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  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 2 O 1 . 5 O 2 Fe 2 O 3 . H 2 O 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.