54
Fundamentals of Corrosion
5. Modification of corrosion inhibitors. Certain bacteria may convert nitrite
corrosion inhibitors used to protect aluminum and aluminum alloys
from nitrate 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 sulfuric acid may be as high as 10 to 12%
2. Production of hydrogen sulfide by sulfate-reducing bacteria
3. Production of organic acids
4. Production of nitric acid
5. Production of ammonia
As mentioned previously, macrobiological organisms are also capable of causing corrosion as well as fouling. These organisms consist of shells, mollusks,
barnacles, etc. In most cases, fouling presents more of a problem than corrosion. Because these organisms remain attached to the metal surface, their
accumulation on the bottom of a ship’s hull increases the drag and power
requirement. Such accumulations in heat exchangers impair heat transfer
and fluid flow, while in pipelines they may clog the pipeline as well as impair
fluid flow. Stagnation and low flow rates are conducive to biofouling.
As the organisms attach themselves to the metal, the underlying metal
is sheltered from dissolved oxygen and a crevice condition is created. The
metabolic by-products of these organisms are often acidic and therefore corrosive. In addition, the anaerobic conditions underneath the macroorganisms can favor the growth of anaerobic bacteria, which in turn accelerates
the corrosion of the metal.
3.8.1 Preventive Measures
There are many approaches that can be used to prevent or to minimize MIC.
Among the choices are:
1. Material change or modification
2. Environmental or process parameter modification
3. Use of organic coatings
4. Cathodic protection
5. Use of biocides
Fundamentals of Corrosion
5. Modification of corrosion inhibitors. Certain bacteria may convert nitrite
corrosion inhibitors used to protect aluminum and aluminum alloys
from nitrate 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 sulfuric acid may be as high as 10 to 12%
2. Production of hydrogen sulfide by sulfate-reducing bacteria
3. Production of organic acids
4. Production of nitric acid
5. Production of ammonia
As mentioned previously, macrobiological organisms are also capable of causing corrosion as well as fouling. These organisms consist of shells, mollusks,
barnacles, etc. In most cases, fouling presents more of a problem than corrosion. Because these organisms remain attached to the metal surface, their
accumulation on the bottom of a ship’s hull increases the drag and power
requirement. Such accumulations in heat exchangers impair heat transfer
and fluid flow, while in pipelines they may clog the pipeline as well as impair
fluid flow. Stagnation and low flow rates are conducive to biofouling.
As the organisms attach themselves to the metal, the underlying metal
is sheltered from dissolved oxygen and a crevice condition is created. The
metabolic by-products of these organisms are often acidic and therefore corrosive. In addition, the anaerobic conditions underneath the macroorganisms can favor the growth of anaerobic bacteria, which in turn accelerates
the corrosion of the metal.
3.8.1 Preventive Measures
There are many approaches that can be used to prevent or to minimize MIC.
Among the choices are:
1. Material change or modification
2. Environmental or process parameter modification
3. Use of organic coatings
4. Cathodic protection
5. Use of biocides