How to Protect Railway Screw Spikes from Corrosion

Railway screw spikes are connected to concrete sleepers by means of sulfur anchoring. After the sleepers are laid on the railway track, with the increase of service time and the exposure in the long-term natural environment, they are easy to cause the loss of material and rust near the thread and the round table. Especially after the severe corrosion around the round table they may be broken due to insufficient resistance to tightening torque and tensile stress. In the daily working environment, railway screw spikes are mainly affected by the following types of pollution:

• A. Manure, urine, and washing waste water from passenger trains
• B. Sulfur anchoring agents are prone to corrosion on spikes.
• C. Local environmental air pollution
• D. Air pollution attached to the environment, causing precipitation to be acidic.
• E. Chemical raw materials, minerals, etc. scattered by freight trains, such as carbon particles.

I. The corrosion reasons of railway screw spikes

Railway screw spikes are made of Q235 carbon structural steel in China. The base metal contains trace elements such as C, Mn, Si, S, and P. The oxidative corrosion of iron metal is a complicated process in the atmospheric environment. According to different corrosion mechanisms, chemical corrosion and electrochemical corrosion can generally be classified. Gas corrosion is the most important form of chemical corrosion, which is the oxidation of metals. The corrosion of railway screw spikes is the gas chemical corrosion in the dry air. In the process of chemical corrosion, the interface reaction forms an oxide film on the metal surface. It separates the environment from iron, and the rate of oxidation is controlled by the diffusion of the reactants through the film. After the spikes are connected with the sleepers by means of the sulfur anchoring method, the surfaces especially near the round tables of spikes will be affixed with a water film under the action of humid atmosphere and waste water discharged from the train, and then a micro battery will be formed on the metal surface. When this layer of water film is >10nm in thickness, the electrolyte membrane the electrochemical corrosion is needed will form. Then CO2, SO2, NO2 in the air will be dissolved in this layer of water film. As the iron are closely contacted with the impurities, the corrosion proceeds continuously. Due to the influence of the position and external environment after the construction and anchoring, the supporting platform on the upper part of the sleepers can retain moisture easily. Here is prone to oxygen-absorbing corrosion.

II. The corrosion types of railway screw spikes

1. Pitting corrosion

As the railway screw spikes are exposed to the atmosphere and the contaminated medium contains active anions (such as Cl- ions in the feces). These living anions are adsorbed at certain points on the surfaces of the spike bases. Thus, this may cause nitriding phenomenon that can damage passivation film of the spike metal surface. Once this layer of passivation film is damaged and the body metal is exposed, the corrosion will occur on the metal surface.

2. Crevice corrosion

When a narrow crevice is formed between iron metals or iron metal and other non-metallic surfaces in the electrolyte retained near the round tables of spikes, the movement of related substances in the crevice is retarded, resulting in a battery and causing localized crevice corrosion. In the medium, the oxygen concentration increases, the crevice corrosion also increases. As PH value decreases, the anode dissolution rate increases, and crevice corrosion also increases. As the concentration of active anions increases, the crevice corrosion sensitivity also increases.

3. Stress corrosion

Steel rails are pressed against railway sleepers by the elastic fasteners. The elastic fasteners are pressed by railway screw spikes. And railway screw spikes are anchored to the sleepers to form a whole with the sleepers. When the train passes through at high speed, it generates strong pressure, friction force, horizontal force and elastic vibration force on the rails. Therefore, the spike bears both the tensile stress at static state and the tensile stress at train running state. The brittle cracking phenomenon below the strength limit, known as stress corrosion, occur the wedge stress generated by crevice corrosion products in a certain corrosive medium and the continuous tensile stress in the fixed direction by the elastic clip. Stress corrosion cracking is the formation of microscopic pits in the sensitive parts of the spikes, resulting in slim cracks, and the cracks are rapidly expanding and can cause serious damage in a short time. The stress concentration area of spikes is prone to corrosion damage.

4. Corrosion fatigue

Corrosion fatigue is caused by the combined effects of corrosive medium and cyclic stress when the train passes. It is caused by the decrease of the corrosion resistance performance of corrosive medium. The stress value is lower than the yield point and the critical cyclic stress is above the fatigue limit, so the failure will occur. But the corrosion fatigue may occur under very low stress conditions, and it is dangerous.

III. Railway screw spike protection

Through the above analysis, we can see that electrochemical corrosion is main reason of corrosion failure of railway screw spikes. In order to protect spikes and avoid or reduce corrosion in use, the following measures can be taken:

1. Spike selection

When a new spike is screwed on a track and the anchoring spike fails to be changed, a spike made of the alloy material or by the carbonitriding treatment can be used. Besides, a spike with an anti-corrosion sleeve provided between the first threaded portion and the second threaded portion can be also used. This can prevent spike corrosion effectively from the source of anti-corrosion spikes.

2. Non-sulfur anchoring materials

At present, railway railway screw spikes are anchored in China by mainly use of sulfur anchoring agents. The sulfur anchoring agents are convenient and quick in anchoring operations. However, once the HS, S2, and SO2 in it are combined with water, the acidic electrolyte solution will be produced. The spikes may be severely corrosive. In addition, when sulphur is used for anchoring operations, it is necessary to use fire to produce sulphur. Using sulphur along the railway line will pollute the environment. By comparison, we choose to use water-hardening cementitious material such as pure silicate 525 white cement to be taken as cementing material and corundum as new water-soluble anchoring agent and the ingredients were all environmentally-friendly materials.

3. Coating long-term anti-corrosion grease

The spikes may be exposed in severe cold, high temperature, rainy, sandy or corrosive areas for a long period of time. They also suffer from axial vibration and radial torsion. Common anti-corrosion greases are far from satisfying anti-corrosion requirements. The special anti-corrosion long-life grease can protect the spikes effectively. It has good water resistance, oxidation resistance, corrosion resistance, lubrication, adhesion, high temperature resistance, and easy coating. It can delay further development of the corrosion, prolong the service life of the spikes.

4. Standard anchoring operation

All the anchoring operations shall be strictly carried out in accord with national standard. We must refuse all the informal or abnormal operations.