Do You Know The Reason For Rusty Metal?

publisher: Lisa Zhong
Time: 2012-05-07

The main factors affecting the corrosion of metal are roughly the following:

(1) the relative humidity of air is an important factor affecting atmospheric corrosivity. The relative humidity refers to the absolute humidity (the amount of water in the air, g / m 3) and the saturated humidity (the amount of water saturated in the air. Beyond this limit, a water vapor that is the formation of water droplets fall) The Metal due to its nature and surface morphology of the different, there is a rust generated by the critical relative humidity value. Steel rust critical relative humidity is generally 75%, so reduce the relative humidity of the atmosphere, is a major anti-rust approach.


(2) changes in air temperature and humidity is also a major factor affecting atmospheric corrosion. Low temperature, high humidity will cause water vapor condensation. Condensation of water film dissolved in harmful gases and salts, it will produce electrochemical corrosion. And the higher the temperature, the faster the rust rate of the metal.


(3) The acidity (pH) of the solution has an effect on the corrosion of the metal, where chlorides have a greater effect on the rate of corrosion. Salt spray, hand sweat, heat treatment residue residue, residual solder after welding and so contain chlorine ions, if the cleaning is not clean, can easily cause rust.


(4) the air corrosive gas pollution on the metal pseudo-impact. The most harmful is the atmospheric sulfur dioxide (SO2), because the SO2 dissolved in water that becomes sulfuric acid H2SO3, if SO2 is oxidized after the formation of sulfur trioxide (SO3), its moisture absorption stronger, more corrosive aqueous solution , Especially in chemical plants such as hydrogen sulfide (H2S), ammonia (NH3) and hydrochloric acid (HC1) and other pollution, will accelerate the electrochemical corrosion of the metal.


(5) other factors also have an impact on the metal. The air contains a lot of dust, including soot, coal ash and carbon, carbides, metal oxides, sand, sodium chloride, sulfuric acid and other salt particles. These substances fall on the metal surface, especially with the water vapor under the action of its corrosion will be greatly enhanced.


Steel in the humid air surface will form a thin layer of water film, water film and dissolved from the atmosphere of carbon dioxide, sulfur dioxide, hydrogen sulfide, and other gases, so that the water film contains a certain amount of hydrogen ions, the results in the steel surface A layer of electrolyte solution of the film, it is iron and iron and a small amount of carbon just constitute the original battery. As the process of corrosion in the hydrogen continue to release, so called hydrogen corrosion, which is caused by corrosion in acidic environments. Under normal circumstances, if the steel surface adsorption of water film is weak or neutral, but dissolved in a certain amount of oxygen, this time will occur oxygen corrosion. In these two electrochemical corrosion process, if there is a salt, it will make electrons in the solution which move faster, played a conductive role, which can speed up the corrosion rate of steel.


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