Corrosion is a physical alteration of a material from electrochemical reaction with its environment that often results in reduction of the mechanical properties of that material. Rock bolts are particularly susceptible to corrosion as they can be exposed in their working environment to ground water. Corrosion increases markedly in sulphide ore bodies due to acid runoff.  Table blow shows different types of corrosion that a rock bolt is likely to undergo when used for ground reinforcement. Of all the types of corrosion, pitting is particularly dangerous as it removes material and subsequent capacity for the bolt to deform with strata movements. Sudden failure of a bolt is likely to occur when pitting corrosion is experienced. The type of corrosion depends on the nature of the ground condition and bolt encapsulation. Generally, the type of corrosion and severity of the corrosion can vary along the bolt.

Historically the subject of steel corrosion has been of interest to civil and construction engineering. In mining, the interest in the topic is relatively new and follows the introduction of bolting for ground support in mines and tunnelling. According to Baxter (1996) the early corrosion studies on rock bolting were carried out by Swedish and Finish researchers. Various publications include: Tuutti, 1982; Sundholm, 1995; Helfrich, 1990; Moving, 1994; Sundholm and Forsen, 1995; Satola and Aromaa, 2005. In the Australian context, the interest in bolt corrosion began in earnest in the late 1990s and the paper by Gray (1998) in which an emphasis was given to Stress Corrosion Cracking (SCC). An ACARP project was initiated in 1999 to address the observed phenomenon of premature failure of rock bolts in a number of Australian coal mines, with a particular focus on the problem of SCC in rock bolts (Hebblewhite, et al., 2002, 2003a, and 2003b). Other Australian publications on corrosion include Gamboa and Atrens (2003), Hassell, et al., (2005), and Vandermaat, et al., (2012). The latter developed an apparatus to study stress corrosion cracking in full sized bolt specimens.

The rate of steel bolt corrosion is influenced by ground water composition, flow rates, water pH, temperature, CO₂ content, surface condition, presence of corrosion inhibitors, applied stresses, residual stresses (from workings, forming or welding operations) and any hydrogen sulphide concentrations (Henthorne, 1972; Spearing, 2010). Accordingly, a specialised test rig was constructed at the University of Wollongong to study the effect of long term exposure on full size bolts, which are of current use in Australian mines. The study was undertaken in an environmentally controlled laboratory under different bolt loading conditions over a period of four years, and the findings  from this has been published  by Aziz, et al., (2013, 2014).

References:

Aziz, N, Craig, P, Nemcik, J and Hai, F, 2014. Rock bolt corrosion – an experimental study, Mining Technology (Trans. Inst, Min. Metall. A), Vol. 123, Issue   2, June,   PP 69-77.  Also visit http://ro.uow.edu.au/coal/448/