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

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

Revised Pages

16.7 Forms of Corrosion • 687

Figure 16.21

Photomicrograph

showing

intergranular stress

corrosion cracking in

brass. (From H. H.

Uhlig and R. W.

Revie,Corrosion and

Corrosion Control,

3rd edition, Fig. 5,

p. 335. Copyright

1985 by John Wiley

& Sons, Inc.

Reprinted by

permission of John

Wiley & Sons, Inc.)

expansion. Also, gaseous and solid corrosion products that are entrapped internally

can give rise to internal stresses.

Probably the best measure to take in reducing or totally eliminating stress cor-

rosion is to lower the magnitude of the stress. This may be accomplished by reducing

the external load or increasing the cross-sectional area perpendicular to the applied

stress. Furthermore, an appropriate heat treatment may be used to anneal out any

residual thermal stresses.

Hydrogen Embrittlement

Various metal alloys, specifically some steels, experience a significant reduction in

ductility and tensile strength when atomic hydrogen (H) penetrates into the ma-

hydrogen terial. This phenomenon is aptly referred to ashydrogen embrittlement;the terms

embrittlement hydrogen-induced crackingandhydrogen stress crackingare sometimes also used.

Strictly speaking, hydrogen embrittlement is a type of failure; in response to applied

or residual tensile stresses, brittle fracture occurs catastrophically as cracks grow and

rapidly propagate. Hydrogen in its atomic form (H as opposed to the molecular form,

H 2 ) diffuses interstitially through the crystal lattice, and concentrations as low as sev-

eral parts per million can lead to cracking. Furthermore, hydrogen-induced cracks

are most often transgranular, although intergranular fracture is observed for some

alloy systems. A number of mechanisms have been proposed to explain hydrogen

embrittlement; most of them are based on the interference of dislocation motion by

the dissolved hydrogen.

Hydrogen embrittlement is similar to stress corrosion (as discussed in the pre-

ceding section) in that a normally ductile metal experiences brittle fracture when

exposed to both a tensile stress and a corrosive atmosphere. However, these two