What is corrosion?

Corrosion is chemical or electrochemical destruction of metal surfaces under the influence of the external environment (atmospheric oxygen, acids, etc.). Chemical corrosion occurs in non-conductive media (gases and oils), and galvanic corrosion usually occurs in aqueous solutions of electrolytes. Corrosion adversely affects the electrical resistance of metal equipment and requires corrosion protection. Equipment expected to operate in a corrosive environment is designed to require minimal corrosion protection or sizing so that corrosion does not cause significant changes in its mechanical properties.

In fact, physicochemical processes between the metal and the environment can lead to damage. As the metals interact with air (and the gases contained in them) and water (and the electrolytes dissolved in them), compounds are formed on the surface of the metals: oxides, sulfates, etc. So, in general, it is an oxidation process in which electrically neutral atoms convert electrons into positively charged ions. Corrosion is a random process. This results in more antioxidant compounds.

Types of corrosion

Chemical corrosion

Chemical corrosion is due to heterogeneous reactions between the metal and the environment in the absence of moisture. It is present in industrial facilities, gas pipelines through which corrosive gas mixtures flow. One of the main factors in its process is the O2 contained in the gas. All metals, including precious metals, are covered with an oxide layer.

For the protective properties of this layer, its thickness is not important, but its density. Some metals such as copper, zinc, lead, nickel, aluminum, etc. Their surfaces are covered with a thin, dense oxide layer that protects them from corrosive gases. Other metals, such as iron and its alloys, also form oxide layers, but freely, which does not stop this aggressive action and the metal collapses in depth. Coating with a thin oxide layer is also known as passivation of the metal. Important factors for corrosion resistance are: the purity of metals, the smooth appearance of metal surfaces is more difficult to corrode, the metal corrodes faster if the temperature is high.

Non-electrolytic corrosion

Non-electrolytic corrosion is particularly important in fuel tanks. Forms iron sulfide.

Electrochemical corrosion

Electrochemical corrosion is the most common damage to metals and alloys and is the result of its action. It occurs in solutions or melts of electrolytes. The ionization process is underway. It is a product of metallic thermodynamic instability, almost independent of the amount of electrolyte.

Biochemical corrosion

Biochemical corrosion occurs under the action of microorganisms, that is, the presence of moisture.

Prevention of corrosion processes

Corrosion prevention in metal structures and pressure vessels is a vast and extremely interesting field. We will briefly mention the main means of protection:

The use of stainless steels (common to all of them is the increased chromium content – not less than 12%) with special sought-after properties

Application of cathodic protection

Certain constructive solutions (possibility of self-drainage, preventing the formation of water pockets, etc.)

Development of a wide range of anti-corrosion coatings with special properties and characteristics. Some metals, such as steel, iron and aluminum, can be coated with a corrosion-resistant protective coating. Selecting the optimal combination of metal and coating requires careful consideration of requirements for strength, reliability, friction, torque, and corrosion resistance.

Material compatibility. When developing products, engineers must consider the possibility of contact with potentially incompatible materials. For example, combinations of copper and stainless steel or bronze and steel can cause electrochemical corrosion. To solve this problem, compatible materials and alloys should be selected, or insulation should be used that prevents the formation of an electrical circuit.

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