Developing Prevention Techniques for Stress Corrosion Cracking Failures of Rock and Cable Bolts

In the past two decades, there has been a notable increase in reports on Stress Corrosion Cracking (SCC) failures of rockbolts and cable bolts in Australian underground coal mines. This trend is not limited to Australia, as similar failures have been reported in underground coal mines where located in the UK, China and the USA.

Through the last 20 years of studying the premature failure of rockbolts and cable bolts in underground coal mines, the root cause of the failure has been defined. The preliminary study has defined that an acidic environment containing hydrogen sulphide (H2S) can cause hydrogen-induced SCC (HISCC) on both rockbolts and cable bolts. It occurs by diffusion of atomic hydrogen into the steel, often referred to as "hydrogen embrittlement". Atomic hydrogen in the environment tends to recombine and form molecular hydrogen, which will then be released into the environment as a gas. However, in HISCC, H2S acts as a poison for the recombination of atomic hydrogen and promotes atomic hydrogen diffusion into steel. While such acidic environments are not generally found in underground mines, environments containing sufficient atomic hydrogen to diffuse into steel may still develop locally, which could eventually lead to HISCC.

This recent research discovered that microorganisms, particularly sulphate-reducing bacteria (SRB), are present in the mines. These microorganisms produce H2S, promoting hydrogen embrittlement in the bolts and ultimately resulting in Microbiologically Induced Stress Corrosion Cracking (MISCC). Therefore, it was necessary to develop new prevention techniques to stop the corrosion failure of bolts from a microbiological and environmental point of view.

The new techniques should be able to prevent the growth of corrosion-causing bacteria in the localised environment where the bolts have been installed or isolate the steel surface from the external environment to stop any bacterial attachment and hydrogen diffusion to the surface.

These requirements have become the main objectives of this proposed project:

• Develop prevention techniques, e.g. anti-microbial coating, as a long-lasting controlling technique to mitigate against SCC in underground coal mines; and
• Develop guidelines for preventing SCC, which is consistent across the industry.

Studies by independent agencies have estimated 276 billion USD per year of corrosion related damages in the USA alone, affecting commercial and residential properties as well as transportation facilities. Similar surveys in the U.K., Japan, Germany and Australia estimate the cost of corrosion to be 1-5% of their gross national products (GNP). Microbiologically-Induced Corrosion (MIC) is believed to account for 20% of the cases. In fact, the annual MIC-related industrial loss in Australia is estimated to be more than 5 billion USD. Therefore, the outcome of this project will benefit the mining industry as below:

• Reducing the maintenance/re-support cost of operations by developing reliable SCC prediction and prevention techniques;
• Mitigation or elimination of downtime associated with SCC;
• Assisting in efficient control and monitoring of SCC failure;
• Increasing safety of underground operations; and
• Providing a framework which is consistent across the industry.

The project outcomes are as follows:

• The background and mechanism of SCC to help geotechnical engineers understand the premature failure of rock and cable bolts have been summarised;
• A framework for rapid on-site bolt failure analysis, presented in the form of a guideline flow-chart has been established;
• A step-by-step process for environmental analysis to identify the corrosion-causing bacteria to predict the MIC failure of bolts has been provided;
• Enriched corrosion-causing bacteria as a seed for future corrosion research for underground mines;
• Developed an anti-microbial compound as a potential admixture in coating and grout as a prevention method for MISCC;
• Developed a barrier coating that has resistance to extremely corrosive environments, showcasing its promising potential as a product designed to prevent corrosion failures of rock and cable bolts for the mining industry.