Page 118 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN005M-206 June 15, 2001 20:25

196 Electrochemistry

IV
Cathode: Mn O 2 (s) + LiX + e − modynamically favored reactions are accelerated by the
presence of salts and acids to cause rapid corrosion:
III
Mn (O)OLi (s) + X − (215)
II
Fe (OH 2 ) 2+
Fe (s) + 2H 2 O/CO 2 6 + H 2
Anode: Li (s) + X − LiX + e − (216)
+ 2 HO(O)O , (221)
−
III
IV
Cell: Mn O 2 (s) + Li (s) Mn (O)OLi (s) (217)
II
2 Fe (s) +·O 2 ·+ 4H 2 O/CO 2 + 10 H 2 O 2Fe (OH 2 ) 2+
6
−
E. Fuel Cells + 4 HO(O)O . (222)
The hydrogen (H 2 )/oxygen (·O 2 ·) anode/cathode combi- This problem has been solved through the use of a sacrafi-
nation is the most highly developed fuel cell. It continues cial metal (usually aluminum) that is attached to the steel
tobeanessentialpowersourceformannedspacemissions, structure. The combination constitutes a Galvanic cell
which accounts for its advanced state of development. Be- with the iron as the cathode (+terminal), the aluminum
yond the practical problem of a gaseous fuel (H 2 ), both as the anode (−terminal), and the seawater as the elec-
electrode reactions require precious-metal catalysts (usu- trolyte. The relevant half reactions and formal potentials
ally platinum supported on porous carbon electrodes). As [Eqs. (223) and (224)] allow formulation of the cell, its
indicated in earlier sections, electrochemistry is limited voltage, and the equilibrium constant, K, for the cell re-
to pathways that involve one electron steps. Hence, the action [Eqs. (225)–(227)].
essential function of the electrocatalysts for H 2 oxidation II 2+ −
and ·O 2 · reduction is to provide such pathways for these Fe (OH 2 ) 6 + 2e Fe (s) + 6H 2 O
multi-electron transformations. E =−0.44 V vs NHE (223)
◦
II
Cathode: ·O 2 · (g) + 2H 2 O + 2 Pt (s) 2Pt (OH) 2 (s); (H 2 O) Al OH + CO 2 + 3e −
III
2+
5
II
+
2Pt (OH) 2 (s) + 4H O + 4e − 2 Pt (s) + 6H 2 O Al (s) +HOC(O)O + 5H 2 O
−
3
(218) E =−1.83 V vs HE (224)
◦
II
Anode: 2 H 2 (g) + 4 Pt (s) 4 PtH (s); Cathode (of battery): Fe (OH 2 ) 2+ + 2e −
6
+
4 PtH (s) + 4H 2 O 4Pt(s) + 4H O + 4e − (219) Fe (s) + 6H 2 O. (225)
3
Cell: ·O 2 · (g) + 2H 2 (g) 2H 2 O (220) Anode: Al (s) + HOC(O)O + 5H 2 O
−
III
2+
(H 2 O) Al OH + CO 2 + 3e − (226)
5
II
VIII. CORROSION; CATHODIC Cell reaction: 3 Fe (OH 2 ) 2+ + 2Al(s) + 2 HOC(O)O −
6
PROTECTION III
3 Fe (s) + 2(H 2 O) Al OH + 8H 2 O
2+
5
All metals (M) react with atmospheric oxygen (·O 2 ·)to + 2H 2 O/CO 2 K (227)
form surface films of metal oxides (MO x ). When this film
E cell = E Fe(II)/Fe − E Al(III)/Al
◦
◦
◦
is formed under controlled conditions, it produces an in-
ert ( passivated ) surface that precludes further reaction =−0.44 V − (−1.83 V) = 1.39 V
and corrosion. However, the oxide films on copper alloys ◦
E cell = [0.05915/n] log K
and structural steel undergo dissolution when exposed
to aqueous media that contain ·O 2 · and salts, acids, or log K = [6/0.05915]1.39 = 141; K = 10 141
bases, and the surface no longer is protected and corrodes
Hence, the steel structure is protected until the aluminum
(dissolves).
anodes are consumed.
A. Structural Steel/Aluminum Anodes
Structual steel in aqueous environments (e.g., oil- SEE ALSO THE FOLLOWING ARTICLES
production platforms in the Gulf of Mexico) undergoes
corrosive dissolution via a number of chemical reactions. ALUMINUM • BATTERIES • CHEMICAL THERMODY-
The protective oxide coating [Fe 2 O 3 ] is especially suscep- NAMICS • CORROSION • ELECTROCHEMICAL ENGINEER-
tible to removal by the acids, salts, and organic matter in ING • ELECTROLYTE SOLUTIONS,THERMODYNAMICS •
seawater, which leaves an exposed iron surface. Two ther- ELECTROLYTE SOLUTIONS,TRANSPORT PROPERTIES •