Corrosion of Steel Strand

Corrosion of high-carbon steel strands is a significant concern in long-term structural engineering applications. However, in mining, severe corrosion of cable bolts is relatively rare due to the short-term nature of open stope support in underground mining. Corrosion issues are more common in long-term support systems in open pits with acidic or saline groundwater, or in underground sulfide deposits. Wet conditions in cut-and-fill applications, where supported stope backs may remain for up to a year, are particularly vulnerable to corrosion. In such cases, corrosion can lead to strand failure, compromising structural integrity.

Understanding Corrosion:

Corrosion is a complex process involving the degradation of metals due to chemical reactions with their environment. Most refined metals, like iron, are unstable and tend to lose electrons, forming more stable compounds such as iron oxide (rust). This electron loss is called an anodic reaction, while the gain of electrons is a cathodic reaction. Both reactions must occur for corrosion to take place. The presence of water, oxygen, acids, or salts accelerates these reactions.

Types of Corrosion:

Corrosion of steel can be categorized into four main types:

1. Dry Corrosion: Occurs when iron reacts with oxygen in the air, forming iron oxide (FeO). This process is slow and typically results in a thin, protective layer. However, this layer can crack, exposing fresh metal to further oxidation.
2. Wet or Atmospheric Corrosion: Accelerated by moisture, this type involves the formation of compounds like iron hydroxide (Fe(OH)₂) and magnetite (Fe₃O₄). These compounds are less adhesive and can weaken the steel.
3. Immersion Corrosion: Occurs when steel is submerged in water or other electrolytes, leading to rapid degradation.
4. Stress-Induced Corrosion: Caused by mechanical stress combined with corrosive environments, leading to cracks and fractures.

Impact of Corrosion on Cable Bolts:

• Dry Corrosion: While generally slow, dry oxidation can slightly improve the bond strength of cable bolts in some cases. However, heavy rust from prolonged exposure to moisture can severely degrade performance.
• Wet Corrosion: This type is more aggressive and can significantly reduce the tensile strength and flexibility of steel strands. The presence of electrolytes like chlorides or sulfates in water accelerates the process, leading to galvanic corrosion.
• Galvanic Corrosion: In mining environments, the high concentration of chloride and sulfate ions in water creates aggressive corrosion cells. This is especially problematic in crevices, such as those formed by the flutes of a cable, where moisture is trapped, and oxygen levels are low.

Prevention and Mitigation:

To prevent corrosion, engineers can:

• Use protective coatings or galvanization to shield the steel from moisture and oxygen.
• Ensure proper drainage to avoid water accumulation.
• Select materials resistant to specific environmental conditions.
• Regularly inspect and maintain structures to detect and address corrosion early.

Sources:

• Illston, J.M., Dinwoodie, J.M., & Smith, A.A. (1979). Concrete, Timber, and Metals.
• Pohlman, S.L. (1987). Corrosion in Mining Environments.
• Goris, J.M. (1990). Cable Bolt Performance in Mining Applications.
• Minick, R.A., & Olson, D.L. (1987). Electrolyte Effects on Corrosion in Mining Environments.

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