Characteristics of chemical corrosion and how to eliminate it
Chemical corrosion is a process that consists in destruction metal when interacting with aggressive external environments.
A chemical type of corrosive process will not be associated with current (electricity) exposure. With this type of corrosion, an oxidative reaction occurs, where the destruction material is simultaneously a reducing agent for the elements of the environment.
The classification of the types of aggressive media will include two types of metal destruction - chemical corrosion to non-electrolyte liquids and gaseous chemical corrosion.
The largest type of chemical corrosion - gas - is a process of corrosive the type that occurs in a gas when the temperature rises.
This problem will be typical for the operation of most types of technological equipment, as well as parts (engines, furnace fittings, turbines, etc.). Moreover, ultra-high temperatures are used for processing metals under high pressure (heating before rolling, forging, stamping, thermal process, etc.).
Features of metals and their state at elevated temperatures will be determined by two properties - heat resistance and heat resistance.
The latter is the degree of stability of mechanical properties at very high temperatures. The stability of mechanical properties can be understood as the retention of strength for a long time and creep resistance. Heat resistance is the resistance of a metal to the corrosive activity of gases at elevated temperatures.
The rate of development of gas-type corrosion is determined by about indicators, among which:
The corrosion process will be greatly influenced by the properties and parameters of the oxide film that appears on the metal surface.
Oxide formation can be divided into a couple of stages (chronologically):
The first stage will be characterized by the formation of an ionic bond, as a result of the interaction of oxygen and atomic surfaces, when the oxygen atom begins to take electrons from the metal. The emerging bond begins to be distinguished by exceptional strength - it is much greater than the bond between oxygen and metal in oxide.
The explanation of this connection will lie in the action of the atomic field on oxygen.As soon as the metal surface begins to fill with an oxidizing agent (and this happens quickly), under low temperature conditions, the adsorption of the oxidizing molecule begins.
The result of the reaction will be the appearance of the thinnest monomolecular film, which becomes thicker after a while, which only complicates oxygen access. At the second stage, a chemical reaction will take place, in which the oxidizing element of the medium begins to take valence type electrons from the metal. Chemical corrosion is the end result of the reaction.
We propose to consider the characteristics of chemical corrosion.
The classification of oxide films has 3 types:
The resulting oxide film has protective capabilities - it will slow down or even fully inhibit the development of corrosion. The presence of a film also increases the resistance of the metal to heat.
But, a really effective film must have the following characteristics:
- Not porous.
- Have a solid structure.
- Have excellent adhesion properties.
- They are distinguished by their chemical interstitiality in relation to the atmosphere.
- Be firm and resistant to wear and tear.
One of the conditions mentioned above is that the solid structure is of particular importance.
The condition of continuity will be the excess of the molecular volume of the oxide film over the volume of metal atoms. Continuity is the ability of the oxide to cover the entire metal surface with a full layer. If the condition is not met, the film will not be protective. But, there are exceptions to this rule - for certain metals, for example, elements of alkaline-earth groups (the exception will be beryllium) and magnesium, continuity is not a critical indicator.
A couple of techniques are used to set the oxide film thickness.
The protective properties of the film can be detected during formation. To do this, one should study the rate of metal oxidation, and the parameters of the rate changes over time. For the already formed oxide, a different method is used, which consists in studying the thickness and characteristics of the protective type of the film. For this, a reagent should be applied to the surface. Further, experts will record the time it takes for the reagent to appear, and based on the data, a conclusion should be drawn about the film thickness.
Please note that even the finally formed oxide film will continue to interact with the oxidizing environment, as well as the metal.
The rate at which chemical corrosion develops will depend on the temperature regime. At high temperatures, oxidation processes begin to develop more rapidly.With this decrease in the role of the thermodynamic factor, the course of the reaction will not affect the process itself. Cooling and alternating heating will be important.
Due to thermal stress, cracks will begin to appear in the oxide film. Through the holes, the oxidation element will reach the surface. As a result, a new layer of the oxide type film appears, and the previous one begins to peel off.
