Corrosion of Rock Anchors in Illinois Coal Basin Mines

Author

Jason David Weber, Southern Illinois University CarbondaleFollow

Date of Award

8-1-2013

Degree Name

Master of Science

Department

Mining Engineering

First Advisor

Spearing, Sam

Second Advisor

Mondal, Kanchan

Abstract

Rock anchors are used extensively in coal mines in the United States and around the world. The conditions in many of these coal mines are conducive towards the corrosion of rock anchors. Despite this, very little research has been performed to analyze corrosion and its effects on rock anchors in US coal mines. The specific aim of this thesis is to analyze typical Illinois Coal Basin (ICB) mine parameters and their effects on corrosion. A well understood relationship between these parameters and corrosion would be useful in locating highly corrosive areas and mitigating risks associated with corrosion. To study ICB mine parameters and their effects on corrosion, three general experiment types were carried out: in situ rock anchor analysis, simulated mine environment testing and analysis, and electrochemical testing and analysis. For in situ rock anchor analysis, open circuit potential (OCP) of rock anchors in coal mines with various conditions were measured. The OCP values can be used to predict whether or not corrosion of the rock anchors has initiated, and to study relationships between mine parameters and corrosion. The readings were also used to assess the validity of experimental setups by comparing experimental OCP values to in situ mine OCP values. The OCP readings obtained were found to vary by locations within the mines indicating corrosion conditions can vary relative to location. No clear trends were found between the OCP values and moisture or rock anchor age likely indicating numerous other factors were affecting the OCP values. These other significant factors will need to be determined and measured in addition to moisture and age in order to develop clear relationships between mine parameters and OCP values. Overall, the experimental OCP values found were slightly more negative than the in situ OCP values. This may have been due to the fact that the corrosion tanks exposed the rock anchors to more moisture. It is also a possible indication that mine rock anchors were not yet significantly corroding. For simulated environments, three tanks were filled with solution made by adding chloride ions, sulfate ions, and iron ions. The concentrations were based on water samples from mines and were the same for each tank. The temperatures of each tank were set at 30°C or 50°C, and the pH levels were set at 5.5 or 9.The tanks were then loaded with thirteen different types of rock anchors. The solution compositions were monitored over time, and the rock anchors were removed at specified intervals and tested to tensile destruction. The decreasing ultimate strengths of anchors were assumed to be related to the corrosion rate. This relationship was used to study the corrosion rates of multiple rock anchor types in varying conditions. Results seemed to indicate higher temperatures caused the highest corrosion rates. Basic pH levels caused a less significant increase in corrosion. With respect to reduced visible surface corrosion, epoxy coatings were the most effective followed by the proprietary coating, and then the galvanized coating. All bolts, cable anchors, and friction bolts with no coating had significant visible corrosion relative to rock anchors with protective coatings. Cable bolts with no coating and friction bolts seemed to have had the largest decrease in strength, and this is hypothesized to be a result of having larger exposed surface areas than other rock anchor types. For electrochemical testing and analysis, potentiodynamic polarization tests were used to estimate the corrosion potential (Ecorr) and corrosion current (Icorr) of rock anchor metals in simulated mine environments. Ecorr can be used as an indication of the conditions necessary for the initiation of corrosion, and Icorr can be used to determine the rate of corrosion. The effects of pyritic sulfur from coal samples in solution, pH of solution, temperature, and the bolt grade on Ecorr and Icorr values were analyzed. No significant trends were observed between electrochemical properties and pyritic sulfur concentrations. pH, temperature, and bolt grade were also parameters that were analyzed, and of these, only temperature had a significant effect on electrochemical properties. The penetration rate increased significantly with respect to increasing temperatures. All corrosion rates in the simulated mine water were considered low according to the standard specifying corrosion rates and characteristics that was used. This research has shown mine corrosion parameters are consistent in localized areas. The results, however show additional parameters other than age and moisture must be accounted for in order to accurately predict the risk of corrosion in a given mine. Simulated mine testing results showed that corrosion prevention coatings are effective in reducing corrosion in the conditions tested. Electrochemical test showed corrosion rates were all low in the mine solutions tested. Temperature had the most significant effect on corrosion rates in both the simulated mine tests and electrochemical tests. The electrochemical tests also showed that increased temperature can only simulate increased corrosion periods up to the point that other reaction factors limit the reaction rate independent of temperature. To fully understand corrosion risks in ICB mines, additional parameters and experimental methods should be considered in order to increase the understanding of relationship between ICB mine parameters and corrosion.