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Immersion of Metals and Alloys
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.
Accelerated Corrosion
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).
Cablebolt storage
-
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.
Installed cablebolts
- 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.
Severe corrosion
-
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).
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