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The Concept Of Rust
Mar 19, 2018

    After exposure to the atmosphere, the metal workpieces that have been cut or formed will inevitably be attacked by moisture, oxygen, or various corrosive substances, and chemical or electrochemical reactions will occur, causing corrosion. In general, corrosion or discoloration caused by the influence of moisture, oxygen, or other corrosive gases (hydrogen sulfide, sulfur dioxide, nitric anhydride, and chlorine gas) in the atmosphere is referred to as rust or rust. Iron and steel in the atmosphere, due to the formation of local micro-battery on the surface, resulting in the electrochemical reaction generated by the hydroxide or oxide-based compounds, broadly referred to as rust.

 

In fact, ferrous metals (iron and steel) are weaker than non-ferrous metals in preventing rust. Because only moisture and oxygen are enough to rust the steel and form loose ferrous hydroxide and iron hydroxide. This process is called rust and its product is "rust." Non-ferrous metals, on the other hand, mainly show corrosion. The rusting process is usually caused by electrochemical corrosion, while the corrosion is generally caused by chemical corrosion.

 

Metal atmospheric corrosion factors

    According to the different media, the corrosion of metal can be divided into atmospheric corrosion, underground corrosion, seawater corrosion and bacterial corrosion. Atmospheric corrosion is the most common metal corrosion and it is an electrochemical corrosion process.

    One of the important factors affecting metal corrosion in the atmosphere is, first of all, the relative humidity of the air, which is related to whether or not a water film and its thickness are formed on the metal surface. Water film formation is a shift with humidity and temperature. When the relative humidity is lower than the critical humidity or lower than 65%, no water film can be formed at any temperature, and the corrosion rate is very small at the time, and almost no corrosion occurs; when the relative humidity is higher than the critical humidity, the water film is formed. The corrosion rate suddenly increases. Even if the temperature increases slowly, the corrosion rate multiplies. For every 10°C increase in temperature, the corrosion rate increases approximately 1 time. The cyclical temperature difference or humidity change is more serious for metal corrosion.

 

Second, oxygen is also a major factor in the corrosion of metals in the atmosphere. Oxygen is easily dissolved and penetrates into the water film. The bottom of the water droplet on the metal surface, due to uneven air penetration and diffusion, causes the oxygen concentration in the central region of the water drop to be low, forming an anode, and the oxygen concentration in the peripheral region is high, forming a cathode, and the ferrous metal is released by the anode reaction in the central region of the water drop. Through the metal flow to the edge of the water droplets, and the cathode reacts to synthesize ferrous hydroxide, deposited on the metal surface between the cathode and the anode. Iron reacts directly with water to form ferrous hydroxide, and ferrous hydroxide continues to dissolve dissolved oxygen in water droplets to form iron hydroxide. Rust is a mixture of [mFe(OH)2, nFe(OH)3, pH2O].

 

Again, in addition to water and oxygen, there are also various pollutant gases in the atmosphere. For example, salt spray, sulfur dioxide, hydrogen sulfide, ammonia, etc. Can make the metal accelerate the corrosion rate in the atmosphere. Dust particles in the atmosphere have capillary condensation. When there is dust on the metal surface, it quickly becomes the core of water droplets condensation. Although their content in the atmosphere is less, but the concentration of dissolved in water is quite high, and has a great impact on metal corrosion.

 

Corrosion and rust prevention

Surface rust will greatly impair the performance and application of metal products and mechanical equipment. Statistics show that the world’s annual loss of corrosion metal accounts for more than 10% of total metal production. It not only affects the appearance, color, and mechanical properties of the metal material itself, but also impairs the quality, precision, and sensitivity of various metal components, directly affecting the use value.

