757 防腐手册 CORROSION PREVENTION MANUAL 第二册 一般信息(9)

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metal couples. Various types of concentration cells, where electron flow
occurs between areas of points of different electrical potential, are
also examples of galvanic corrosion. Pitting results when, in the pres-ence of a conducting solution, electron flow occurs between different
metals or between different points or areas on a metal surface exhibit-ing different electrical potential (Fig. 3 and 4).

(5)Filiform -- A thread-like type of corrosion characterized by unpredict-able directional growth which develops under protective coatings on cer-tain metals. It is usually found adjacent to fastener head and skins
edges (Fig. 5).

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(6)Stress Corrosion --This type usually manifests itself as fine cracks.
It occurs in alloys that are susceptible to cracking when exposed to a
corrosive environment while under a tensile stress (Fig. 6).

(7)Fretting Corrosion -- This type of corrosion damage occurs between close
fitting parts which rub together. The wear or chafing breaks down the
protective surface on either or both parts (Fig. 7).

(8)Microbial -- Occurs in integral fuel tanks and is caused by the presence
of bacteria and fungus in jet fuel. The fungus grows at the fuel/water
interface, and the metabolic products formed corrode the metallic struc-ture (Fig. 8).

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Exampleof Pitting Corrosion
Figure 1
Example of Exfoliation Corrosion
Figure 2

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GROUP  I  Magnesium and its alloys. Aluminum Alloys 1100,5052,5056,5356, 6061,6063 
GROUP  II  Cadmium, zinc, aluminum and their alloys (including Group I aluminum alloys) 
GROUP  III  Iron, lead, tin, and their alloys (except corrosion resistant steel) 
GROUP  IV  Chromium, nickel, titanium, copper and their alloys. Corrosion resistant steel, silver, graphite, tungsten [1] 

NOTE
MEMBERS OF ANY ONE GROUP ARE SIMILAR.
MEMBERS OF DIFFERENT GROUPS ARE DISSIMILAR.

[1] CORROSION RESISTANT STEELS CONTAIN 14% OR MORE CHROMIUM

Classification of Dissimilar Metals
Figure 3

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METAL ION CONCENTRATION CELL RIVETED LAP JOINT
OXYGEN CONCENTRATION CELL
ACTIVE -PASSIVE CFU
Galvanic Corrosion Concentration Cells
Figure 4

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BEFORE PAINT REMOVAL
ENLARGED DETAIL
AFTER PAINT REMOVAL
Example of Filiform Corrosion
Figure 5

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Example of Stress Corrosion Cracking
FIGURE 6

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