Environmental Engineering Reference
In-Depth Information
direction with respect to the corrosion potential of the metal N and the corrosion
rate increases to
i
corr(M-N)
.
Galvanic coupling may not always increase the corrosion rate of the anodic
member. Coupling of titanium or chromium with platinum or palladium drasti-
cally reduces the corrosion rate of the active metals through active-passive transi-
tion (Section 2.4.2). Polarization behavior of metals also influences galvanic cor-
rosion. For example, titanium shows excellent resistance in seawater, yet the
galvanic corrosion of a less resistant metal coupled to titanium does not increase
appreciably because titanium cathodically polarizes readily in seawater.
The selective attack shows a marked environmental dependence at times. In
a copper matrix-tantalum filament composite, the copper matrix has been re-
ported [1] to be selectively attacked after a brief exposure to 70% nitric acid,
whereas in 45% hydrochloric acid the tantalum filament has been attacked. The
passivation of tantalum in nitric acid and the lack of it in hydrochloric acid ex-
plains the difference in behavior.
3.3.2 Galvanic Series
The limitation of the emf series in the prediction of galvanic corrosion because
of kinetic factors involved has been discussed in the previous section. There are
other limitations as well. The emf series lists the half-cell potentials of metals
measured under standard conditions of the electrolytes, i.e., solutions containing
unit activity of ions of the respective metal and at a constant temperature. In
practical situations, galvanic coupling rarely occurs between metals in equilib-
rium with their ions. Thus, the actual driving force for galvanic corrosion is quite
different from the potential difference calculated from the emf series data. More-
over, the emf series lists only pure metals as it is not possible to establish a
reversible potential for alloys containing two or more reactive components. In
practice, alloys are widely used and the prediction of galvanic corrosion from
the emf series becomes difficult in couples involving an alloy or where both
members are alloys.
Passivity is another factor that influences the galvanic behavior of a couple
as explained in the previous sections. In the passive state, the metal behaves like
a nobler metal. The emf series does not provide for the passive state of metals.
The galvanic series has been formulated to overcome these difficulties.
The galvanic series is an arrangement of metals and alloys according to their
actual measured corrosion potentials in a given environment. Since the series is
meant for obtaining qualitative information about the tendencies for galvanic
corrosion, the measured potentials are not always indicated. Table 3.1 illustrates
the galvanic series of commercial metals and alloys in seawater.
It should be noted that alloys have been included in the series, as has a non-
metal of electrochemical interest, i.e., graphite. Active-passive metals and alloys
occupy different positions in the series. The positions of platinum and gold have
