Asuransi Central Asia (ACA)
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Corrosion problems observed by the authors in mining environments were typically in long term support in open pits where the groundwater was acidic or saline and in long term support in underground sulphide deposits. Cut and fill applications in wet conditions where fractured stope backs could remain (supported) for up to a year were notably susceptible to corrosion. Serious failure, due to corrosion and rupture of the strand, can occur in such applications.
The nature of corrosion is extremely complex and a fundamental discussion is beyond the scope of this book. It is the intent here to discuss some of the important factors involved in corrosion so that the engineer may assess the potential for problematic corrosion and take steps to prevent it or make the appropriate design allowances for it.
Most common refined metals are inherently unstable ionic materials composed of arrays of single atoms which possess a full compliment of electrons.
Metals such as iron normally tend to give up electrons at room temperature (gold is a notable exception) and become involved in reactions leading to the formation of more stable compounds such as iron oxide or iron hydroxide (rust). The release of electrons is termed an anodic reaction and the acceptance of electrons a cathodic reaction. Both reactions must occur for corrosion to take place. Since metals such as the iron found in steel cable are normally willing to give up their electrons, it is normally the presence of a cathode which determines the corrosion potential.
The cathodic reaction (involving the consumption of electrons released anodically from the iron) can be made possible by the presence of an acid, sulphate, water and/or oxygen.
Corrosion of steel (iron) can be divided into four basic categories (Illston et al., 1979; Pohlman, 1987):
- Dry corrosion
- Wet corrosion
- Corrosion of immersed metals and alloys Induced or accelerated corrosion (includes influence of stress)
The following discussion is confined to corrosion of cablebolts and as such is incomplete as a comprehensive examination of general corrosion.