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Four Types of Screw Spike Corrosion for Railway Concrete Sleepers
May 28, 2021
(1) Spike pitting
Because the screw spike is exposed to the atmosphere, after being polluted, when the medium contains active anions (such as Cl- ions in feces), these active anions are adsorbed on certain points on the surface of the root of the railway spike, thereby causing nitriding the passivation film on the metal surface of the rear spike is damaged. Once this passivation film is destroyed and leaks out the body metal, the metal surface will corrode. This is because the iron leaking out of the body is in an activated state, while the passivation film is still in a passive state, thus forming an active-passive corrosion galvanic cell. Since the area of the anode is much smaller than the area of the cathode, the anode current density is very large. Therefore, the corrosion develops deep, and the metal surface is quickly corroded into small holes, which is commonly referred to as pitting corrosion.
When the chemical raw materials or acid rain scattered on the train cause the pH value of the electrolyte to decrease and the temperature rises, it will increase the tendency of pitting corrosion. Conditions for the accumulation and concentration of active anions are easily formed in the stagnant active anion-containing medium, which further promotes the formation of pitting corrosion. Although the pitting corrosion has little weight loss, the corrosion rate is very fast due to the small area of the anode. Pitting corrosion will aggravate stress corrosion and corrosion fatigue. In many cases, pitting corrosion is the origin of these types of corrosion.
(2) Crevice corrosion of screw spikes
When a narrow gap is formed between iron and iron or iron and other non-metallic surfaces in the electrolyte remaining near the spike round table, the movement of related substances in the gap is blocked, forming a concentration battery, and local crevice corrosion is generated. In the medium, the oxygen concentration increases, the crevice corrosion amount increases; the PH value decreases, the anode dissolution rate increases, and the crevice corrosion amount also increases; when the concentration of active anions increases, the crevice corrosion sensitivity also increases.
For example, when there are carbon blocks scattered from the train at the round table of the spike, a very small gap (with a width between 0.025 and 0.1 mm) is formed between the ferrous metal and the non-metal, so that the medium in the gap is in a stagnant state, causing the gap corrosion of inner iron. At the beginning, oxygen absorption corrosion is carried out both inside and outside the crevice. Due to stagnation, it is difficult to replenish the oxygen consumed in the crevice. The inside and outside of the crevice constitute a macroscopic oxygen concentration cell. The lack of oxygen in the crevice is the anode, and the oxygen-rich outside the crevice is the cathode. As the pits deepen and expand, the corrosion will accelerate.
(3) Stress corrosion of screw spikes
The steel rail is pressed against the sleeper by a elastic clips fastener, the elastic clips fastener is pressed by a threaded spike, and the threaded spike is anchored to the sleeper to form a whole with the sleeper. When the train passes at high speed, it generates strong pressure, friction, horizontal force, and elastic vibration force on the rail. Therefore, the spikes bear both the static tensile stress and the cyclic stress when the train passes.
The brittle cracking phenomenon below the strength limit of the spike in the specific corrosive medium and under the action of the continuous tensile stress in the fixed direction generated by the elastic strip and the wedge stress of the crevice corrosion product is called stress corrosion. The stress corrosion cracking is the formation of small pits in the corrosion-sensitive parts of the screw spikes resulting in elongated cracks, and the cracks propagate very quickly, which can cause serious damage in a short period of time. The stress concentration area of the road stud is prone to corrosion damage. The existence of stress causes the crystal lattice to be distorted, the atoms are in an unstable state, the energy rises, the electrode potential drops, and it becomes the anode in the corrosion battery and is the first to be destroyed.
The stress corrosion process of railway spikes can generally be divided into three stages. The first stage is the incubation period. In this stage, due to the localization of the corrosion process and the effect of tensile stress, the cracks nucleate; the second stage is the corrosion crack development period. After the cracks nucleate, the corrosion medium and metal Under the combined action of the middle tensile stress, the crack propagates; in the third stage, due to the local concentration of the tensile stress, the crack grows rapidly and causes the damage of the part.
(4) Corrosion fatigue of railway spikes
Corrosion fatigue is caused by the combined action of the corrosive medium and the cyclic stress when the train passes. It is the reduction in corrosion fatigue resistance caused by the corrosive medium. The stress value of fatigue damage is lower than the yield point, and is at a certain critical cyclic stress. The fatigue failure occurs only when the value (fatigue limit or fatigue life) is above. However, corrosion fatigue may break under very low stress conditions, so it is very dangerous.
