It is the differential electrical potential between the anode (+) and the cathode (-) which is key to the moist corrosion example described above. This differential is primarily generated by the difference in oxygen availability between the edge and the centre of the water droplet.
Differential potentials can also be generated by the presence (and contact) of dissimilar metals immersed in an oxygenated electrolyte solution (Illston et al., 1979; Bryson, 1987). Corrosion induced by such a coupling can be extremely aggressive and can result from the designed use of dissimilar metals (steel cables with aluminum plates or anchors) or from the presence of cablebolts in a rich sulphide ore. Indeed, rock bolts in sulphide ore bodies have significantly reduced service lives (Hoey and Dingley, 1971; Gunasekera, 1992).
Corrosion cells can also be generated on cablebolt surfaces at the point where abrupt transitions in environment occur. These include differential grout coverage, for example, at the borehole collar, at penetrating cracks in the grout, where the cable crosses a local water table, or within voids in the grout column. Oxygen (atmospheric or dissolved) is the critical component of the cathodic reaction discussed so far.
The concentration of oxygen is therefore a critical factor governing the rate of corrosion. In aqueous environments with high levels of acidity or low pH, however, the hydrogen (H ) ions in the acid solution react +cathodically with the free electrons in the steel to form hydrogen gas (H ). This 2 reaction is countered as before by the release of iron ions from the steel and does not require the presence of oxygen. While oxygen concentration normally controls corrosion rate (loss of iron ions), the acid (H ) reaction dominates below a pH of +4 and can become extremely aggressive.
Although it is not as common as oxygen related corrosion, acid corrosion can pose a serious hazard to mine support (Gunasekera, 1992) due to its accelerated rate. Sampling of groundwater and/or mine water for pH is relatively simple so the risk can be easily determined. In Canada, mine water with a pH of 2.8 has been recorded in underground mines, and measurements of 3-4 are not uncommon (Minick and Olson, 1987). Acidic mine water can often be linked to the oxidation of sulphide ores (primarily pyrite and marcasite) resulting in the generation of sulphuric acid and pH levels as low as 1.5-2 (Gunasekera, 1992).
In addition, there are many species of bacteria which flourish in the underground environment and which greatly accelerate the breakdown of sulphides to form sulphuric acid. Different species are active with and without the presence of oxygen. Such bacteria can accelerate the production of acid in mine waters by a factor of four with a related increase in corrosion rate.
Of primary consideration in cablebolting is the acceleration of any of these corrosion processes at points of excessive strain in the cablebolt. As steel is strained in tension or in shear across a joint in the rock by rockmass movement, or bent by improper plate installation, the susceptibility to all forms of corrosion increases. Any protective surface rust is cracked by such strain exposing fresh surfaces. Microscopic cracks formed in areas of high strain create corrosion conduits beyond the steel surface. In addition, the strained ionic bonding in the metal increases the potential for iron-electrolyte interaction and hydrogen embrittlement (Littlejohn and Bruce, 1975).
This so-called stress corrosion cracking is important because cables will tend to corrode much more rapidly in aggressive environments exactly when and where their mechanical integrity is most tested and is most critical. In the case of grouted cablebolts, load concentrations along the cable length are usually related to full cracking and separation across the grout column. This allows direct and focussed attack on the stressed steel by corrosive agents. Stress corrosion is often the final mechanism in cablebolt failure in corrosive environments.
Cablebolt Geometry Effects
In general, the high carbon steels used in the manufacture of cablebolt strand are more corrosion resistant than the steels used in conventional rock bolts. Nevertheless, certain features of the grouted cablebolt which increase its potential for detrimental corrosion include the presence of flutes (v-grooves), internal channels between the outer wires and the king wires, as well as the formation of concentrated corrosion sites at separation planes in the rock and grout. Voids and bubbles in the grout column also create potential corrosion cells.
Summary Recommendations for Corrosive Environments
Corrosion is rarely a problem in open stope cable support, simply due to the short service life involved. Cut and fill stopes can be open for up to a year or more and overhead cables should, therefore, not be allowed to corrode to unacceptable levels during this time. Fractured, sulphide ore bodies require special attention in this regard. Corrosion of cablebolts (and other steel support) in permanent mine openings can cause serious problems in terms of safety and rehabilitation. In addition to normal capacity reduction, corroded cables tend to become brittle and can suffer reduced effectiveness in dynamic loading situations. The factors which contribute to corrosion are often complex, are compounded in an underground environment, and are very difficult to combat in areas of high severity. Nevertheless, the following is a brief list of remedial measures for use when corrosion has been identified as a problem (Littlejohn, 1990; Gunasekera, 1992).
- Store cablebolts in a dry location, preferably moving them underground to the working site only when required. Long-term storage outside, under the sun or exposed to the elements should also be avoided.
- Do not allow water to collect on the cablebolts. Corrosion will quickly fill the flutes reducing bond strength and potentially pitting the steel.
- High humidity accelerates corrosion. Good ventilation at all times can help to reduce this factor.
- Use caution when installing cables in areas with flowing water.
- Avoid any use of cements, mixing water or admixtures containing chlorides, sulphides or sulphites.
- Grout voids and bubbles increase corrosion potential.
- Request that plates, barrels and wedges, and other fixtures are electro-chemically compatible with the high strength carbon steel used in strand.
- Long rust stalactites growing rapidly from the ends of uphole cables indicates potentially severe strand corrosion up the hole.
- Sulphate resistant grouts are alkaline and can counteract acidic mine waters. The use of this cement does not permit the use of such waters for grout mixing.
- Epoxy-encapsulated cables are available for use in corrosive environments (Windsor, 1992). Note that such coatings may not be resistant to all forms of corrosion and that the coating must penetrate the strand, encapsulating the king-wire to prevent focussed corrosion down the centre of the strand.
- Galvanized cable would be of use against non-acidic corrosion.
- Grease can protect ungrouted lengths of cable (at the collar, for example).
Other more costly measures such as cathodic protection are discussed in Littlejohn and Bruce (1975) and Littlejohn (1990; 1993).