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Main factors affecting metal corrosion

May 20, 2019

(1) Internal factors of metal corrosion
From a thermodynamic point of view, the metal has higher energy than its corrosive products, is unstable, and has a tendency to spontaneously convert to low-energy corrosion products, that is, metal corrosion is a natural phenomenon. The vast majority of metals in the industry are multi-component alloys with different metallographic structures, impurities, uneven machining distortion, uneven stress distribution, etc. These physical, chemical and electrochemical inhomogeneities, when surface Chemical or electrochemical corrosion inevitably occurs when a corrosive medium or surface forms a water film.
(2) External factors of metal corrosion
1. Relative temperature
The higher the relative humidity in the air, the thicker the water film on the metal surface, and the oxygen in the air acts through the water film to the metal surface. When the relative humidity reaches a certain value, the corrosion rate increases sharply. This value is called critical relative humidity, and the critical relative humidity of steel is about 70%.
2. Temperature effect
The ambient temperature is related to the relative humidity. In a dry environment (desert), the temperature is higher and the metal is not prone to rust. When the relative humidity reaches a critical value, the effect of temperature is significantly increased. For every 10 °C increase in temperature, the rust rate is increased by two times. Therefore, in wet tropical or rainy seasons, the higher the temperature, the more severe the rust.
3. Oxygen effect
The rust process is represented by the following reaction formula:
Fe+H2O→Fe(OH)2
Fe(OH)2+ H2O+O2→Fe(OH)3
Fe+ H2O+O2→Fe(OH)3
It can be seen that without water and oxygen, the metal will not rust, and 20% of the volume in the air is oxygen, which is pervasive.
4. The influence of other substances in the atmosphere
When the atmosphere contains salt spray, sulfur dioxide, hydrogen sulfide and dust, it will accelerate corrosion. Therefore, the difference in corrosion between different environments is obvious, the city is higher than the rural area; the industrial area is higher than the living area; the coastal area is higher than the inland; High dust is higher than low dust.
(3) Corrosion factors during processing
The raw materials have been rusted, and the removal has not been completed after processing; the processing materials such as cutting fluid and lubricant are contacted during processing; the neutralization and cleaning are not performed after pickling; the heat treatment oxidative decarburization or residual salt cleaning is not clean; the operation during inspection, handling and assembly The sweat left by the person; improper stressing or dehydrogenation annealing; unsuitable for rust prevention between processes and storage.

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