Corrosion behavior and failure mechanism of rock bolt(cable) in underground coal mine

Premature failure of rock bolt (cable) due to stress corrosion cracking (SCC) is a phenomenon that has reported to occur in underground environments. In this study, the corrosion behavior and failure mechanism of rock bolt (cable) samples obtained from underground coal mine were examined and discussed. Macroscopic observation and weight loss test were carried out for the corrosion characteristics of the bolts without failure. It is found the corrosion form is uniform corrosion for the bolts with short service time (S-1, S-2). But for the bolts with longer service time (S-3, S-4 and S-5), experience different degrees of pitting corrosion and the number of corrosion spots on the surface of the sample increases with increase of the service time. The corrosion amount of different parts of the bolts shows the rule of bolt head > bolt end > free section, and the corrosion amount increases year by year with the service time, but the corrosion growth rate is fast in the early stage and slows down in the later stage. At the same time, the failure mechanism of the failure sample S-6, which has been in service for 6 years, was analyzed with macro and micro methods. The results show that S-6 have been in O, Cl and S surrounding rock environment for a long time, resulting serious pitting corrosion and spalling of the surface material of the steel strand, which reduces the outer diameter of the rock cable and its bearing capacity, finally leads to the instantaneous SCC fracture of the sample S-6.


Introduction
The prestressed rock bolt (cable) technology has been used for more than 80 years since 1934 (Hu et al.1987), and is widely applied in geotechnical engineering for its advantages like light structure, flexible layout, fast construction and low cost (Zhang et al. 2009;Kang et al.2010). However, as a kind of reinforced material with the steel bar or steel strand, the durability is threatened for that the prestressed bolt (cable) in the prestressed and severe surrounding rock environment for a long time.
Recently, the durability of the prestressed bolt (cable) has been highly concerned by scholars at home and abroad. The Fé dé ration Internationale de la Pré contrainte has collected and analyzed some major anchor bolt (cable) failure cases (Zeng et al. 2004;Zhang et al. 2001;Liu et al.2001 ); With the support of the US Department of Energy, Chandra et al.(2007) carried out the study of corrosion characteristics and mechanism of different anchor cables under the influence of nuclear waste in Yucca Mountain geological environment; Gamboa et al. (2003) studied the influence of environmental factors on the stress corrosion failure of the prestressed bolt through the indoor test for the first time. The research on the prestressed bolt started relatively late in China. Zheng et al. (2010) carried out the experimental research on the influence of environmental factors on mechanical properties of the bolt. Zhao et al.(2007) studied the factors affecting the durability of the bolt under the stress and environmental corrosion condition by indoor simulation test and field sampling test; Li et al. (2008) quantitatively described the influence of the pH value, time, stress level and other factors on the corrosion amount per unit length of the specimen through indoor accelerated corrosion test; Li et al. (2017) analyzed the influence characteristics of chloride ion concentration, time, pH value and other factors on the corrosion of the cables and the mutual coupling relationship between different influencing factors through the indoor test; Zeng et al. (2011) studied the strength index and apparent characteristics of the anchor cable under strong corrosion environment; Zhang et al. (2018) observed the corrosion damage morphology of the prestressed anchor cable under different time, established the relationship between the indicators like corrosion pit area and depth and the time, and analyzed the surface damage evolution law of the stress corrosion.
At present, most of the research on rock bolt (cable) durability is limited to the indoor test of simulated corrosion environment, which is quite different from the actual working environment of the anchor bolt (cable). In this paper, the bolt samples obtained from a underground coal mine were used as the experimental object to study the corrosion characteristics through the macro observation and indoor weight loss method. Meanwhile, macro and micro means were adopted to analyze the failure mechanism of a certain cable sample in this mine. The samples obtained from underground coal mine were shown in Fig. 2 , which numbered as S-1, S-2, S-3, S-4, S-5, S-6 respectively. The details of bolt (cable) samples see Table 1.

Corrosion behavior of bolt samples
The corrosion degree of five bolt samples which were not failed was analyzed, and the weight loss was measured on 2 cm samples taken respectively from the bolt head, free section and anchorage section. The bolt was derusted by acid pickling. Fig. 3 shows the schematic diagram of the bolt samples before and after derusting. The mass loss can directly and quantitatively characterize the corrosion degree of the bolt, which is calculated by Eq. (1), and the corrosion rate of the bolt is calculated by Eq. (2).
Where ΔW is the weight loss per unit area of the bolt; 0 w is the mass before derusting of the bolt; 1 w is the mass after derusting; 0 As shown by the appearance of the bolt samples taken, all samples have slight corrosion, which can be divided into uniform corrosion and local pit corrosion. S-1 and S-2 are characterized by uniform corrosion, and pit corrosion occurs in different parts of S-3, S-4 and S-5. The number of corrosion spots on the surface of each samples increases with the increase of service time. For example, a large area of corrosion spots can be found on the surface of S-5 bolt which has been used for 8 years, and the depth of some pits reaches 0.18 mm.
(a) Bolt sample before derusting (b) Bolt sample after derusting year. The corrosion amount of different parts is characterized by the rule of anchor head > anchor end > free section. This is because the anchor head has been exposed to the air for a long time, and the O2 in the air accelerates the corrosion of the anchor head, while other parts are wrapped by concrete mortar for a long time with less contact with corrosive media, so the corrosion is relatively light in degree. The failure analysis of the anchor cable sample S-6 in use for six years in Qishan Coal Mine was conducted. Fig. 6 shows the overall morphology of S-6. The anchor cable has brown surface and consists of seven steel strands. The corrosion morphology of each area of the broken anchor cable sample is shown in Fig. 7. The macro analysis results show that the whole anchor cable is characterized by uniform oxidation corrosion (Fig. 7a). Pit corrosion is found in some areas of the steel strand in the region from about 900mm away from the anchorage to the fracture, as shown by the arrow in Fig. 7 (b). However, near the fracture (about 1200mm away from the anchorage), the number and degree of pit corrosion are relatively serious, and some places even have spalling (Fig. 7c). As shown in Fig. 7(c), the length of the steel strand at the breaking point of the anchor cable is different. It is an inclined fracture formed under tensile stress with a certain angle to the axial direction, and the longitudinal length of the fracture is about 25 mm. There is no obvious necking before fracture of these steel wires, and it can be observed that the steel strands crack from the outer surface with more serious oxidation and corrosion.   (2) Analysis of corrosion products on fracture and steel strand profile The metallographic samples of transverse section were taken from the pit corrosion area about 900 mm away from the anchorage near the non-fracture area.
After sample grinding and polishing, the morphology was observed by the scanning electron microscope and the EDS energy spectrum composition was analyzed. Fig. 10 shows the low speed fracture of the steel wire, and Fig. 11   there is a shallow crack perpendicular to the applied load in the fracture initiation area (Fig 13a). Then the crack deflects and propagates along the longitudinal direction in second stage (Fig 13b). Lastly, the sample was suddenly fractured with higher deviation angle in third stage (Fig 13c). This type of failure has also been observed in other research results (Proverbio et al. 2003;Toribio et   The fracture path profile of S-6

Conclusions
The corrosion behavior and failure mechanism of bolt (cable) samples obtained from a certain mine were analyzed. The conclusions are as follows: ( (3) The failure mechanism of sample S-6 is that it has been in the surrounding rock environment of O, Cl and S for a long time, resulting serious pitting corrosion and spalling of the surface material of the steel strand, which reduces the outer diameter of the rock cable and its bearing capacity, finally leads to the instantaneous SCC fracture of the sample S-6.