ISO 9223-1992 classifies atmospheric corrosion into five categories: C1 - very low corrosion; C2 -- low; C3 -- medium; C4 -- high; C5 -- very high.

 

However, some scholars have evaluated the atmospheric corrosion of tropical humid climates (Cuba, Mexico, Venezuela, etc., in the Caribbean) and found that the atmospheric corrosion of the seaside is higher than the ISO standard C5. Research by J. Molales et al. shows that the corrosion rates of carbon steel, copper and zinc on the Canary Islands exceed the C5 level specified in ISO 9223-1992.

 

Therefore, ISO 9223-2012 classifies atmospheric corrosivity into 6 categories, adding a CX level to the first 5 categories - applied to specific Marine and Marine/industrial environments, the specific atmospheric corrosivity level comparison table is as follows:

 

 

Metal surface state

 

The brightness of the metal surface has a great impact on atmospheric corrosion, and the smooth surface is not easy to form a continuous water film, or even the water film cannot be attached, so it is not easy to produce electrochemical corrosion. On the contrary, if the metal surface is uneven or very rough, then it is conducive to the formation of water film, so that the metal surface produces electrochemical corrosion, even in the case of low relative humidity corrosion may occur.

 

Atmospheric components and impurities

 

01 The effect of oxygen on atmospheric corrosion of metals

 

The air contains a great deal of oxygen. In the electrochemical corrosion process of microcells under electrolyte film, oxygen acts as a cathode depolarizer to corrode the metal.

 

02 Effect of atmospheric sulfur dioxide on corrosion

 

The degree of atmospheric corrosion is mainly determined by the composition of the atmosphere, temperature and humidity, so the corrosion of various atmospheres on metals will vary in a large range according to various different factors. The highest and most severe degree of corrosion is the wet and polluted industrial atmosphere, and the lowest degree of corrosion is the dry and clean atmosphere. For most industrial structure alloys, the most accelerated corrosion process is sulfur containing pollutants such as sulfur dioxide and hydrogen sulfide. In a polluted atmosphere, when the critical humidity is below, there is no water film on the surface of the metal. The corrosion is caused by chemical action, and the corrosion rate is very small. When the critical humidity is higher, a thin water film can be formed, and serious electrochemical corrosion occurs, in which sulfur dioxide promotes and accelerates the corrosion.

 

03 Effect of atmospheric salt on metal corrosion

 

The ocean atmosphere contains a lot of tiny salt particles and salt spray, when these salts deposited on the surface of the metal, it will produce serious corrosion. But the extent depends on the amount of salt, which in turn depends on wind and wave conditions, height from sea level, and time of exposure. Sea salt is chloride, especially magnesium chloride and calcium chloride are very hygroscopic salts, it is easy to absorb water on the surface of the metal to form an electrolyte film.

 

hazard

 

After the metal is corroded by the atmosphere, the first is that its surface color becomes gray and gradually changes color, and rust can be seen when it is serious, destroying the appearance of its surface. There may not be any major problems in the short term, but its danger is latent, and when the corrosion reaches a certain degree, it will produce obvious damage. Metal corrosion damage accidents are frequent, from the smallest screws to large metal components can occur. For example, fasteners such as screws and nuts used in factory buildings often increase the load per unit area because of corrosion and rust, and cause huge economic losses such as wind exposure when the limit is exceeded.

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