Corrosion Behavior and Failure Mechanism of Prestressed Rock Bolts (Cables) in the Underground Coal Mine

Premature failure of rock bolts (cables) due to stress corrosion cracking (SCC) is a phenomenon that has been reported to occur in the underground environment. In the 1990s, many failure accidents of bolts which occurred in the United Kingdom were caused by SCC [1]. In this study, the corrosion behavior and failure mechanism of rock bolt (cable) samples obtained from the underground coal mine were examined and discussed. Macroscopic observation and weight loss tests were carried out for the bolts’ corrosion characteristics without failure. 0e results show that the bolts with short service time (1.5–2 yrs) underwent uniform corrosion. However, bolts with longer service time (3–8 yrs) experienced different pitting corrosion degrees. 0e corroding degree of different parts of bolt samples shows the following decreasing trend: bolt head > bolt end > free section. 0e absolute corrosion degree increased with the service time, while the corrosion rate was the highest in the early stage and dropped down in the later stage. At the same time, the macroand micromethods were used to analyse the failure mechanism in the broken cable sample. Failure of one cable sample with a medium service life (6 yrs) was found to be controlled by the SCC. It was induced by long-term action of O, Cl, and S in the surrounding rock environment and resulted in pitting corrosion. 0e pitting corrosion reduced the outer diameter of the rock cable and its bearing capacity, leading to the final fracture.


Introduction
e prestressed rock bolt (cable) technology has been used since 1934 [1,2]. It was widely applied in geotechnical engineering for its advantages, such as light structure and low cost [3][4][5][6]. However, since prestressed bolts (cables) are materials reinforced with steel bars or strands, their durability is strongly affected by the long-term action of the surrounding rock environment.
us, several researchers have collected and analysed some rock bolt (cable) failure cases [7][8][9]. Chandra and Jack [10] carried out the study of corrosion characteristics and mechanism of different rock cables under the influence of nuclear waste in the Yucca Mountain geological environment. Erwina [11] studied the influence of environmental factors on the stress corrosion failure of the prestressed bolt through the indoor test.
In China, Zheng et al. [12] carried out experimental research on the influence of environmental factors on the bolt's mechanical properties. Zhao et al. [13] studied the factors affecting the durability of the bolt under the stress and environmental corrosion condition. Li [14] 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 the indoor accelerated corrosion test. Li [15] analysed 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. [16] studied the strength index and apparent characteristics of the rock cable under the corrosion environment; Zhang et al. [17] observed the corrosion damage morphology of the prestressed rock cable under different times, established the relationship between the indicators such as the corrosion pit area (depth) and the time, and analysed the surface damage evolution law of the stress corrosion. Aziz et al. [18] conducted a stress corrosion test for 3.5 years on four rock bolts and found that the strength of the four rock bolts decreased 21% to 39%. Wu et al. [19] conducted a stress corrosion cracking test on the bolts by simulating the stress state and local chemical environment. e results showed that the stress intensity had a significant influence on the corrosion damage of the specimen.
At present, most of the research studies on the durability of the rock bolt (cable) were limited to the indoor test of the simulated corrosion environment, which was quite different from the actual working environment of the rock bolt (cable). In this paper, the bolt samples obtained from an underground coal mine were experimentally investigated to study the corrosion characteristics through the macro-observation and weight loss test. Meanwhile, macro-and micromethods were adopted to analyse the failure mechanism of the rock cable.

Examination of Rock Bolts (Cables)
Obtained from the Underground Coal Mine 2.1. Sampling Condition. e samples were taken from Qishan Coal Mine in Xuzhou, China, which was put into operation in December 1959. e influencing environmental factors such as temperature, pH, and harmful ions were tested. It was found that the surrounding rock environment of the bolts (cables) is nearly the same at the same horizontal stratum, which provides the possibility to study the corrosion behavior of the rock bolts with different service years in the underground coal mine. Synthetically considering the service time and the difficulty of on-site sampling, six full-length adhesive samples were drilled out in the −700 m East Wing return airway. Among them, five samples were undamaged rock bolts, and one was a failed rock cable. Figure 1 shows the fielding sampling diagram. After the bolt (cable) samples were drilled out, they were immediately packed and sealed with the plastic wrap to avoid accelerated corrosion. e samples obtained from the underground coal mine are shown in Figure 2, which were numbered as S-1, S-2, S-3, S-4, S-5, and S-6, respectively. e details of bolt (cable) samples are listed in Table 1.

