Rock anchor corrosion potential determination in US underground coal mines

Transcription

1 The Southern African Institute of Mining and Metallurgy 6 th South African Rock Engineering Symposium SARES 2014 A.J.S. Spearing, G. Bylapudi, K. Mondal, and A.W. Bhagwat Rock anchor corrosion potential determination in US underground coal mines A.J.S. Spearing*, G. Bylapudi*, K. Mondal*, and A.W. Bhagwat *Southern Illinois University Carbondale, Illinois, USA Orica Limited, Bowerston, Ohio, USA According to the National Institute of Occupational Safety and Health (NIOSH), about 100 million rock anchors were installed in the USA mining industry during Rockbolt usage in the US mining industry fell from 100 million to 68 million by 2005, and was still assumed to be close to that number for rock anchors consumed currently. Most coal mines have conditions conducive for corrosion of rock anchor and ancillaries (such as plates), yet the effects of this on the performance of the anchors have not been well researched, and its effects are not fully understood in the USA. Corrosion studies in the mining industry are more complicated and take more time than in the civil industry, due to the constraints of the working environment. A methodology has been developed and tested for determining the corrosion potential in any underground mine. This involves measuring the open circuit potential (E OC ) between an installed rock bolt and a reference electrode. The open circuit potential (E OC ) data of the roofbolts from the field is then compared with the corrosion potential (E corr ) data of the roofbolt material from laboratory electrochemical tests. From this comparison, the seriousness of corrosion is determined. This paper details the test method and presents some preliminary results. Introduction Corrosion is potentially a serious problem in a mining industry that uses steel roof support systems in a working environment that is typically humid. Minimizing the effects of corrosion on the rock anchors makes them more durable per the manufacturers performance specifications, which in turn enables them to perform their desired function over a given amount of time. Reliable rock anchors/ground control systems are fundamental for safe underground operations over the design life of an excavation. The objective of this paper is to outline the test developed to identify potentially corrosion-prone areas in underground mines, through measurements of the rock anchors open circuit potential versus a standard copper sulphate reference electrode. For several reasons, research on corrosion of roofbolt/roof support systems is not extensive in the USA. Corrosion research is time-consuming and requires large amounts of data to draw reliable conclusions, making corrosion research projects challenging (Spearing and Mondal, 2010). In the USA, rock anchor corrosion studies relevant to the mining industry are currently conducted only by the research group at Southern Illinois University. The initial studies were started in 2008, after considering the seriousness of the problem seen in countries like Australia, South Africa, and UK. They were supported by Illinois Clean Coal Institute (ICCI) and a roofbolt manufacturer (Spearing, 2012). Australian studies conducted by Hebblewhite et al. (2004) indicate that stress corrosion, in particular, may be a significant factor in rock anchor failure. Hassell et al. (2004) performed several experiments to assess the corrosion mechanisms in ground control systems in Australian mines by recording and analysing the water flow rates, sources, temperature, ph, dissolved oxygen content, and total dissolved solids that appear to accelerate corrosion. The initial studies at SIUC gave some meaningful conclusions, and justified further research, with support from Orica North America in In the initial phase of this research project, in situ measurements were conducted on installed bolts in specific coal mines, using the open circuit potential test method to determine the corrosion potential of the bolts (E OC ). In the second stage, laboratory electrochemical studies were performed. Electrochemical test studies determine corrosion potential and corrosion rates of the steel roofbolts in their respective corrosion environments. Potentiodynamic polarization tests were performed by using a linear sweep technique with a complete e-daq electrochemical test setup and equipment. Potentiodynamic polarization studies were used to estimate the corrosion potential (E corr ) of the steel (ASTM 2004a, ASTM 2009a, and ASTM G5). This paper discusses the test method and in-situ and laboratory results to demonstrate their importance in the mine ground control. 1

