What we usually call corrosion or rust is a chemical or electrochemical oxidation reaction process on the metal surface. To do a good job of rust resistance, we must understand the types of rust.
Classification of rust
Steel, cast iron, stainless steel, and other metals are red metal oxides formed in the air at room temperature due to humidity, wind, and rain.
Colored or black metal oxides formed by steel, cast iron, stainless steel, copper, and other metals under high temperatures (hot rolling, casting, annealing, quenching, welding, etc.).
Copper rust: red copper, brass, bronze, white copper, etc. in the air and soil (underground), due to the corrosion of the environment, various metal oxides and oxide films such as green, blue, red, and colored are formed.
White hair of aluminum, zinc, and magnesium: white metal oxides formed in the air at room temperature due to humidity, wind, and rain.
Aluminum oxide film: an oxide film formed after aluminum is treated by anodic oxidation, chemical oxidation, etc.
Cause of rust
At the same temperature, the percentage of the water vapor content of the atmosphere and its water vapor saturation content is called relative humidity. Below a certain relative humidity, the metal corrosion rate is very small, but above this relative humidity, the corrosion rate increases sharply.
This relative humidity is called critical humidity. The critical humidity of many metals is between 50% and 80%, and that of steel is about 75%. Atmospheric relative humidity has the greatest influence on metal corrosion.
When the atmospheric humidity is higher than the critical humidity, a water film or water droplets will appear on the metal surface. If the harmful impurities contained in the atmosphere dissolve in the water film or water droplets, it will become an electrolyte, which will aggravate the corrosion.
Atmospheric temperature and humidity both correlate with metal corrosion. This has the following main conditions: first, the water vapor content of the atmosphere increases with the increase of temperature; second, high temperature promotes the intensification of corrosion, especially in humid environments, the higher the temperature, the faster the corrosion rate.
When the relative humidity is low, the effect of temperature on corrosion is not obvious, but when the relative humidity is higher than the critical humidity, the amount of corrosion increases sharply with the increase in temperature.
In addition, if there is a temperature difference between the atmosphere and the metal, the condensed water will form on the metal surface at low temperatures, which will also cause the metal to rust.
Among the corrosive gases in polluted air, sulfur dioxide has the greatest impact on metal corrosion, especially on steel, copper and their alloys. Sulfur dioxide in the atmosphere mainly comes from the combustion of coal.
At the same time, the combustion product carbon dioxide also has a corrosive effect. Corrosive gases are mixed in the atmosphere around the plant. Such as hydrogen sulfide, ammonia gas, hydrochloric acid gas, etc. are all factors that promote metal corrosion.
The atmosphere contains a lot of dust, such as smog, coal ash, chloride and other acid, alkali, salt particles, etc., some of which are corrosive in themselves, or condensation nuclei of water droplets, which are also rust factors, such as chloride is considered It is the sworn enemy of corroding metals.
Anti-rust methods and products
The principle of metal anti-rust: form a physical covering film or chemical conversion film on the metal surface to achieve the purpose of short-term stock anti-rust or long-term anti-rust and anti-corrosion.
Common anti-rust methods:
1. Paint (paint) method: apply paint (paint) on the metal surface to form a physical shielding protective film.
2. Plating method (electroplating, chemical plating, immersion plating, spray plating, ion plating): plating is another method of protecting metal on the metal surface, such as galvanizing, chrome plating, etc.
3. Anti-rust agent method: apply anti-rust liquid to form a physical or chemical protective film on the metal surface.
4. Electrochemical protection method: implement anodic protection on the protected metal, and achieve the purpose of protecting the equipment by sacrificing the anode (such as zinc plate).
5. Anti-rust oil: A uniform and continuous anti-rust oil film is formed on the metal surface. The anti-rust time is longer than that of water-based preparations. It also has an anti-rust effect in wet conditions, but it is easy to rust after rain.
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