Laboratory Testing of Rock Bolt Stress Corrosion Cracking

The incidence of Stress Corrosion Cracking (SCC) in rock bolts has not been quantified and its magnitude has not been addressed. A laboratory test has been achieved that causes a tensile sample to fail in a manner similar to the failure mode observed from service failures, namely slow SCC followed by fast brittle fracture. The laboratory tests involve subjecting a tensile sample to a linearly increasing stress at a slowly applied stress rate whilst the specimen in exposed to a dilute sulphate solution of pH 2.1. Detailed fractography of SCC fracture features from the LT has shown that these fracture surfaces have the same features as fracture surfaces of service failures. An SCC velocity can be calculated from these tests. This SCC velocity can be used to evaluate the benefit provided by a material with a higher fracture toughness. The SCC velocity measured from the laboratory tests indicates that the SCC lifetime is increased only marginally by the use of a rock bolt materials with a higher fracture toughness Laboratory tests are being used to evaluate the threshold stress for rock bolts of various metallurgies and the environments causing stress corrosion cracking. A hydrogen embrittlement mechanism for the SCC is indicated by the particular restricted range of conditions for which SCC occurs in the laboratory. In particular, SCC only occurs in the laboratory for the restricted range of environmental conditions corresponding to acid conditions at the open circuit potential (pH of 2.1 or more acid) or at negative applied electrochemical potentials corresponding to copious hydrogen evolution at the steel surface. This is consistent with reports from the USA indicating rock bolt failure due to the presence of H2S in the mine atmosphere. Similarly, this failure mechanism is consistent with bacterial corrosion of the rock bolt surface during service producing acid conditions leading to SCC. Water chemistry analyses carried out for a number of Australian mines (including one coal mine) visited during the 2002 suggest that SCC in a coal mine would be caused by bacterial corrosion locally decreasing the mine water pH down to a pH of 2.1.