2 6 th South African Rock Engineering Symposium SARES 2014 Test methodology Two types of testing, open circuit potential and electrochemical, were conducted in order to determine the corrosion potentials in environments similar to those found in underground coal mines. The field tests focused on collecting the open circuit potential data from installed roofbolts in the mines, while the laboratory experiments focused on testing the rock anchor steels in water electrolyte samples, simulating corrosion environments in US underground coal mines. In-situ open circuit potential (E OC ) measurement The open circuit potential (E OC ) is the observed potential due to the anodic and cathodic reactions that develop spontaneously when a specimen is exposed to a particular electrolytic environment. The ASTM standard test method (ASTM 2004b) for corrosion potentials of uncoated reinforcing steel in concrete was adapted and modified for E OC measurements of roofbolts in underground mines. Although there were several other complex parameters (roof strata, etc.) in mining that influence corrosion, this test method could become predominant and standard by applying more constraints and limitations to the modified method for the mine conditions. This requires more data collection from different mine conditions and more research to standardize this methodology and to apply it in the industry. The personnel working on this have to have a clear understanding of all the typical rules and conditions while performing the tests. Figure 1. In situ open circuit potential measurement schematic 2

3 Rock anchor corrosion potential determination in US underground coal mines In situ open circuit potential test protocol E OC voltage readings between a rockbolt and the coal mine roof were taken by establishing the circuit shown in Figure 1. The MSHA-approved multi-meter is used to measure the voltage or potential difference between a rockbolt and the standard copper sulphate reference electrode. E OC voltage measurements were recorded for different roofbolt types (if used) and for different roof conditions (wet, humid, and dry areas). Corrosion is more likely to be severe in wet and humid areas. The data was recorded after each separate reading. Water samples were also collected from mine test sites where E OC measurements were taken. The water samples were analysed to determine ph, sulphates, nitrates, and chlorides, which are responsible for the corrosion and the resulting corrosion potential of roofbolts underground. Laboratory corrosion potential measurement The ASTM standard test methods (ASTM G 102, ASTM G59 and ASTM G5) were used in the electrochemical testing to determine the corrosion potentials of the bolt steel specimens and corresponding corrosion currents and corrosion rates. Laboratory electrochemical test setup The experimental setup consists of the following material and equipment: Test cell Thermometer Multimeter Working electrode (roofbolt steel sample) Counter-electrode (platinum coil) Standard calomel reference electrode (SCE) Water, isopropyl alcohol, distilled water Chemicals (ferric nitrate, sodium chloride, sodium hydroxide, and sodium sulphate) for electrolyte preparation e- DAQ potentiostat and e-corder set PC (Data recorder) Machine shop equipment for sample preparation (lathe, drill, cutter, grinding, polisher M/Cs, and soldering machine) Precision water bath control unit. The electrochemical cell was composed of three electrodes the working electrode, the reference electrode, and the counter-electrode. The working electrode was the actual sample material being interrogated and is shown in Figure 2. The reference electrode provides a stable reference against which the applied potential/voltage may be accurately measured. The reference electrode provides a means to accurately measure the potential applied to the working electrode. In general, the reference electrode is based upon a reaction that is more or less at equilibrium and is not influenced by changes occurring at the working electrode that is being monitored. An SCE was used as the reference electrode. The counter-electrode is used to provide the applied current, and as such should be composed of a highly corrosion-resistant material, such as platinum. A heated water bath with a control system was used to control and maintain the water temperature. The ph meter and thermometer were used to monitor the solution. A multimeter was used to find the open circuit potential of the sample in the solution. Figure 3 shows the actual test cell, with the three main electrodes. Figure 4 and Figure 5 show the complete test setup schematic and actual laboratory setup respectively, with electrochemical test cell, temperature control unit, potentiostat, e-corder (data recorder), and a data storage device. Figure 2. Steel working/test electrode 3

4 6 th South African Rock Engineering Symposium SARES 2014 Figure 3. Electrochemical test cell Figure 4. Laboratory electrochemical test setup schematic 4

5 Rock anchor corrosion potential determination in US underground coal mines Figure 5.Electrochemical test setup Laboratory electrochemical test protocol The test was conducted using the edaq potentiostat equipped with a data acquisition system (e-corder) and e-chem software (edaq Inc). The prepared test specimen (rockbolt steel) was immersed in the electrolyte solution prepared to match that found at a specific test site underground. The E OC was recorded after stabilizing the equipment for 10 minutes without any external voltage applied to it. The stable value after 10 minutes is considered to the E OC. (E OC + 300) mv was set as the initial voltage setting, and final voltage setting value for the potentiodynamic scan was set at a value equal to (E OC - 300) mv. The potentiodynamic scans were conducted at a scan rate of 1 mv/s. Figure 6 shows the snapshots of the electrochemical test input parameters and the recorded linear sweep from the software. From the recorded corrosion current with respect to the input potential, the potentiodynamic polarization scans were plotted in Microsoft Excel and then Tafel analysis was performed per the edaq manual (edaq, n.d) and used for reading the corrosion potential (E corr ) in millivolts. 5

