Main Principles and Several Types of Rail Spike Corrosion
Main Principles of Rail Spike Corrosion
After a long-term exposure in the atmosphere environment, the external environment of rail spikes(include dog spike and screw spike, etc) is worse. Rail spikes must experience rain, the excrement and urine from the passenger train, wastewater and chemical, mineral from freight train during transport process. With the temperature and climate changes constantly, they are prone to be corrosive. After the spikes are anchored, the corrosion reasons are very complex. The following several reasons of corrosion on spike and rail in daily working environment are illustrated:
- A. The excrement, urine and washing wastewater from the passenger train
- In the excrement, urine and washing wastewater, there contains all kinds of organics, microbe, bacteria, compounds and inorganic salts (calcium, iron, magnesium), etc., which provides the environment for electrolyte solution for spike corrosion.
- B. Sulfur and sulfide of anchoring agent or in the atmosphere are easy to cause the corrosion on the rail spikes.
- C. The air pollution affected by local environment.
- In the nearby regions of railway lines passing through, water soluble acid and liquid particles, for example, SO2, ammonia gas, emitted from the chemical plants emissions of, may cause the corrosion on the surfaces of rail spikes.
- D. In recent years, because of the atmospheric pollution, the rain is acidic.
- E. The chemicals and minerals from the freight train.
Rail spike made in China is generally made of Q235 carbon steel. The base material contains C, Mn, Si, S, P and other elements.
Steel oxidation is a complex process in atmosphere corrosion. According to the different corrosion mechanism, it can be usually divided into chemical corrosion and electrochemical corrosion.
The most main chemical corrosion
The most main chemical corrosion is the air corrosion, which is also the metal oxidation process. Rail spikes in dry air are the air chemical corrosion. Under the very dry conditions, the thickness of water film on their surfaces is 10nm. If there is no continuous electrolyte membrane, the corrosion rate is very low, which is mainly chemical oxidation.
In the process of chemical corrosion, because of the interface reaction, the oxidation film is formed on the metal surface. This will separate the environment from the steel. The oxidation speed is controlled by the diffusion of the reactant through the membrane. At the early stage, after the contact of the metal and the oxygen, the oxygen is attached on the surface by the physical adsorption, and then it will decompose into atomic oxygen molecules and have the chemical reaction with the steel. This will generate a layer of single molecule oxide (FeO) with chemical bonding. This is so-called metal oxidation film. the film growth is the process of electrochemical reaction. Therefore, the chemical corrosion is not a major reason of the failure of the root corrosion of rail spikes.
The electrochemical corrosion
When rail spikes are fixed by the sulfur anchor with the connection with concrete sleepers, the surface of cone frustum of rail spike will be attached to a layer of water film affected by the discharged wastewater in the humid air. The metal surface will form a micro battery, namely corrosive battery. As the water layer is more than 10nm, the electrolyte membrane the electrochemical corrosion must be needed will be formed, which makes CO2, SO2, NO2, etc. dissolved in the water layer, and forms weak acid electrolyte solution. The steel soaked in the solution layer is also contains graphite, carburizing body (Fe3C) and other metals and impurities. The formation of corrosive battery will take the steel as the anode and the impurity as the cathode. In addition to the close contact of the steel and the impurities and, the corrosion process will be on constantly.
Because the impacts of construction structure, anchoring position and external environment, the upper part of rail bearing platform of the sleeper is easy to retain water. When the surface of cone frustum of rail spike exist the electrolyte membrane, it generally shows the weak acid or neutral environment, which is prone to absorb the oxygen, cause the corrosion and generate Fe(OH)2 that can be oxidized, resulting in Fe(OH)3. Fe(OH)3 can get Fe2O3 by dehydration.
Several Types of Rail Spike Corrosion
Because the rail spike is directly exposed in the atmospheric environment, as the medium contains the active anion (Such as Cl-anion), the anion is adsorbed on the some points of surface of the root part of the spike after the pollution so that the nitrided passive film on the surface of the spike fails. Once the passive film is destroyed, the inner metal is exposed and begins the surface corrosion. This is the reason that the iron there is active, but the passive film still is in passive condition. This forms the active-passive corrosive battery. The anode area is much smaller than the cathode area, and the anode current is dense, so the corrosion gets deeper and deeper. The metal surface forms the holes. This is commonly named pitting corrosion. When the chemical raw materials from the train or the acid rain caused the PH value of the electrolyte gets decreased, or makes the temperature increase, they must add the tendency of pitting corrosion.
In the electrolyte remaining on the surface of cone frustum of rail spike, when The crevice has in the narrow gap formed between the irons or iron and other non-metallic surfaces, the related substances inside the crevice are blocked, forming the concentration difference. This will cause local crevice corrosion. In the medium, the oxygen concentration increases, crevice corrosion gets worse. The PH value decreases, the anodic dissolution rate increases. The crevice corrosion gets much worse. When the concentration of the active anion increases, the sensitivity of the crevice corrosion will be also increased.
As we know, the steel rail is fixed on the sleeper by rail fasteners. The elastic fasteners are screwed tightly by the stress of screw spike that is anchored to the sleeper and form a whole with the latter. When a train passes, it will have strong friction force, horizontal force and elastic vibration force on the rail. Therefore, rail spike bears not only the tensile stress under static state but also the cyclic stress as the train passes. Under the affection of the stress, rail spike may bring about the brittle cracking phenomenon as it is lower than the ultimate strength. This is known as stress corrosion. Stress corrosion is formed from small pits in the sensitive parts of the spike, producing slender crevices. And the crevice develops very quickly can cause serious damage in a short period.
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