Steel Designers Manual - TheBestFriend.org

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Steel Designers' Manual - 6th Edition (2003)
reactions
at the anode
at the cathode
combined
Fe 2*
further oxidation
woter oir
H20 02
Fe —+ Fe2 + 2e
02 + 2H20 + 4e 4(0H)
Fe2 + 2(OH) Fe)OH)2
(ferrous hydroxide)
Fe)OH)2 —+ Fe2O3H20
The corrosion process 1031
(hydrated ferric oxide or rust)
Fig. 35.1 Diagrammatic representation of the corrosion of steel
metallic structure to the adjacent catholic sites on the surface, where they combine
with oxygen and water to form hydroxyl ions. These react with the ferrous ions
from the anode to produce ferrous hydroxide, which itself is further oxidized in air
to produce hydrated ferric oxide: red rust (Fig. 35.1).
The sum of these reactions is described by the following equation:
4Fe + 3O2 + 2H2O = 2Fe2O3H2O
(iron/steel) + (oxygen) + (water) = rust
Two important points emerge:
(1) For iron or steel to corrode it is necessary to have the simultaneous presence of
water and oxygen; in the absence of either, corrosion does not occur.
(2) All corrosion occurs at the anode; no corrosion occurs at the cathode.