Page 253 - Corrosion Engineering Principles and Practice

226 C h a p t e r 7 C o r r o s i o n F a i l u r e s , F a c t o r s , a n d C e l l s 227

FIGURE 7.16 Mangled communication tower fallen due to loss of the anchor
shown in Fig. 7.15. (Courtesy of Anchor Guard)

7.4.1 Galvanic Cells
It is important to realize that galvanic corrosion effects can be
manifested not only on the macroscopic level but also within the
microstructure of a material. Certain phases or precipitates will
undergo anodic dissolution under microgalvanic effects. Because
the principle of galvanic corrosion is widely known, it is remarkable
that it still features prominently in numerous corrosion failures.
Figure 7.17 illustrates the main factors affecting the formation of a
galvanic cell [14].
Any two metals can be used to make a galvanic cell. Whether a
metal will behave as an anode or a cathode in combination with
another metal in the same environment can usually be determined by
its relative position on a galvanic series. Figure 7.18 shows the
galvanic series of many metals and alloys in slow-moving seawater
and Table 7.5 presents the galvanic series which was constructed for
metals exposed to neutral soils and water [15].
Are these corrosion cells common? The answer is yes. Whenever
a copper pipe service line is directly connected to a cast iron gas or
water main, a galvanic cell is formed (Fig. 7.19) the soil is the
electrolyte, the copper service line is the cathode, the iron (or steel)
main is the anode, and the connecting circuit is completed by attaching
the line to the main. Such cells may be relatively harmless when the
anode or corroding metal occupies a much larger surface than the
cathode so that the attack is spread out over a large area.