74
Fundamentals of Corrosion
internal stress is produced that splits off the overlying layers of metal, hence
the name exfoliation.
This is a dangerous form of corrosion because the splitting off of uncorroded metal rapidly reduces load-carrying ability. The splitting action continually exposes film-free metal, so the rate of corrosion is not self-limiting.
Exfoliation requires elongated (parallel-shaped) grains, a susceptible grain
boundary condition, and a relatively severe environment. Exfoliation corrosion is mostly found in certain alloys and tempers of aluminum. The most
damaging natural environments are those with high chloride ion content,
such as de-icing salts or a seacoast atmosphere. The presence or absence of
an applied stress has no significant effect. Coatings can delay exfoliation but
the best procedure is that of resistant temper.
3.12.1 Preventive Measures
Exfoliation can be minimized by the use of extended aging cycles for aluminum-copper alloys, the use of organic and sprayed metal coatings, by avoiding graphite-bearing lubricants that act as cathodes, and by promoting an
equiaxed grain structure at the surface and throughout the alloy.
3.13 Corrosion Fatigue
Corrosion fatigue is the cracking of a metal or alloy under the combined action
of a corrosive environment and repeated or fluctuating stress. As in stress
corrosion cracking (SCC), successive or alternate exposure to stress and corrosion does not lead to corrosion fatigue.
Metals and alloys fail by cracking when subjected to cyclic or repetitive
stress, even in the absence of a corrosive medium. This is known as fatigue
failure. The greater the applied stress, the fewer the number of cycles required
and the shorter the time to failure. In steels and other ferrous metals, no failure occurs for an infinite number of cycles at or below a stress level called the
endurance limit (also called the fatigue limit). In a corrosive medium, failure occurs at any applied stress if the number of cycles is sufficiently large.
Corrosion fatigue can therefore be defined as the reduction in fatigue life of
a metal in a corrosive environment. Unlike SSC, corrosion fatigue is equally
prevalent in pure metals and their alloys, and is not restricted to specific
environments. Any environment causing general attack in a metal or alloy is
capable of causing corrosion fatigue. For steels, the minimum corrosion rate
required is approximately 1 mpy.
Corrosion fatigue increases almost proportionately with the increase
of general aggressiveness of the corrodent. Consequently, an increase in
Fundamentals of Corrosion
internal stress is produced that splits off the overlying layers of metal, hence
the name exfoliation.
This is a dangerous form of corrosion because the splitting off of uncorroded metal rapidly reduces load-carrying ability. The splitting action continually exposes film-free metal, so the rate of corrosion is not self-limiting.
Exfoliation requires elongated (parallel-shaped) grains, a susceptible grain
boundary condition, and a relatively severe environment. Exfoliation corrosion is mostly found in certain alloys and tempers of aluminum. The most
damaging natural environments are those with high chloride ion content,
such as de-icing salts or a seacoast atmosphere. The presence or absence of
an applied stress has no significant effect. Coatings can delay exfoliation but
the best procedure is that of resistant temper.
3.12.1 Preventive Measures
Exfoliation can be minimized by the use of extended aging cycles for aluminum-copper alloys, the use of organic and sprayed metal coatings, by avoiding graphite-bearing lubricants that act as cathodes, and by promoting an
equiaxed grain structure at the surface and throughout the alloy.
3.13 Corrosion Fatigue
Corrosion fatigue is the cracking of a metal or alloy under the combined action
of a corrosive environment and repeated or fluctuating stress. As in stress
corrosion cracking (SCC), successive or alternate exposure to stress and corrosion does not lead to corrosion fatigue.
Metals and alloys fail by cracking when subjected to cyclic or repetitive
stress, even in the absence of a corrosive medium. This is known as fatigue
failure. The greater the applied stress, the fewer the number of cycles required
and the shorter the time to failure. In steels and other ferrous metals, no failure occurs for an infinite number of cycles at or below a stress level called the
endurance limit (also called the fatigue limit). In a corrosive medium, failure occurs at any applied stress if the number of cycles is sufficiently large.
Corrosion fatigue can therefore be defined as the reduction in fatigue life of
a metal in a corrosive environment. Unlike SSC, corrosion fatigue is equally
prevalent in pure metals and their alloys, and is not restricted to specific
environments. Any environment causing general attack in a metal or alloy is
capable of causing corrosion fatigue. For steels, the minimum corrosion rate
required is approximately 1 mpy.
Corrosion fatigue increases almost proportionately with the increase
of general aggressiveness of the corrodent. Consequently, an increase in