Page 186 - Corrosion Engineering Principles and Practice

160 C h a p t e r 6 R e c o g n i z i n g t h e F o r m s o f C o r r o s i o n 161

150 µm

(a)

30 µm

(b)

FIGURE 6.16 Photomicrograph of a section through an edge of the
8090-T851 panel immersed in seawater during four months; (a) at 64× and
(b) at 320× to illustrate the intergranular nature of the corrosion attack.

Pitting corrosion can be assessed by various methods, including
simple visual examination of a corroded specimen or monitoring coupon.
When the sites of attack are numerous, the tedious job of counting pits
can be facilitated by using rating charts, as shown in Fig. 6.18 [7].
Engineers concerned with soil corrosion and underground steel
piping are aware that the maximum pit depth found on a buried
structure is somehow related to the percentage of the structure
inspected. Finding the deepest actual pit requires a detailed inspection
of the whole structure. As the area of the structure inspected decreases,
so does the probability of finding the deepest actual pit.