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

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

Revised Pages

684 • Chapter 16 / Corrosion and Degradation of Materials

Figure 16.17 The pitting of a 304 stainless

steel plate by an acid-chloride solution.

(Photograph courtesy of Mars G. Fontana.

From M. G. Fontana,Corrosion Engineering,

3rd edition. Copyright©c1986 by

McGraw-Hill Book Company. Reproduced

with permission.)

Concept Check 16.6

Is Equation 16.23 equally valid for uniform corrosion and pitting? Why or why not?

[The answer may be found at http://www.wiley.com/college/callister (Student Companion Site).]

Intergranular Corrosion

intergranular As the name suggests,intergranular corrosionoccurs preferentially along grain

corrosion boundaries for some alloys and in specific environments. The net result is that a

macroscopic specimen disintegrates along its grain boundaries. This type of corro-

sion is especially prevalent in some stainless steels. When heated to temperatures

between 500 and 800◦C (950 and 1450◦F) for sufficiently long time periods, these al-

loys become sensitized to intergranular attack. It is believed that this heat treatment

permits the formation of small precipitate particles of chromium carbide (Cr 23 C 6 )

by reaction between the chromium and carbon in the stainless steel. These particles

form along the grain boundaries, as illustrated in Figure 16.18. Both the chromium

and the carbon must diffuse to the grain boundaries to form the precipitates, which

Grain boundary

Zone depleted of chromium

Cr 23 C 6 precipitate particle

Figure 16.18

Schematic

illustration of

chromium carbide

particles that have

precipitated along

grain boundaries in

stainless steel, and

the attendant zones

of chromium

depletion.