6 6 th South African Rock Engineering Symposium SARES 2014 Figure 6. Sample snapshots of input parameters to the e-chem application (top) and recorded data (linear sweep) using e-chem application (bottom) The corrosion potential (E corr ) and corrosion current density (i corr ) were found using the polarization curves (edaq, n.d.). The computer records the applied voltage to the working electrode versus the reference and the corresponding applied current to the counter-electrode versus the reference. The voltage was plotted on the vertical axis and the natural log of the absolute value of the current on the horizontal axis. An idealized experimental plot is shown in Figure 7. The sharp point in the plot represents the voltage at which the current flow changes direction and this is the E corr. The slopes of the linear portions of the curves represent the cathodic and anodic reaction rates. The intersection of these lines extrapolated from the linear sections occurs at the equilibrium potential termed the corrosion potential (E corr ) in millivolts. At this corrosion potential (E corr ), the anodic and cathodic reactions are in equilibrium. The current density at this point is the corrosion current density (i corr ) in milliamperes per square centimetre. 6

7 Rock anchor corrosion potential determination in US underground coal mines Figure 7. Idealized potentiodynamic polarization plot Test results and discussion The open circuit potential data was recorded at selected locations underground. A minimum sample size of six was used, unless there were insufficient bolts for stable readings at that particular location. The data primarily includes sample ID number, location ID, open circuit potential value (E OC ), and approximate number of days since the roofbolt had been installed (obtained through communication with mine personnel). The bolt type, steel grade and roof condition in terms of moisture were also recorded. The moisture condition of roof was recorded as: Wet water was visible on the roof and or dripping Dry no sign of moisture was observed Humid the roof was slightly moist due to humidity in the mine atmosphere. Relative humidity greater than 60% was recorded as humid. If the relative humidity values were less than 60%, the roof condition was recorded as dry. Later, the collected E OC ) readings were compared with the E corr values for the respective steel grade, found from the laboratory electrochemical tests (corrosion potential tests). Some important results and the correlation are explained briefly in the following sections. Open circuit potential results and discussion E OC readings of the Grade 60 roofbolts, which were collected from an underground coal mine and classified according to the three moisture conditions, are discussed in this section. The potential values recorded show a large difference per the installation environment. Figure 8, Figure 9, and Figure 10 show the roofbolt potential data from dry, humid, and wet areas, respectively. All the bolts tested were older than 2 years. From the results, it was evident that the potential values are lower in a dry roof than in the humid and wet areas. The higher the potential value, the more corrosion-prone is the area. By comparing those values with the corrosion potential data from the laboratory tests, the possibility and seriousness of corrosion can be determined. From several mine visits and a large number of observations, a stringent set of rules was formulated to ensure the validity of the conclusions drawn. One of the rules can be inferred from the data in Figure 8, Figure 9, and Figure 10 that is the distance between the bolt and reference copper-copper sulphate reference electrode used. From the data we can see the variance in the potential data when the distance was varied between 12 inches (30 cm) and 24 inches (60 cm), which could give inaccurate data leading to an incorrect interpretation. Therefore, the distance between the bolt and the reference copper-copper sulphate reference electrode was set to 12 inches (30 cm) or less for stable and reliable results which are compared to the laboratory corrosion potential (E corr ) data. For in-situ OCP testing, the bolts need to be isolated from any surrounding metal meshes, which may create noise, resulting in errors Another rule was the use of the reference electrode, which is sensitive to several factors such as temperature and light intensity. The inaccurate usage of the reference electrode without proper understanding could result in errors. Therefore the reference electrode needs to be taped to avoid erroneous measurements. Several other parameters, such as the resistivity of the strata, also play a major role in determining the E OC data. In addition, the variable underground mine stresses could also play a role in the erroneous potential data when compared to a general civil structures, which exhibit constant or stable stresses. Hence, more studies are being conducted considering the above-mentioned factors. 7

8 6 th South African Rock Engineering Symposium SARES 2014 Figure 8. Open circuit potential data from a dry roof Figure 9. Open circuit potential data from a humid area roof 8

