Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

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Revised Pages

16.2 Electrochemical Considerations • 669

SinceVis negative, the spontaneous reaction direction is the opposite to

that of Equation 16.22, or

Ni^2 ++Cd→Ni+Cd^2 +

That is, cadmium is oxidized and nickel is reduced.

The Galvanic Series

Even though Table 16.1 was generated under highly idealized conditions and has

limited utility, it nevertheless indicates the relative reactivities of the metals. A more

galvanic series realistic and practical ranking, however, is provided by thegalvanic series,Table 16.2.

This represents the relative reactivities of a number of metals and commercial alloys

in seawater. The alloys near the top are cathodic and unreactive, whereas those at

the bottom are most anodic; no voltages are provided. Comparison of the standard

emf and the galvanic series reveals a high degree of correspondence between the

relative positions of the pure base metals.

Most metals and alloys are subject to oxidation or corrosion to one degree or

another in a wide variety of environments; that is, they are more stable in an ionic

state than as metals. In thermodynamic terms, there is a net decrease in free energy

Table 16.2 The Galvanic Series

Platinum

Gold

Graphite

Titanium

Silver

316 Stainless steel (passive)

304 Stainless steel (passive)

[

Inconel (80Ni–13Cr–7Fe) (passive)

Nickel (passive)

[

Monel (70Ni–30Cu)

Increasingly inert (cathodic) Copper–nickel alloys

Bronzes (Cu–Sn alloys)

Copper

Brasses (Cu–Zn alloys)

⎡

⎢⎢

⎢

⎢

⎣

Inconel (active)

Nickel (active)

[

Tin

Lead

Increasingly active (anodic) 316 Stainless steel (active)

304 Stainless steel (active)

[

Cast iron

Iron and steel

[

Aluminum alloys

Cadmium

Commercially pure aluminum

Zinc

Magnesium and magnesium alloys

Source:M. G. Fontana,Corrosion Engineering,3rd edition. Copyright 1986 by McGraw-Hill

Book Company. Reprinted with permission.