Because the screw spike is exposed to the atmosphere, after being polluted, when the medium contains active anions (such as Cl- ions in feces), these active anions are adsorbed on certain points on the surface of the root of the railway spike, thereby causing nitriding the passivation film on the metal surface of the rear spike is damaged. Once this passivation film is destroyed and leaks out the body metal, the metal surface will corrode. This is because the iron leaking out of the body is in an activated state, while the passivation film is still in a passive state, thus forming an active-passive corrosion galvanic cell. Since the area of the anode is much smaller than the area of the cathode, the anode current density is very large. Therefore, the corrosion develops deep, and the metal surface is quickly corroded into small holes, which is commonly referred to as pitting corrosion.
When the chemical raw materials or acid rain scattered on the train cause the pH value of the electrolyte to decrease and the temperature rises, it will increase the tendency of pitting corrosion. Conditions for the accumulation and concentration of active anions are easily formed in the stagnant active anion-containing medium, which further promotes the formation of pitting corrosion. Although the pitting corrosion has little weight loss, the corrosion rate is very fast due to the small area of the anode. Pitting corrosion will aggravate stress corrosion and corrosion fatigue. In many cases, pitting corrosion is the origin of these types of corrosion.
(2) Crevice corrosion of screw spikes
When a narrow gap is formed between iron and iron or iron and other non-metallic surfaces in the electrolyte remaining near the spike round table, the movement of related substances in the gap is blocked, forming a concentration battery, and local crevice corrosion is generated. In the medium, the oxygen concentration increases, the crevice corrosion amount increases; the PH value decreases, the anode dissolution rate increases, and the crevice corrosion amount also increases; when the concentration of active anions increases, the crevice corrosion sensitivity also increases.
For example, when there are carbon blocks scattered from the train at the round table of the spike, a very small gap (with a width between 0.025 and 0.1 mm) is formed between the ferrous metal and the non-metal, so that the medium in the gap is in a stagnant state, causing the gap corrosion of inner iron. At the beginning, oxygen absorption corrosion is carried out both inside and outside the crevice. Due to stagnation, it is difficult to replenish the oxygen consumed in the crevice. The inside and outside of the crevice constitute a macroscopic oxygen concentration cell. The lack of oxygen in the crevice is the anode, and the oxygen-rich outside the crevice is the cathode. As the pits deepen and expand, the corrosion will accelerate.
(3) Stress corrosion of screw spikes
The steel rail is pressed against the sleeper by a elastic clips fastener, the elastic clips fastener is pressed by a threaded spike, and the threaded spike is anchored to the sleeper to form a whole with the sleeper. When the train passes at high speed, it generates strong pressure, friction, horizontal force, and elastic vibration force on the rail. Therefore, the spikes bear both the static tensile stress and the cyclic stress when the train passes.
The brittle cracking phenomenon below the strength limit of the spike in the specific corrosive medium and under the action of the continuous tensile stress in the fixed direction generated by the elastic strip and the wedge stress of the crevice corrosion product is called stress corrosion. The stress corrosion cracking is the formation of small pits in the corrosion-sensitive parts of the screw spikes resulting in elongated cracks, and the cracks propagate very quickly, which can cause serious damage in a short period of time. The stress concentration area of the road stud is prone to corrosion damage. The existence of stress causes the crystal lattice to be distorted, the atoms are in an unstable state, the energy rises, the electrode potential drops, and it becomes the anode in the corrosion battery and is the first to be destroyed.
The stress corrosion process of railway spikes can generally be divided into three stages. The first stage is the incubation period. In this stage, due to the localization of the corrosion process and the effect of tensile stress, the cracks nucleate; the second stage is the corrosion crack development period. After the cracks nucleate, the corrosion medium and metal Under the combined action of the middle tensile stress, the crack propagates; in the third stage, due to the local concentration of the tensile stress, the crack grows rapidly and causes the damage of the part.
(4) Corrosion fatigue of railway spikes
Corrosion fatigue is caused by the combined action of the corrosive medium and the cyclic stress when the train passes. It is the reduction in corrosion fatigue resistance caused by the corrosive medium. The stress value of fatigue damage is lower than the yield point, and is at a certain critical cyclic stress. The fatigue failure occurs only when the value (fatigue limit or fatigue life) is above. However, corrosion fatigue may break under very low stress conditions, so it is very dangerous.