Page 387 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

CORROSION DAMAGE, DEFECTS, AND FAILURES 365

Figure 5.55 Well-developed pits impinging on one another (19).

Figure 5.56 Pits typical of microbiologically induced corrosion (19).

in the water resulted in saucer-sized pits, smooth sided pits, and bright shiny cop-
per to matte-red clean areas. The black deposits – corrosion products from carbon
steel – may cause underdeposit corrosion resulting in failure. Treatment of water with
biocide may minimize microbiologically induced corrosion.

Failure of Rock Bolts A typical Swellex rock bolt shown in Figure 5.57 has the
following modes of support: (i) beam building; (ii) suspension; (iii) pressure arch;
(iv) support of discrete bolts.
The corrosion modes of rock bolts are: (i) uniform corrosion; (ii) pitting or crevice
corrosion; (iii) galvanic corrosion.
The factors involved in corrosion are: low pH mine water because of oxidation of
iron sulfides, or bacterial oxidation such as sulfate-reducing bacteria, carbon dioxide,
chloride, and sulfate; mine air also contains SO and NO .
2
2
A bolt fails when it can no longer provide the support it is designed for. Fracture
is the separation of a solid body into two or more parts under the action of stress. It
consists of crack initiation and propagation. Figure 5.58 shows the visual examination