As practice shows, rust (corrosion) affects almost 12% of the world's iron production. Due to the fact that the environment is becoming more aggressive, including due to its pollution, metals are exploited in increasingly difficult conditions. Sulfur or nitrogen oxides released from the atmosphere form microdrops of nitric or sulfuric acid, which leads to a significant increase in the rate of rust.
The chemical basis of the process of corrosion of iron
Iron is a chemically active metal. It in the presence of oxygen and water undergoes oxidation, while forming various compounds: oxides, hydroxides, hydrates of oxides. Khimki state that there is no specific formula for rust. What is rust? This is corrosion, which is formed due to oxidation of iron. It has a variable composition, which depends on the environment.
Rust strikes iron over its entire surface. However, the most vulnerable are the internal and external components of the product, welds, threaded joints. Structures of rusty iron are characterized by a significant degree of friability. Rust lacks any adhesion to metal. Due to the fact that the surface of highly porous corrosion freely retains atmospheric moisture, optimal conditions are created for the further destruction of iron.
Usually the color of rust is red-brown, brown, which does not allow to assess the state of iron under a layer of corrosion. Under rust, metal can be permanently destroyed. If you do not take measures to prevent its spread, then the effects of corrosion on iron can be catastrophic, leading to complete destruction of structures. This is especially dangerous if rust has corroded the power transmission towers or the bottom of a marine vessel. What rust is for a car, and what harm it does, is known to every motorist.
Causes of Rust
Rust begins to appear when the metal is in contact with oxygen, water, oxidizing agents or acids. One of the conditions for a metal to undergo rust is the presence of impurities or additives in it. If there is contact of iron with external stimuli in the presence of salt (salt water), then corrosion destroys it much faster in view of the onset of electrochemical reactions.
If iron is pure, without impurities, then it is much more resistant to the effects of oxygen and water. Just like their metals, such as aluminum, a dense oxide coating (passivation layer) forms on its surface, which protects the bulk of the iron from deeper oxidation. However, this layer can also be destroyed if the interaction of iron with oxygen and water begins together.
Other factors that actively destroy iron are carbon dioxide in water and sulfur dioxide. Under their influence, various types of iron hydroxide are very actively formed. They, unlike iron oxides, cannot protect the metal. Hydroxide, forming, begins to exfoliate from the surface of the iron, after which the lower layer, which also exfoliates, is negatively affected. And this process continues until all the metal is destroyed, or oxygen, carbon dioxide, sulfur and water remain in the environment.
If iron, being burned in air, comes into contact with oxygen, then iron oxide ii.
When burned in pure oxygen, oxide IV.
Iron oxide iii is formed when air or oxygen passes through a metal in the molten state.
Rust composition
The rust that forms under normal conditions is usually a mixture of 3 iron oxides. They are formed at more than one moment and have different physical and mechanical properties. Iron oxides from the bottom layer towards the surface are a combination of the following components:
- Wustite (iron oxide) is a soft structure that depends on the conditions in which the metal is located. If the storage temperature is high, then this layer is the largest.
- magnetite (magnetic iron ore) - iron oxide-oxide, which has a higher porosity than the thick-walled layer, and less hardness. This structure has pronounced magnetic properties.
- Hematite (red iron ore) is usually a red-gray structure, a solid abrasive substance. Hematite has a higher density, corrodes metal and increases the coefficient of friction in contact with surfaces.
Before starting work on the elimination of rust, it is necessary to find out the composition of the metal, especially on its surface, and also to establish the conditions that contributed to its appearance. With such information, it’s easy enough to find the best option for removing iron oxide and choose the most effective rust control agents.
Classification of ways to combat corrosion
Given the basic components of corrosion, methods for removing rust are divided into the following:
- Mechanical - the elimination of the oxide layer is carried out by means of hard metal brushes, sandpaper, etc.
- Thermal - carried out by exposure to corrosion of high temperatures, usually in combination with water and (or) air currents.
- chemical - removal of iron oxides is carried out due to exposure to them with special tools that dissolve rust, when applied to the metal surface.
Please note that the effectiveness of the above methods is different. So, if the process of corrosion formation is established in a timely manner, and this is a small spot, then the surface of the iron can be effectively treated with a steel brush, emery coarse paper, angle grinder with the corresponding nozzle.
However, if it is established that rust has captured large surfaces, then chemical methods will be optimal.
If the areas of rusty metal are very large, they cannot be transported, then heat treatment is considered optimal, but it is associated with high complexity.
Typically, the processing of metal to remove rust is carried out by combined methods in which various methods are applied in a certain sequence.
Mechanical methods
The choice of a specific method of machining depends on the type of surface of the iron. So for coils of steel wire apply its rewinding from one carrier to another. In this case, during bending, the rust is separated from the surface of the metal.
When mechanically removing corrosion, hard brushes made of steel bristles or sandpaper (coarse-grained) are usually used.
The disadvantages of mechanical methods of getting rid of rust include the fact that traces formed by a cleaning tool remain on the surface of the iron. Therefore, it is recommended that the surface of the purified iron be polished to give it its former appearance.
Thermal cleaning
Thermal removal of rust requires special installations (industrial steam generators or building hair dryers). The method of cleaning iron oxides is based on the fact that the contact of rust with the base metal is not strong. The effect of elevated temperature and hot moisture at high speed airflow is such that rust is almost completely removed.
This method is most effective when hot steam is also supplied to the surface to be treated. The steam-air mixture in a stream, which is supplied to a metal surface under pressure, leads to softening of the rust, crushing into individual fragments, which are removed from the surface of the iron by an air stream.
These methods are especially effective when it is necessary to remove rust from steel doors, ventilation structures, metal structures, which are impossible or difficult to dismantle.
Chemical cleaning
Currently, methods of chemical cleaning of metal surfaces from rust are very diverse. However, all are based on one process - the removal of corrosion through the chemical action of acid solutions on it.
The most effective ways to get rid of iron oxides include exposure to rust with hydrochloric acid, especially when its concentration in the solution is at least 15%. If the concentration is less, then the dissolution of rust is significantly slowed down.
Acidic compositions made on the basis of sulfuric acid should not be used, since as a result of exposure to the surface of iron, a layer of hydrides is formed, which increase the fragility of the metal.
If it is necessary to carry out chemical cleaning of the metal at home, it is possible to use non-aggressive substances, such as lemon, vinegar, etc. The principle of corrosion is the same. These substances dissolve rust quite well, which is then easily removed with rags. What is rust and how to remove it, probably knows most housewives.
The use of other equipment to remove metal corrosion
It is not always possible to use mechanical methods to combat rust, especially if metal products have complex shapes.
Chemical methods also have certain disadvantages. If you do not follow safety precautions, you can get a chemical burn or poisoning. There are difficulties with the disposal of waste solutions.
As a consequence, the most optimal is the use of the so-called soft blasting method. Its principle is that a stream of compressed air is sent to the surface of a metal damaged by rust, which contains abrasive components.
By changing the pressure in the stream, you can adjust the depth of the layer that is removed. This leads to the fact that only rust is removed, while the metal itself remains intact. Granules that act on corrosion consist of fine soda and chalk, and very fine sand can be used.