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What factors cause metal to rust despite rust-preventive treatments?
Release date:
2016/06/21
(1) Atmospheric relative humidity
At the same temperature, the percentage of the actual water vapor content in the atmosphere relative to its saturation capacity is called relative humidity. Below a certain relative humidity, the corrosion rate of metals is very low; however, once the relative humidity exceeds this threshold, the corrosion rate increases sharply. This specific relative humidity is referred to as the critical humidity. For many metals, the critical humidity ranges between 50% and 80%, with steel having a critical humidity of approximately 75%. Atmospheric relative humidity has the greatest impact on the corrosion of metals treated for rust prevention. When atmospheric humidity surpasses the critical humidity, a thin film of water or water droplets forms on the metal surface. If harmful impurities present in the atmosphere dissolve into this water film or droplets, they become an electrolyte, thereby accelerating the corrosion process.
(2) Temperature and Humidity
Atmospheric temperature and humidity are both closely linked and influence metal corrosion. There are several key factors involved: First, the water vapor content in the atmosphere increases as air temperature rises; second, higher temperatures accelerate corrosion, especially in humid environments—where the higher the temperature, the faster the corrosion rate. At relatively low relative humidity levels, the effect of temperature on corrosion is not yet pronounced. However, once the relative humidity exceeds a critical threshold, the amount of corrosion increases dramatically with rising temperature. Additionally, if there is a temperature difference between the atmosphere and the metal surface, condensation will form on the cooler metal surface, also leading to metal rusting.
(3) Corrosive gas
Corrosive gases in polluted air pose the greatest threat to metals, with sulfur dioxide being the most damaging, particularly to steel, copper, and their alloys. Sulfur dioxide in the atmosphere primarily originates from the combustion of coal. At the same time, carbon dioxide—a byproduct of combustion—also exhibits corrosive effects. The atmosphere surrounding industrial plants often contains corrosive gases such as hydrogen sulfide, ammonia, and hydrochloric acid, all of which accelerate metal corrosion. Of course, oxygen in the atmosphere is the most common factor contributing to metal corrosion, and its corrosive action occurs constantly and everywhere—this goes without saying.
(4) Other factors
The atmosphere contains large amounts of particulate matter, such as smog, soot, chlorides, and other particles of acids, bases, and salts. Some of these particles are inherently corrosive themselves, while others serve as condensation nuclei for water droplets—both of which contribute to corrosion. For instance, chlorides are considered the "arch-enemy" of metallic corrosion.
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