Corrosion Behavior of Bolt Samples
e corrosion degree of undamaged bolt samples was analysed. Firstly, the appendages on the sample surface, such as soil and resin, were decontaminated by the ultrasonic vibration. en, the weight loss was measured by acid pickling on 2 cm-long samples, which were taken from the bolt head, free section, and anchorage section, respectively. Figure 3 shows the schematic diagram of bolt samples before and after derusting. e mass loss can directly and quantitatively characterize the bolt's corrosion degree, which is calculated by equation (1), while the bolt's corrosion rate is calculated via equation (2): where ΔW is the weight loss per unit area of the bolt; w 0 is the mass before derusting of the bolt; w 1 is the mass after derusting; A 0 is the surface area of the bolt; ΔW U is the weight loss per unit area of the original stainless bolt after pickling, and ΔW U � W U0 − W U1 ; W U0 is the mass per unit area of the original stainless bolt before pickling; W U1 is the mass per unit area after pickling; v corr is the corrosion rate of the bolt; and t is the service time of the bolt. As shown by the bolt samples' appearance, 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. e number of corrosion spots on the surface of each sample increases with service time. For example, a large area of corrosion spots can be found on the S-5 bolt's surface, which has been used for eight years, and the depth of some pits reaches 0.18 mm. Figure 4 shows the time-mass loss relation diagram of different parts of the bolt. As shown in Figure 4, with an increase in service time, the corrosion degree of different bolt parts increases year by year. e rule of the anchor head characterizes the corrosion amount of different parts > anchor end > free section. is is because the anchor head has been exposed to the air for a long time, and O 2 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. Figure 5 shows the variation curve of the corrosion rate of different parts of the bolt. It can be seen that each part of the bolt's corrosion rate increases rapidly in the early stage and slows down in the later stage. e reason is that, with the increase of service time, the corrosion on the surface of the bolt is accumulated, which avoids the large area of contact between the bolt body and external corrosion factors, thus reducing the growth rate of bolt corrosion.

Macroscopic Analysis of the Sample Fracture.
e failure analysis of anchor cable sample S-6 in use for six years in Qishan Coal Mine was conducted. Figure 6 shows the overall morphology of S-6. e anchor cable has a brown surface and consists of seven steel strands. e corrosion morphology of each area of the broken anchor cable sample is shown in Figure 7. e macroanalysis results show that the whole anchor cable is characterized by uniform oxidation corrosion (Figure 7(a)). Pit corrosion is found in some steel strand areas in the region from about 900 mm away from the anchorage to the fracture, as shown by the arrow in Figure 7(b). However, near the fracture (about 1200 mm away from the anchorage), the number and degree of pit corrosion are relatively serious, and some places even have spalling (Figure 7(c)). As shown in Figure 7(c), the steel strand's length 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. ere 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.

Microscopic Analysis of Corrosion
Products of the Sample Fracture

Analysis of Corrosion Products on the Fracture Surface and
Its Vicinity. e macroscopic and microscopic fracture morphology analyses were carried out after the position marked with white words was cleaned in Figure 7. e microscopic corrosion product morphology and EDS energy spectrum composition analysis on the fracture surface are shown in Figure 8. e fracture surface's microscopic analysis results show that the fracture surface is mainly covered by iron oxide, and sulfur corrosion products are detected in the local area. e morphology of microscopic corrosion products on the fracture edge surface and EDS energy spectrum composition analysis are shown in Figure 9. e results show that the corrosion products are mainly in fluffy needle and ball shape.
e corrosion products' main components as detected are Fe, O, Cl, S, Si, Mn, Cr, etc. e content of element Cl in the corrosion products is as high as 4.4%, and the mass content of element S is 0.6%.

Analysis of Corrosion Products on the Fracture and Steel Strand Profile.
e transverse section's metallographic samples were taken from the pit corrosion area about 900 mm away from the anchorage near the nonfracture area. After sample grinding and polishing, the morphology was observed by using the scanning electron microscope, and the EDS energy spectrum composition was analysed. Figure 10 shows the low-speed fracture of the steel wire, while Figures 11(a)-11(c) show the microscopic morphology and composition analysis of corrosion products of the area near the matrix, near the surface, and inside the matrix of the reinforcement in the area with pit corrosion. Figure 12 shows the microscopic morphology and composition analysis of corrosion products near the matrix, near the surface, and inside the reinforcement matrix in the area without pit corrosion. e analysis results show that the depth of some pits reaches 0.9 mm. In the pit corrosion area, the corrosion products containing O, Cl, and S or O and S are found near the steel strand matrix and the surface; in the area without pit corrosion, the corrosion products containing O are mainly found near the steel strand matrix and the surface.

Sample Failure Mechanism
As known from the macroscopic and microscopic analysis of the samples, the failure samples are not characterized by obvious plastic deformation or necking, indicating that the sample's failure mode is a brittle failure. e sample's fracture path profile can be divided into three stages, as shown in Figure 13. ere is a shallow crack perpendicular to the applied load in the fracture initiation area (Figure 13(a)). en, the crack deflects and propagates along the longitudinal direction in the second stage (Figure 13(b)). Lastly, the sample was suddenly fractured with a higher deviation angle in the third stage (Figure 13(c)). is type of failure has also been observed in other research results [20][21][22], which is considered to be a typical feature of SCC. To sum up, the failed samples have been in the O, Cl, and S surrounding rock environment for a long time, which results in pit corrosion, causes the corrosion and spalling of the surface material of the steel strand, reduces the outer diameter of the anchor cable and its bearing capacity, and leads to one-time SCC fracture of the anchor cable.       the service time, but the corrosion rate increases rapidly in the early stage and slows down in the later stage. (3) e 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 in 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 and finally leads to the instantaneous SCC fracture of the S-6 sample.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.