Transcript Localized corrosion - Coal Mine Ground Control Conference

29th INTERNATIONAL CONFERENCE ON GROUND CONTROL
A METHOD TO DETERMINE THE
CORROSION POTENTIAL OF
ROCK BOLTS ON COAL MINES
A.J.S. (Sam) Spearing
Kanchan Mondal
Gopi Bylapudi
SIUC
Joe Hirschi
ICCI
Contact: [email protected]
618-9251718
Background
According to a NIOSH report, about 100
million rock anchors are installed
annually in coal mines in the USA.
Most coal mines have conditions that
encourage corrosion of the rock anchor
and ancillaries (such as plates).
Coal mining corrosion
Corrosion is an issue
on coal mines in
Australia where
stress corrosion
failure has been
found to be an
issue causing falls
of ground.
Why not in US coal
mines ?
The problem
The major contributors in the water that
exacerbate the corrosion problem are:
• pH (acid and alkali conditions both cause
problems).
• The concentration of chloride (Cl-) and sulfate
(SO42-) ions.
• The dissolved oxygen concentration.
• The concentration of hydrogen (H+) ions.
• The concentration of calcium (Ca2+) and
magnesium (Mg2+) ions.
Perception
There is a perception that
with full column resin
grouted bolts, corrosion is
not an issue.
This is not correct because
resin shrinks slightly on
curing; and under shear
and tensile loading the
resin column is also
damaged allowing
corrosion.
Micro-cracking on setting
Corrosion types
The types of corrosion that can adversely affect
rock bolts are:
• Aqueous corrosion caused by the presence of
water.
• Atmospheric corrosion mainly caused by the
humid conditions.
• Stress corrosion cracking which is the interaction
of corrosion and stresses resulting in cracking
and subsequent failure.
Influencing factors
The following factors influence the magnitude of the
corrosion potential:
• The presence and quality of the water especially the pH,
the water flow rate and the concentrations of dissolved
oxygen, chlorides and sulfates.
• The rock mass quality and competence.
• The metallurgy of the steel rock bolt.
• The quantity, properties and type of grout used.
• The temperature because increased temperatures
increase the corrosive chemical reactions.
Corrosion phenomenon
Definition: Deterioration of materials by chemical
reaction of material surface and the
surrounding environment.
• Types:
 Uniform corrosion
 Localized corrosion
1. Pitting corrosion
2. Crevice corrosion (including corrosion
under deposits and tubercles (large,
impervious structures))
3. Galvanic corrosion
4. Cavitation corrosion
5. Intergranular corrosion
6. Stress corrosion
7. De-alloying
8. Corrosion fatigue
Different types of corrosion that
may attack rock anchors [9]
Corrosion In Mines
A heavily corroded Split Set
Examples
Micrograph picture of stress
corrosion cracking (SCC)
causing failure in rock bolt
steel (after Hebblewhite)
Typical failure due to SCC
(after Hebblewhite)
A failed corroded cable
strand.
The project details:
• Destructive tests every 3 months for a
year with bolts in corrosion tanks.
• Underground field measurements.
• Small scale lab calibration tests.
The corrosion tanks
Five corrosion tanks
were set-up to
evaluate and quantify
the potential problem,
using a range of
conditions that
covered that found in
underground roof
egress water.
The tank conditions
Tank 1
Tank 2
Tank 3
pH
5.5
7.0
9.0
Iron
2.8 mg/L
2.8 mg/L
2.8 mg/L
Chlorides
950 mg/L
950 mg/L
950 mg/L
Sulfates
1150 mg/L
1150 mg/L
1150 mg/L
Ambient
Ambient
Ambient
Temperature
Tanks 4 and 5 had the same conditions as Tank 3 but were heated to 50°C
for accelerated results.
These conditions were selected based on water samples from about 12
mines.
The tensile testing rig
Destructive testing was
carried out every 3
months at Jennmar Corp.
as part of the research.
Sample results – Iron content (ppm)
Other water monitored results included pH, sulfates, chlorides and nitrates.
Corrosion after 9 months
Almost all the rock bolts used in coal
mines are made from steel, and
hence the corrosion of iron needs to
be understood.
The corrosion of iron is an
electrochemical phenomenon with
the overall equation:
2 Fe  H 2O  1.5O2  Fe2O3 .H 2O
Hydrated ferric oxide (Fe2O3.H2O) is
known as rust.
Localized corrosion
As an example, the corrosion rate is
calculated to be 0.0516 mm/year under
corrosive conditions of a pH of 5.1 and a
temperature of 22˚C.
Localized corrosion is therefore generally
not a major issue.
Forged head vs threads
Thread failure (under load/SCC)
Destructive tests - corrosion at
the threads
Does a resin column protect
reliable corrosion protection ?
If confined in a hole, would the corrosion still be as severe ?
Underground and lab evaluation
Tafel plot
The corrosion takes place at
a rate determined by a
balance between opposing
electro-chemical reactions.
The first is an anodic
reaction which oxidizes the
metal releasing electrons
into the metal.
The second is the opposite
cathodic reaction in the
solution in which electrons
are removed from the metal.
When these reactions are in
balance, there is no net flow
of electrons.
Field evaluation
Open voltage potential:
Measuring open circuit voltage
potential of roof bolts
installed in underground
coal mines (it wet and dry
conditions) can be useful for
finding the corrosion
potential of rock anchors
and can help determine if
corrosion is a potential longterm issue for a mine.
Sample preparation
Lab testing
Procedure
• The open circuit potential (OCP) was recorded after 10 minutes.
• Scans: Initial Voltage setting :OCP mV (after 10 minutes)
Final Voltage setting : OCP-400 mV
• Scan rate of 0.5 mV/s
• From the scans , Ecorr (mV) and corresponding Icorr (mA) is read
which are used for corrosion rate
•
Calculations and comparison with the OCP (mV) readings of the
mine roof bolts.
Polarization resistance
It is an electrochemical
method used to find the
corrosion rate of steel
used in roof bolts.
Test set-up including
electrochemical cell,
potentiostat, e-corder
(data recorder), and data
storage device (PC).
Lab and field results can
then predict the corrosion
potential.
Sample results
Results from 3 sites at a mine
Sample result
For the #6 Grade 60 roof bolt steel, the
uniform corrosion rate is calculated to be
0.0516 mm/year under the corrosive
conditions of a pH of 5.1 and a
temperature of 22.2˚C.
Under normal circumstances this would not
be an issue except in a threaded bolt
where localized corrosion is common.
Conclusions
Based on water quality samples from coal mines, corrosive conditions
can exist for rock bolts.
• Uniform corrosion does not appear to be an issue, but localized
corrosion can be.
• Corrosion would only seem to be a potential issue for the support of
long life excavations. Under most conditions it however does not
appear to be a serious design and safety issue.
• Bolts with forged heads are less corrosion prone than threaded
bolts.
• Full column resin grouting does not provide total corrosion protection
to a bolt as has been generally assumed in the past.
• If corrosion is anticipated to be a problem, a larger diameter rebar
could be used, ancillaries protected and cable bolts could be
galvanized or epoxy coated for example.
The effects of corrosion can
therefore be mitigated/avoided if
they are identified!
Future work
• Longer term corrosion tests needed (2 to 3
years).
• Bolts that appear prone to corrosion
should be “extracted” and destructively
tested to confirm that the corrosion is
actually taking place.
Acknowledgements
The authors would like to thank:
• The Illinois Department of Commerce and
Economic Opportunity
• The Illinois Clean Coal Institute
• Jennmar Corp
for funding the research.