passivated. With the exception of lead, as econdary barriero fc orrosion
products is not formed. Noblem etals do not provide cathodic protection
for the steel substrate because their corrosion potential is more noble than
those of iron and steel in an atural environment. Refer to Table 7.5.I n
multilayer coating systems, as mall difference in potential between coating
layersr esults in galvanic action in coating layers.
Noblecoating metals that provide corrosion protection by means of EMF
control include copper,silver,platinum, gold,and their alloys. The standard
single potentials of these metals are more noblet han those of hydrogen
(refer to Table7.6). Therefore, the oxidizer in corrosioncells formed on these
metalsi nanatural environment, containing no other particular oxidizers,
E c
E a
E corr
of
iron
O
2 + 2 H
2 O + 4 e
4 O H −
Ri corr of
coated iron
F e
F e
2 + +
2 e
Electrode potential
i corr of
coated iron
i corr of
uncoated
iron
Log current density
FIGURE 7.7
Resistance control protection. (From I. Suzuki. 1989. Corrosion Resistant Coatings Te chnology,
New York: Marcel Dekker.)
TABLE 7.5
Corrosion Potential of Noble Metals
CorrosionP otentials(V, SCE)
pH
2.9
6.5
Chromium
K 0.119
K 0.186
Nickel
K 0.412
K 0.430
Tin
K 0.486
K 0.554
Lead
K 0.435
K 0.637
Steel
K 0.636
K 0.505
Corrosion of Linings and Coatings
422
CAT8247—CHAPTER 7—17/10/2006—15:02—TRGANESH—XMLM ODEL B–pp. 403–485