    Since metal rust has such drawbacks, in order to effectively prevent corrosion of metal parts and equipment, it is an effective method to isolate the metal from the surrounding environment. One of them is permanent protection such as: change the internal structure of the metal (add other elements when making steel, make it stainless steel); use chemical methods such as oxidation, phosphatization, etc. on the metal surface to transform the cover; use nitriding, infiltration of metal and other methods Metal surface alloying; also used for electroplating, spraying, electroless plating and other methods to change the cover; or use enamel, ceramics, rubber, paint and other non-metallic materials coating. However, in the process of machining metal raw materials, a large number of metal surfaces are not allowed to give permanent preservation. For example, metal surfaces such as machine tool guide surfaces, cutting tools, measuring tools, bearings, and metal sections after machining need to maintain the original surface, which requires temporary anti-corrosion, that is, temporary rust prevention. There is a temporary protection against corrosion in the lubricant category. The term "temporary" here means that the rust-proof material can be removed after the product has been rust-proof for a certain period of time, and does not mean the rust-proof period of such products.

 

Temporary rust-proof materials (antirust oil, grease)

     Anti-rust grease is formulated by adding oil-soluble corrosion inhibitors, film-forming materials and other auxiliary additives to petroleum base oils. The role of anti-rust grease is as follows:

(1)  Adsorption According to adsorption theory, rust-proof grease can play a role in rust prevention mainly because its adsorption on the metal surface reduces the metal ionization tendency, thereby reducing the reaction energy of the metal surface atoms, making the metal surface tend to be stable status. The rust inhibitors in rust preventive greases are polar organic compounds, and they are also surfactants. They consist of asymmetric polar and non-polar groups. When rust-proof grease is applied to the metal surface, polar parts of the rust inhibitor such as -OH, -COOH, -SO3, -NH2, etc. form a directional adsorption at the oil-metal interface, and the adsorption strength depends on the strength of the polar group. weak. The non-polar portion of the rust inhibitor, the hydrocarbon group, forms a hydrophobic protective film on the surface of the metal, hinders the movement of charges or substances involved in the corrosion reaction, and slows the corrosion rate. The rest of the rust inhibitor molecules have their non-polar groups arranged outward in the oil, and the polar groups are aligned inwards to form micelles present in the oil. This micelle melts adsorbed water and insoluble impurities. Insoluble rust inhibitor helps dissolve. The directional molecular film formed on the oil-air interface by the rust inhibitor molecule can also reduce the solubility of oxygen in the oil and the penetration of other corrosive substances to a certain extent.

(2) Prevent Corrosion of Corrosive Media from Metals Generally, the adsorption force of mineral oil itself and metal is not strong, and the formed oil film is not strong, and it is easy to absorb and dissolve some water and oxygen, and it cannot prevent the media from corroding the metal. When the rust inhibitor is added, the rust preventive micelles contained in the oil play a role of solubilizing the water, so that the water entering the oil is trapped in the micelles, which slows down the water permeability of the oil layer. Plays a role in inhibiting rust.

 (3) Replacement of water film Some rust inhibitors in rust preventive oils have the property of replacing the  water film and can replace the water film adsorbed on the metal surface, thereby slowing the metal corrosion  rate. Water is also a polar substance. The polarity of the rust inhibitor is stronger than that of the water  molecule, and it has a greater affinity with the metal and thus has the ability to replace water.

 (4) Neutralization displacement and solubilization of polar substances Some anti-rust oils neutralize and  solubilize polar substances. They adsorb very densely on the metal surface, making it difficult for the corrosive  medium to penetrate the metal surface. Also due to the adsorption and trapping of the rust inhibitor above the  critical micelle concentration, the corrosive medium can be trapped in the micelles. To rust effect.

 (5) The carrier oil of the base oil The base oil can act as a carrier in the rust-preventing oil to fully disperse the  rust inhibitor in the oil; the molecules of the base oil penetrate deep into the directional adsorbed rust inhibitor  molecules and adsorb on the adsorbent. Physical adsorption is carried out in a few places, together with rust  inhibitor molecules to fill gaps, making the adsorption membrane more compact and effectively protecting the  metal.


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