hydrogen equal to zero, AH=0: A,=0, it is possible define the standard potential of the partial reduction reaction of with respect to the standard hydrogen electrode. The standard potential of an electrode reaction that corresponds to the overall reaction MARCEL DEKKER. INC 270 Madison Avenue. New York. New York 10016
32 Chapter 2 icorr V = (5) corr nF The corrosion potential lies between the equilibrium potentials of the anodic and cathodic partial reactions. The equilibrium potential of the partial reactions is predicted by electrochemical thermodynamics. The overall stoichiometry of any chemical reaction can be expressed by 0 = vi i (6) where designates the reactants and the products. The stoichiometric coefficients vi of the products are positive and of the reactants negative. The free enthalpy of reaction G is G = vi i (7) where i is the chemical potential of the participating species. If Reaction (6) is conducted in an electrochemical cell, the corresponding equilibrium potential Erev is given by G = nFE (8) rev Under standard conditions (all activities equal to one), 0 0 G = nFE (9) where G0 represents the standard free enthalpy and E0 represents the standard potential of the reaction. Electrode reactions are commonly written in the form vox,i ox,i red,i red,i ne = v (10) where vox,i represents the stoichiometric coefficient of the ‘‘oxidized’’ species, ox,i appearing on the left side of the equality sign together with the free electrons, and vred,i indicates the stoichiometric coefficients of the reducing species, red,i appearing on the right side of the equality sign, opposite to the electrons. Equation (10) corresponds to a partial reduction reaction and the stoichiometric coefficients vox,i and vred,i are both positive. By setting the standard chemical potential of the solvated proton and of the molecular hydrogen equal to zero, = 0; = 0, it is possible to 0 0 H H2 define the standard potential of the partial reduction reaction of Eq. (10) with respect to the standard hydrogen electrode. The standard potential of an electrode reaction that corresponds to the overall reaction n v H = v nH (11) ox,i ox,i 2(PH = 1 bar) red,i red,i (aH =1) 2 2