corrosion of steel

 Corrosion of steel

Corrosion of steel refers to the destruction of its surface due to chemical reaction with the surrounding medium. Corrosion can occur in many media that cause corrosion, such as moist air, soil, industrial waste gas, etc. As the temperature rises, the chemical reaction accelerates, and the degree of corrosion is accelerated.

The steel is severely corroded during storage, not only the cross-sectional area becomes smaller, the material is reduced or even scrapped, but also the derusting work is very expensive. Derusting during use not only reduces the stressed area, but also causes local rust pits to cause stress concentration and promote early failure of the structure. Especially when the structure bears repeated loads, the steel will produce corrosion fatigue phenomenon, which will greatly reduce the fatigue strength and cause brittle fracture.

Corrosion is classified according to the different effects of the steel surface and the surrounding medium

1) chemical corrosion

Chemical corrosion is a kind of pure chemical corrosion caused by the chemical reaction between steel and electrolyte solution or various dry gases (such as O2, CO2, SO2, etc.) in the service environment. There is no current action in the corrosion process. Most of this corrosion is oxidation, forming loose oxides on the surface of the steel. This kind of corrosion is greatly affected by the environment, and it progresses slowly in a dry environment, but it progresses quickly in an environment with high temperature and humidity.

2) Electrochemical corrosion

The corrosion that occurs when steel is in contact with an electrolyte solution to generate an electric current and form a primary battery is called electrochemical corrosion. Since the electrode potentials of different components in the steel are different, it is easy to form two poles of the electrode. For example, when steel is in contact with moist air, water, and soil, the surface is covered with a layer of water film, and various ions from the air are dissolved in the water, thus forming an electrolyte. Through a series of physical and chemical actions, the ferrite in the steel is oxidized into iron hydroxide and iron oxide, etc., resulting in corrosion.

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