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: ajsspearing@yahoo.com

618-9251718

29th INTERNATIONAL CONFERENCE ON GROUND CONTROL

BackgroundAccording 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 corrosionCorrosion 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 problemThe 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.

PerceptionThere 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.

Definition: Deterioration of materials by chemical

reaction of material surface and the

surrounding environment.

• Types:

Uniform corrosion

Localized corrosion1. Pitting corrosion2. Crevice corrosion (including corrosion

under deposits and tubercles (large, impervious structures))

3. Galvanic corrosion4. Cavitation corrosion5. Intergranular corrosion6. Stress corrosion7. De-alloying8. Corrosion fatigue

Different types of corrosion that may attack rock anchors [9]

Corrosion phenomenon

Corrosion In Mines

A heavily corroded Split Set

Examples

Typical failure due to SCC (after Hebblewhite)

Micrograph picture of stress corrosion cracking (SCC) causing failure in rock bolt steel (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 conditionsTank 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

Temperature Ambient Ambient Ambient

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:

Hydrated ferric oxide (Fe2O3.H2O) is known as rust.

OHOFeOOHFe23222

.5.12

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 plotThe 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 long-term 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.

ConclusionsBased 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.