9 Rock anchor corrosion potential determination in US underground coal mines Figure 10. Open circuit potential data from a wet roof Corrosion potential results The corrosion potential (E corr ) of the Grade 60 steel rebar was determined from the potentiodynamic polarization test by conducting a linear sweep. The recorded corrosion current for the potential applied to the steel per the protocol leads to a potentiodynamic polarization plot from which a Tafel analysis can be made, as shown in Figure 11. This allows determination of the E corr of the sample tested. Figure 11. Sample Tafel plot 9

10 6 th South African Rock Engineering Symposium SARES 2014 From the sample Tafel plot, it is evident that the recorded E corr of the Grade 60 test sample is -646 mv. This calculated value was used as a base for comparison with the measured open circuit potential of the similar grade steel bolts from the underground mines. Open circuit potential and corrosion potential comparison The recorded E OC values of the bolts from the mines and the E corr value of the steel sample in the laboratory can be compared. Figure 12 shows the sample data described in the previous section, compared to the E corr from the laboratory test, which was calculated to be -646 mv. The samples in location 1, with lower potential values, could be considered safe when compared to the samples from locations 2 and 3. Although the samples from location 2 exhibit potential values close to the corrosion potential they were considered to be safe, but corrosion possibility should not be ignored. Two samples (2 and 5) from location 3 with higher potential values of -655 mv are considered to be more at risk and in the serious corrosion phase compared to the other samples in the same region. Action should therefore be taken in case of location 3. Although corrosion is visible at locations 2 and 3, the location with higher potential readings should be considered serious, and the location closer to the corrosion potential value needs to be monitored on a regular basis. As discussed in the previous sections, several other factors might affect the potential values of the bolts, and hence more research is deemed necessary for delivering a reliable and standard test method. Figure 12. Corrosion potential determination chart with E OC and E corr comparison Conclusions Open circuit potential (OCP) values from wet roof conditions are higher compared to dry roof values, and suggest that higher the OCP (negativemillivolts), the higher the corrosion potential. Roof conditions with relative humidity greater than 60% also have favorable conditions for corrosion, and the OCPs observed in these regions need to be analysed accordingly. OCP recordings of the roofbolts, from the bolt head to reference electrode placement distance, were standardized to be less than 30 cm (12 inches). The in situ corrosion potential determination method developed here is considered to be useful based on the tests conducted to date. It does not consider other factors that could be significant (such as the induced stress in the bolt), but is quick to conduct and is a useful initial evaluation. Acknowledgements The authors gratefully acknowledge the funding provided by the Illinois Department of Commerce and Economic Opportunity through the Office of Coal Development and the Illinois Clean Coal Institute. They would also like to 10

11 Rock anchor corrosion potential determination in US underground coal mines acknowledge the collaboration, including the test materials and the destructive testing equipment that were provided by Orica North America. References ASTM International. 2009b. Standard Test Method for Corrosion Potentials of Uncoated Reinforcing Steel in Concrete. ASTM Standard C876. West Conshohocken, PA. DOI: /C ASTM International. 2004b. Standard Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements. ASTM Standard G5, West Conshohocken, PA. DOI: /G R04. ASTM International. 2009a. Standard Test Method for Conducting Potentiostatic and Potentiodynamic Anodic Polarization Measurements. ASTM Standard G59. West Conshohocken, PA. DOI: /G R09. ASTM International. 2004a. Standard Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements. ASTM Standard G102 West Conshohocken, PA. DOI: /G R04E01. Dolinar, D. B. (2000). Trends in roof bolt application. Proceedings: New Technology For Coal Mine Roof Support (pp ). Pittsburgh: NIOSH. Hassell, R. V. (2004). Corrosion assessment of ground support systems. Ground support in mining and underground construction, Hebblewhite, B. F. (2004). Premature bolt failures in Australian coal mines due to stress corrosion cracking. International Ground Control Conderence, (pp ). Perth. Spearing, A. J. (2010). A Method to Determine the Corrosion Potential of Rock Bolts on Coal Mines. 29th International Conference on Ground Control in Mining. Morgantown: ICGCM. Spearing, A. J. (2012). Corrosion of Rock Anchors in Coal Mines. Carterville: Illinois Clean Coal Institute. Tadolini, S. M. (2006). Understanding Roof Bolt Selection and Design Still Remains Priceless. 25th International Conference on Ground Control, (pp ). Morgantown. The Author 11