Corrosion Analysis of Railway Screw Spike

After six major speed increases, China's railways are developing in the direction of high-speed and heavy-duty transportation. To realize high-speed and heavy-load transportation, railway rails are required to have high stability and balance, so as to ensure the smooth operation of trains. In the application of heavy rails, sufficient buckling force must be provided to the rails to ensure the relative stability of the rail frame. Whether the spiral spike is in good condition is the key to the effective function of the fastener system.

Screw spikes and concrete sleepers are connected by sulfur anchoring. After the sleeper is laid on the line, with the increase of the use time on the road, the long-term exposure in the natural environment will easily cause the corrosion of the thread and the material near the round table, especially after the serious rust near the round table is not enough to resist the torque and tensile stress, break off. How to slow down the corrosion rate of spikes, prolong their service lifeis a issue we face in our daily work.

1. Introduction of screw spikes
The screw spike is manufactured in accordance with the China’s railway industry standard (TB 564-92), and the material is Q235 steel, that is, a carbon structural steel with a yield point of 235 MPa. The total length of the screw spike is 195mm, the upper thread is the basic size of M24, the upper nail rod length is 75mm, the thread length is 45mm; the diameter of the central round table is 28mm. The screw spike shall not be broken when the load is 130KN during the physical tensile test.

2. Analysis of the principle of screw spikes corrosion
After the spikes are laid on the road with the sleepers, they are exposed to the atmosphere for a long time. After several years of use, they are continuously affected by rainwater, direct excrement of passenger trains, waste water, and chemicals and minerals left during the transportation of freight trains. The climate is changing, the temperature is constantly changing, and the external environment is harsher. The reasons for the corrosion of the upper thread and the round platform after anchoring are very complicated. The spikes are mainly polluted by the following types in the daily working environment: ①The feces, urine, and washing water discharged from passenger trains. Stool and urine mainly contain various organic matter, microorganisms, bacteria, sulfur-containing, chlorine-containing compounds and inorganic salts (calcium, iron, magnesium), which provide an electrolyte solution environment for the corrosion of spikes. ②Sulfur and sulfide in the atmosphere and sulfur anchoring agent are easy to corrode the spikes. ③ It is greatly affected by local environmental air pollution. The SO2, ammonia and other acid gases and water-soluble liquid particles discharged from chemical plants near the area where the line passes are attached to the surface of the spikes to cause corrosion. ④In recent years, air pollution has been attached to the environment, which has caused acidic precipitation. ⑤ Chemical raw materials and minerals, such as carbon particles, scattered by the freight train.

Screw Spikes are made of Q235 carbon structural steel, and the base metal contains trace elements such as C, Mn, Si, S, and P. Oxidation and corrosion of iron metals in the atmosphere is a complicated process. According to different corrosion mechanisms, it can usually be divided into chemical corrosion and electrochemical corrosion.

(1) Chemical corrosion
The most important form of chemical corrosion is gas corrosion, which is the oxidation process of metal (chemical reaction with oxygen). The corrosion of road studs in dry air is gas chemical corrosion. Under the condition of very dry air, the thickness of the water film adsorbed on the surface is £10nm, there is no continuous electrolyte film, and the corrosion rate is very low, mainly due to chemical oxidation. In the process of chemical corrosion, an oxide film is formed on the metal surface due to the interface reaction, which separates the environment from the iron, and the rate of oxidation is controlled by the diffusion of the reactants through the film. During the initial oxidation of the road stud, after the iron metal contacts and collides with oxygen, the oxygen molecules are first physically adsorbed on the surface, and then the oxygen molecules are decomposed into atoms and chemically reacted with the iron to combine with chemical bonds to form a layer of monomolecular oxide (FeO), which is a metal oxide film, and the subsequent growth of the film has an electrochemical reaction process. Therefore, chemical corrosion is not the main cause of corrosion failure at the root of screw spikes.

The reaction is as follows:

3Fe + 20₂ →Fe304

(2) Electrochemical corrosion
After the road stud is connected to the concrete pillow by the sulfur anchoring method, its surface, especially near the road stud round platform, will adhere to a water film under the action of the moist atmosphere and the waste water discharged from the train, and a micro battery will be formed on the metal surface , Which is to corrode the primary battery. When this layer of water film ﹥10nm, an electrolyte film necessary for electrochemical corrosion is formed, so that CO2, SO2, NO2 in the air are dissolved in this layer of water film to form a weak acid electrolyte solution. The iron immersed in this layer of solution is actually an alloy, and also contains graphite, cementite (Fe3C) and other metals and impurities, most of which are not as active as iron. The anode of the corrosion battery formed in this way is iron, and the cathode is impurities, and since iron is in close contact with the impurities, the corrosion continues.

Due to the influence of the structure and the position after anchoring and the external environment, the upper part of the sleeper bearing platform is very easy to retain water. When there is an electrolyte film on the surface near the spike round platform, it generally appears as a weak acid or neutral environment, which is prone to oxygen corrosion.
Negative electrode (Fe): Fe-2e-=Fe2+
Positive electrode: O2+2H2O+4e-=4OH-
Total reaction: 2Fe+O2+2H2O=2Fe(OH)2
Then Fe(OH)2 produced by oxygen absorption corrosion is oxidized by oxygen, and Fe(OH)3 is produced and dehydrated to produce Fe2O3 rust.
That is: 4Fe(OH)2 + O2 + 2H2O = 4Fe(OH)3
The corrosion of screw spikes in the atmosphere is mainly oxygen-absorbing corrosion.