The components of the gaseous medium will play an important role. This factor is individual for different types of metals and will be consistent with temperature fluctuations.
For example, copper will corrode quickly if it comes into contact with oxygen, but it is also resistant to the process in a sulfur oxide environment. For nickel, the oxide is destructive, and resistance is visible in oxygen, carbon dioxide and water. But chromium is capable of showing resistance to all environments that are listed. If the level of the dissociation pressure of the oxide exceeds the pressure of the oxidation element, then the process itself will stop and acquire thermodynamic stability.
Alloy components will also affect the oxidation reaction rate.
For example, sulfur, manganese, phosphorus and nickel will not contribute to the oxidation of iron in any way. But silicon, aluminum and chromium greatly slow down the process. Copper, iron oxidation, cobalt, titanium and beryllium make this even stronger. Additives of tungsten, vanadium and molybdenum help to make the process more intensive, which is explained by the volatility and low melting point of such metals. The slowest chemical corrosion processes take place with an austenitic structure because it is best adapted to high temperatures.
Another factor on which the speed will depend is the characteristics of the treated surface. A smooth surface will oxidize more slowly, and an uneven surface much faster.
To non-conductive liquid media (more precisely, non-electrolyte liquids) include such organic substances, for example:
A small amount of inorganic liquids, for example, liquid bromine and sulfur, which is molten, are also considered to be such liquids. It should be noted that organic-type solvents by themselves will not react with metals, but, with a small volume of impurities, an intense process of interactions appears. The corrosion rate is increased by sulfur-containing elements in the oil.
Also, high temperatures are required to enhance corrosive processes. Moisture will intensify the development of electromechanical corrosion.
Another factor in the rapid development of corrosion is bromine in liquid form. At normal temperatures, it will be particularly destructive to high carbon steels, titanium and aluminum.The effect of bromine on nickel and iron is less significant, and tantalum, lead, platinum and silver will show the greatest resistance to liquid type of bromine.
Molten sulfur will enter into aggressive reactions with almost all metals, and primarily with tin, lead and copper. Sulfur will affect carbon grades of titanium and steel less, and almost completely destroys aluminum.
Protective actions for metal structures that are in non-conductive liquid-type media are carried out by adding a metal resistant to a certain environment (for example, steels with a high chromium content). Special protective coatings are also used (for example, in an environment where there is a lot of sulfur, aluminum coatings are used).
Methods of Corrosion Control will include:
The selection of a certain material will depend on the potential efficiency (here there is a form of financial and technological) of its application.
Modern principles for the protection of metal from chemical corrosion of metal will be based on the following methods:
These techniques will be subdivided into two groups:
Anti-corrosion electroplating is one of the most commonly used metal protection methods, but this is not economically viable with a large surface area.
The reason is the large expenditures on the preparation process. The leading place among the methods of protection will be occupied by metal coating with paint and varnish material.
The popularity of this method of combating corrosion is due to a combination of factors:
- High protection properties (liquid repulsion, hydrophobicity, low gas permeability and vapor permeability).
- Great possibilities for decorative type solutions.
- Economic justification.
At the same time, the use of widely available materials also has disadvantages:
- Incomplete respect for the metal surface.
- The adhesion of the coating to the base metal, the coating against corrosion, is broken, and will begin to promote corrosion.
- Porosity, which results in increased moisture permeability.
And yet, the painted surface protects metals from corrosion processes even with local damage to the film, while imperfect galvanic coatings can even accelerate corrosion.
For high-quality corrosion protection, it is recommended to use metals with a high level of hydrophobicity, impermeability to gas, water and vapor media. Organosilicates can be classified as such materials. Chemical corrosion is hardly applicable to organosilicate materials. The reasons for this will lie in the increased chemical stability of the compositions, their resistance to light, a low level of water absorption and hydrophobic qualities.
Posted By: Work Style |13, Nov 2020
Posted By: Work Style |08, Nov 2020
Posted By: Work Style |09, Nov 2020
Posted By: Work Style |13, Nov 2020
Posted By: Work Style |08, Nov 2020