Accelerated corrosion investigations on mechanical hybrid rockbolt steel alloying elements can be used to meet the mechanical performance requirements of steel and rockbolts. In underground mining environments, rockbolts are exposed to a range of corrosive environments. Atmospheric corrosion is controlled by factors such as temperature, relative humidity, airflow, and the presence of pollutants (Hadjigeorgiou et al., 2008). As the water vapour condenses onto the rockbolt, it triggers an electrochemical reaction. Aqueous corrosion occurs due to exposure to underground mine water. Rockbolts exposed to an aqueous environment are generally more susceptible to corrosion than those undergoing atmospheric corrosion. Along with the water temperature, various parameters that indicate the presence of chemical species exacerbating the corrosion rate can be used to characterise the environment, such as pH, conductivity, total dissolved solids (TDS), the dissolved oxygen (DO) level, and concentration of aggressive anions (Cl-, SO4-2, etc.). In addition, microorganisms present in the water can facilitate or inhibit the corrosion of rockbolts (Craig et al., 2016; Dorion, Hadjigeorgiou, 2020).The pH measures the concentration of H+ions in the solution, which also indicates the acidity/alkalinity of the solution. A solution with a higher concentration of H+ will lead to a lower pH level. The dissolved oxygen (DO) is a measure of oxygen, a strong oxidising agent in the solution. The rate of mass transfer for chemical species is temperature-dependent. As the corrosion reaction involves an electron transfer step, the rate of electricity passing through a solution (conductivity) can also be an indicator. The total dissolved solid (TDS) measures the concentration of the dissolved substance in the solution. As the previously mentioned parameters increase, the corrosivity of the environment will consequently increase, resulting in an elevated corrosion rate. The influence of aggressive anions, e.g., Cl- and SO4-2, affect not only the general corrosion rate but are also associated with localised forms of attack such as pitting corrosion. Since pitting corrosion rates may be orders of magnitude greater than general corrosion rates, the latter may not be a representative indicator for long-term bolt integrity. The lowered rate is due to the decreased solubility of oxygen in the presence of the dissolved NaCl. A significant mass loss occurring at the pit will negatively affect the structural integrity of the metal by acting as a point of stress concentration and crack nucleation. The susceptibility of any rockbolt to corrosion is a complex process influenced by multiple parameters, including the corrosive environment, the rockbolt configuration, and the material properties. The corrosion rate of rockbolt steel depends on various factors when the rockbolts are exposed to an aqueous corrosive environment. The influencing factors can be generally divided into two: the corrosivity of the aqueous environment or the solution and the corrosion resistance of the steel. The corrosivity of the aqueous environment, or the external factors to the rockbolt material, is associated with the presence of chemical species and microbacteria or temperatures that affect the kinetics behaviour of the corrosion reaction. The corrosion resistance of the steel material is attributed to the presence of a protective coating, the alloying elements, and the resultant microstructure upon fabrication. Dorion and Hadjigeorgiou (2014) provided a high-level comparison between ground support elements and their susceptibility to corrosion. Although these guidelines are used at mine sites to select appropriate rockbolts in a range of corrosive environments it is limited in that it does not include MHR. Given the short history of MHR there are no empirical data at this time to assess their corrosion susceptibility. This is of concern given that they employ a two-component system (friction unit and rebar/rod) that, in the absence of a resin, is likely to be susceptible to corrosion. This may limit the application of mechanical hybrid rockbolts to relatively short-term applications in corrosive environments, thus, the driving force for ongoing work at the University of Toronto on the long-term performance of MHR in corrosive environments. The present work reports on the results of a research programme focused on the influence of the chemical composition and the resultant steel microstructure of the main components of MHR using accelerated corrosion experiments. Experimental investigations The objective of this investigation was to provide a first set of data on the potential corrosion susceptibility of the recently developed MHR. To this end, two MHR from different suppliers have been subjected to a series of electrochemical tests. Of interest is that mechanical hybrid rockbolts comprise two distinct main components, the rebar/rod and the friction unit, which use different Table 1 Chemical composition of steels for MHR (in weight%) Element MHR #1 MHR #2 C (%) Mn (%) P (%) S (%) Si (%) Cu (%) Ni (%) Cr (%) Tendon/bar 0.229 1.44 0.024 0.025 0.59 0.02 0.01 0.04 Friction unit 0.188 0.83 0.025 0.021 0.34 0.02 0.01 0.03 Tendon/bar 0.230 1.46 0.013 0.004 0.62 0.10 0.07 0.10 Friction unit 0.091 1.14 0.008 0.003 0.20 0.01 0.01 0.02 V (%) Mo (%) Al (%) Co (%) Ti (%) Pb (%) Nb (%) Fe (%) Tendon/bar 0.05 <0.01 <0.01 <0.01 <0.01 <0.01 0.01 Balance Friction unit 0.02 <0.01 <0.01 0.01 <0.01 <0.01 0.02 Balance Tendon/bar 0.07 0.02 <0.01 <0.01 <0.01 <0.01 0.01 Balance Friction unit <0.01 <0.01 0.04 <0.01 0.02 <0.01 0.03 Balance Element MHR #1 MHR #2 The Journal of the Southern African Institute of Mining and Metallurgy VOLUME 126 AUGUST 2026 493