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

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

Revised Pages

682 • Chapter 16 / Corrosion and Degradation of Materials

2.Avoid an unfavorable anode-to-cathode surface area ratio; use an anode area

as large as possible.

3.Electrically insulate dissimilar metals from each other.

4.Electrically connect a third, anodic metal to the other two; this is a form of

cathodic protection cathodic protection,discussed presently.

Concept Check 16.4

(a)From the galvanic series (Table 16.2), cite three metals or alloys that may be used

to galvanically protect nickel in the active state.

(b)Sometimes galvanic corrosion is prevented by making an electrical contact be-

tween both metals in the couple and a third metal that is anodic to these other

two. Using the galvanic series, name one metal that could be used to protect a

copper-aluminum galvanic couple.

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

Concept Check 16.5

Cite two examples of the beneficial use of galvanic corrosion.Hint:One example is

cited later in this chapter.

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

Crevice Corrosion

Electrochemical corrosion may also occur as a consequence of concentration differ-

ences of ions or dissolved gases in the electrolyte solution, and between two regions

of the same metal piece. For such aconcentration cell,corrosion occurs in the locale

that has the lower concentration. A good example of this type of corrosion occurs

in crevices and recesses or under deposits of dirt or corrosion products where the

solution becomes stagnant and there is localized depletion of dissolved oxygen. Cor-

crevice corrosion rosion preferentially occurring at these positions is calledcrevice corrosion(Figure

16.15). The crevice must be wide enough for the solution to penetrate, yet narrow

enough for stagnancy; usually the width is several thousandths of an inch.

Figure 16.15 On this plate, which was immersed in seawater, crevice corrosion has

occurred at the regions that were covered by washers. (Photograph courtesy of LaQue

Center for Corrosion Technology, Inc.)