1/4/2010
Corrosion
Not all spontaneous redox reaction are beneficial.
Corrosion
Natural redox process that oxidizes metal to their oxides and
sulfides runs billions of dollars annually. Rust for example is
not the direct product from reaction between iron and oxygen
but arises through a complex electrochemical process.
Rust: Fe2O3 • X H2O
Anode: Fe(s) g Fe+2 + 2eE = 0.44 V
Cathode:
O2 (g) + 4H+ + 4e- g 2H2O (l)
E = 1.23 V
Net: Fe+2 will further oxidized to Fe 2O3 • X H2O
Chapter 9.6
Conditions for Corrosion
Conditions for Iron Oxidation:
Iron will oxidize in acidic medium
SO2 g H2SO4 g H+ + HSO4+
Anions improve conductivity for oxidation.
Cl- from seawater or NaCl (snow melting) enhances rusting
Conditions for Prevention:
Iron will not rust in dry air; moisture must be present
Iron will not rust in air-free water; oxygen must be present
Iron rusts most rapidly in ionic solution and low pH (high H+)
The loss of iron and deposit of rust occur at different places on objects
Iron rust faster in contact with a less active metal (Cu)
Iron rust slower in contact with a more active metal (Zn)
Iron Corrosion; Chemistry
Most common and economically destructive
form of corrosion is the rusting of iron. Rust
is not a direct product of the reaction
between iron and oxygen but arises through
complex electrochemical process. The
features of a voltaic cell can help explain this
process.
Iron will not rust in dry air; moisture must be present.
Iron will not rust in air-free water; oxygen must be present
Iron rusts most rapidly in ionic solutions and at low pH (High H+)
The loss of iron and the
depositing of rust often occur at
different places on the same
object.
Iron rust faster in contact with a
less active metal (such as Cu)
and more slowly in contact with
a more active metal (such as Zn).
Corrosion Prevention
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