Environmental factors associated with premature corrosion failure of rockbolts in Australian underground coal mines

Premature failure of rockbolts from stress corrosion cracking (SCC) has been a problem in a limited number of Australian coal mines since into the late 1990’s. Laboratory research into the susceptibility of different steel grades to SCC has been inconclusive with conflicting results where failure could only be achieved in highly acid solutions. Based on anecdotal field evidence, the industry moved to adopting a HSAC840 grade steel having a higher impact toughness of 16 J being more than double that used previously. After several years, mines began reporting significant numbers of failed HSAC840 rockbolts. The industry sought answers that would address the underlying causes. Around 200 broken rockbolts were collected from across a range of underground coal mines in New South Wales and Queensland. The steel grade of the majority of rockbolt was found to be HSAC840. It was found that SCC was the dominant failure mode, but localised pitting corrosion often occurred at the same locations. Underground studies were completed using survey and non-destructive test methods to determine the extent of the problem. It was found that only two mines had significant problems and they became the focus of further research. Groundwater dripping from rockbolts was collected for analysis from 12 mine sites. Mine groundwater with a DIN 50929 corrosivity classification of medium to high corresponded with a limited number of broken bolts, but the groundwater from the two mines with significant problems was not rated corrosive. Claystone mineralogy showed no swelling clays, however they were prone to softening with exposure to water to seal off around rockbolts to retain moisture. Microbiological testing of the groundwater indicated the presence of bacteria that has a known influence on steel corrosion.An in-hole rockbolt coupon was designed for in-situ testing. The unique design allows the coupon to fit inside a borehole exposing it to the same environment as an installed rockbolt. The in-hole coupon experiments were successful in achieving SCC together with localised pitting along the surface of the rockbolt which is a world’s first. Testing of the surface crust formed on coupons that had been exposed for up to 566 days revealed sulphate reducing bacteria in significant concentrations. These results provide the best evidence to date that SCC may be caused by hydrogen embrittlement resulting from the action of hydrogen sulphide producing bacteria