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

GTBL042-16 GTBL042-Callister-v2 September 17, 2007 17:32

Revised Pages

662 • Chapter 16 / Corrosion and Degradation of Materials

Corrosion processes are occasionally used to advantage. For example, etching

procedures, as discussed in Section 5.12, make use of the selective chemical reactivity

of grain boundaries or various microstructural constituents.

16.2 ELECTROCHEMICAL CONSIDERATIONS

For metallic materials, the corrosion process is normally electrochemical, that is, a

chemical reaction in which there is transfer of electrons from one chemical species

to another. Metal atoms characteristically lose or give up electrons in what is called

oxidation anoxidationreaction. For example, the hypothetical metal M that has a valence of

n(ornvalence electrons) may experience oxidation according to the reaction

M→Mn++ne− (16.1)

Oxidation reaction

for metal M

in which M becomes ann+positively charged ion and in the process loses itsn

valence electrons;e−is used to symbolize an electron. Examples in which metals

oxidize are

Fe→Fe^2 ++ 2 e− (16.2a)

Al→Al^3 ++ 3 e− (16.2b)

anode The site at which oxidation takes place is called theanode;oxidation is sometimes

called an anodic reaction.

The electrons generated from each metal atom that is oxidized must be trans-

ferred to and become a part of another chemical species in what is termed a

reduction reductionreaction. For example, some metals undergo corrosion in acid solutions,

which have a high concentration of hydrogen (H+) ions; the H+ions are reduced as

follows:

2H++ 2 e−→H 2 (16.3)

Reduction of

hydrogen ions in an

acid solution

and hydrogen gas (H 2 ) is evolved.

Other reduction reactions are possible, depending on the nature of the solution

to which the metal is exposed. For an acid solution having dissolved oxygen, reduction

according to

O 2 +4H++ 4 e−→2H 2 O (16.4)

Reduction reaction

in an acid solution

containing dissolved

oxygen

will probably occur. Or, for a neutral or basic aqueous solution in which oxygen is

also dissolved,

O 2 +2H 2 O+ 4 e−→4(OH−) (16.5)

Reduction reaction

in a neutral or basic

solution containing

dissolved oxygen

Any metal ions present in the solution may also be reduced; for ions that can exist

in more than one valence state (multivalent ions), reduction may occur by

Mn++e−→M(n−1)+ (16.6)

Reduction of a

multivalent metal ion

to a lower valence

state