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

GTBL042-16 GTBL042-Callister-v2 September 13, 2007 13:10

Revised Pages

690 • Chapter 16 / Corrosion and Degradation of Materials

Ground level

Earth

environment

Coated copper wire

Backfill

Rectifier

Tank

Current

Current Backfill

_ + Current

Gravel

Steel

pipe anodeMg

Anode

(a) ( b)

Figure 16.22 Cathodic protection of (a) an underground pipeline using a magnesium

sacrificial anode, and (b) an underground tank using an impressed current. (From M. G.

Fontana,Corrosion Engineering,3rd edition. Copyright©c1986 by McGraw-Hill Book

Company. Reproduced with permission.)

some situations, completely stop corrosion. Again, oxidation or corrosion of a metal

M occurs by the generalized reaction

Oxidation reaction M→Mn++ne− (16.1)

for metal M

Cathodic protection simply involves supplying, from an external source, electrons to

the metal to be protected, making it a cathode; the reaction above is thus forced in

the reverse (or reduction) direction.

One cathodic protection technique employs a galvanic couple: the metal to be

protected is electrically connected to another metal that is more reactive in the par-

ticular environment. The latter experiences oxidation, and, upon giving up electrons,

sacrificial anode protects the first metal from corrosion. The oxidized metal is often called asacrificial

anode,and magnesium and zinc are commonly used as such because they lie at the

anodic end of the galvanic series. This form of galvanic protection, for structures

buried in the ground, is illustrated in Figure 16.22a.

The process ofgalvanizingis simply one in which a layer of zinc is applied to the

surface of steel by hot dipping. In the atmosphere and most aqueous environments,

zinc is anodic to and will thus cathodically protect the steel if there is any surface

damage (Figure 16.23). Any corrosion of the zinc coating will proceed at an extremely

slow rate because the ratio of the anode-to-cathode surface area is quite large.

Zn2+Zn2+ Zn2+Zn2+

e– e–

e– e–

Corrosive environment

Steel

Cathode region

Zinc coating

(anode)

Figure 16.23 Galvanic protection of steel as

provided by a coating of zinc.