50
Fundamentals of Corrosion
must be applied uniformly; it will not be effective if pitting occurs on the
compressive layer.
The use of inhibitors may reduce or even eliminate SCC by moving the
corrosion potential outside the range of cracking. For example, the addition
of a small amount of nitrates to concentrated NaOH prevents SCC of steel.
Substances such as Η 3 ΡΟ 4 , Na 2 O 4 , and CO(NH 2 ) 2 retard or prevent the cracking of iron in nitrates.
Another option, when practical and economically feasible, is to select a
different material of construction — one that is not susceptible to SCC in the
particular environment.
3.8 Biological Corrosion
Biologically influenced corrosion refers to the degradation of metals caused
by the activity of living organisms. Contributing to the corrosion are both
micro- and macroorganisms in a variety of environments, including domestic
and industrial fresh waters, soils, groundwater, seawater, natural petroleum
products, and oil-emulsion cutting fluids. Biologically influenced corrosion
does not represent a special form of corrosion but rather the aggravation
of corrosion under environmental conditions in which corrosion rates are
expected to be low.
Corrosive conditions can be developed by living microorganisms as a
result of their influence on anodic and cathodic reactions. The metabolic
activity can directly or indirectly cause deterioration of a metal by the corrosion process; this activity can:
1. Produce a corrosive environment
2. Create electrolytic cells on the metal surface
3. Alter the resistance of surface films
4. Have an influence on the rate of anodic or cathodic reactions
5. Alter the environmental composition
Because this form of corrosion gives the appearance of pitting, it is first necessary to diagnose the presence of bacteria. This is also referred to as microbial-induced corrosion (MIC).
The term “microorganism” covers a wide variety of life forms, including
bacteria, blue-green cyanobacteria, algae, lichens, fungi, and protozoa. All
microorganisms may be involved in the biodeterioration of metals. Pure
cultures never occur under natural conditions; rather, mixed cultures prevail. Of the mixed cultures, only a few actually become involved in the process of corrosion. The other organisms support the active ones by adjusting
the environmental conditions to support their growth. For example, in
Fundamentals of Corrosion
must be applied uniformly; it will not be effective if pitting occurs on the
compressive layer.
The use of inhibitors may reduce or even eliminate SCC by moving the
corrosion potential outside the range of cracking. For example, the addition
of a small amount of nitrates to concentrated NaOH prevents SCC of steel.
Substances such as Η 3 ΡΟ 4 , Na 2 O 4 , and CO(NH 2 ) 2 retard or prevent the cracking of iron in nitrates.
Another option, when practical and economically feasible, is to select a
different material of construction — one that is not susceptible to SCC in the
particular environment.
3.8 Biological Corrosion
Biologically influenced corrosion refers to the degradation of metals caused
by the activity of living organisms. Contributing to the corrosion are both
micro- and macroorganisms in a variety of environments, including domestic
and industrial fresh waters, soils, groundwater, seawater, natural petroleum
products, and oil-emulsion cutting fluids. Biologically influenced corrosion
does not represent a special form of corrosion but rather the aggravation
of corrosion under environmental conditions in which corrosion rates are
expected to be low.
Corrosive conditions can be developed by living microorganisms as a
result of their influence on anodic and cathodic reactions. The metabolic
activity can directly or indirectly cause deterioration of a metal by the corrosion process; this activity can:
1. Produce a corrosive environment
2. Create electrolytic cells on the metal surface
3. Alter the resistance of surface films
4. Have an influence on the rate of anodic or cathodic reactions
5. Alter the environmental composition
Because this form of corrosion gives the appearance of pitting, it is first necessary to diagnose the presence of bacteria. This is also referred to as microbial-induced corrosion (MIC).
The term “microorganism” covers a wide variety of life forms, including
bacteria, blue-green cyanobacteria, algae, lichens, fungi, and protozoa. All
microorganisms may be involved in the biodeterioration of metals. Pure
cultures never occur under natural conditions; rather, mixed cultures prevail. Of the mixed cultures, only a few actually become involved in the process of corrosion. The other organisms support the active ones by adjusting
the environmental conditions to support their growth. For example, in