Protection against steel corrosion
Protection against steel corrosion
In actual engineering, there are three main methods of protection against steel corrosion.
1) Protective film method
The protective film is used to isolate the steel from the surrounding medium, thereby avoiding or slowing down the damaging effect of the external corrosive medium on the steel. Such as spraying paint, enamel, plastic, etc. on the surface of steel; or using metal plating as a protective film, such as zinc, tin, chromium, etc.
2) Electrochemical protection method
According to the specific cause of corrosion, it is divided into no-current protection method and applied current protection method.
The no-current protection method is also called the sacrificial anode method. It is to connect a metal that is more active than steel on the steel structure, such as zinc and magnesium. Because the potential of zinc and magnesium is lower than that of steel, zinc and magnesium become the anode of the corrosion battery. to damage (sacrificial anode), while the steel structure is protected. This method is often used for places where it is not easy or impossible to cover the protective layer, such as steam boilers, ship shells, underground pipelines, port structures, road and bridge buildings, etc.
The impressed current protection method is to place some scrap steel or other refractory metals, such as high-silicon iron and lead-silver alloy, near the steel structure, connect the negative pole of the external DC power supply to the protected steel structure, and connect the positive pole to the refractory On the metal, after electrification, the refractory metal becomes the anode and is corroded, and the steel structure becomes the cathode and is protected.
3) Alloying
Alloying elements that can improve corrosion resistance, such as nickel, chromium, titanium, copper, etc., are added to carbon steel to make different alloy steels.
The above-mentioned methods can be used to prevent the corrosion of steel bars in reinforced concrete, but the most economical and effective method is to increase the compactness and alkalinity of concrete and ensure that the steel bars have sufficient protective layer thickness.
In cement hydration products, due to the presence of about 1/5 of calcium hydroxide, the pH value of the medium reaches about 13. The presence of calcium hydroxide causes a passivation film on the surface of the steel bar to form a protective layer. At the same time, calcium hydroxide can also interact with CO2 in the atmosphere to reduce the alkalinity of concrete, and the passivation film may be destroyed, making the steel surface in an activated state. In a humid environment, electrochemical corrosion begins to occur on the surface of the steel bar, resulting in cracking of the concrete along the bar. Therefore, the anti-carbonation performance of concrete should be improved by improving the compactness of concrete.
In addition, chloride ions (C1 − ) have the effect of destroying the passivation film. Therefore, when preparing reinforced concrete, the use of chloride salts should be limited.
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