Corrosion and erosion of metals: causes and methods of protection

2024 Author: Howard Calhoun | [email protected]. Last modified: 2023-12-17 10:16

Chemical, mechanical and electrical external influences often occur in metal product operating environments. As a result, with improper maintenance of such elements, as well as ignoring safety standards, there may be risks of deformation and damage to structures and parts. This is due to the emerging processes of corrosion and erosion of metals, which in the long term contribute to the complete destruction of the structure of the product.

Why does rust appear

In a corrosive reaction, the conditions for the structural destruction of metal products are created by contact with chemical and electrochemical media. In the first case, the material comes into contact with oil products, coal, s alt and other minerals. Electric current is not involved in this case. The main operating medium can be a dry gas or a non-conductive liquid. Light varieties have the greatest destructive effect.petroleum products like kerosene and gasoline. In particular, the hull of an offshore transport tanker may be affected by sulfur compounds and acidic residues present in their composition.

In electrochemical corrosion, there is also an effect of current. Complex destruction with metal erosion will be accompanied by mechanical wear. The situation can be complicated by the fact that the external influences themselves will be determined by the characteristics of the natural environment - for example, chemical reactions with electrolytes can take place in sea water. The metal body itself is a heterogeneous material in structure, which leads to the presence of microgalvanic pairs. It is they, together with the metal sections of the structure, that act as anodes, creating conditions for the corrosion process.

Causes of erosion

In general terms, erosion refers to mechanical wear, as a result of which the product can change in size, shape, weight and other characteristics. What is the cause of metal erosion? Physical processes of external influence that reduce the strength of microvolumes of the surface layer of a structure or a separate part. Moreover, the acting media are not only mechanical factors, such as direct contact with hard abrasives.

These can be thermal, gas, and chemical active media, and they can act both independently and as an additionalwear factor. For example, gas flows contribute to the movement of solid particles in the circuit for the delivery of working mixtures through the pipeline, which has an indirect destructive effect on metal surfaces.

Methods for protecting metals from corrosion

Practice shows that 80% of the conditions for protecting materials from corrosion are laid down at the stage of surface preparation. The remaining 20% are already provided during operation. Approximately the same ratio of the effectiveness of protective measures is observed in the erosion of metals, when the means of minimizing the wear of workpieces are used.

The main areas of anti-corrosion protection include structural, passive and active. Structural protection is due to the use of special alloys based on stainless steel, Corten steel and non-ferrous metal. Active methods involve changing the structure of the material with a double electric layer - a method of electrochemical protection. As for passive methods, they involve the use of special coatings that prevent the formation of a corrosive element.

Varieties of metal heat treatment

A group of methods for the technological processing of metal blanks, which also focuses on a structural change in the surface layer in order to protect against corrosion damage. The following types of such processing are distinguished:

• Annealing. Heat treatment, which heats the metal, followed by gradual cooling.
• Hardening. ATsteels and their alloys can serve as target products. During hardening, the structure recrystallizes, and after holding the material at a critical temperature, cooling follows. A non-equilibrium structure is formed in a part that has undergone such processing, which is a limiting factor in choosing this method.
• Vacation. An alternative method of heat treatment of metal in relation to hardening, which can also act as an auxiliary step in changing the structure. In any case, during its implementation, excessive stresses on the steel are removed, which leads to an increase in anti-corrosion qualities.
• Normalization. Processing similar to annealing. The difference is that during annealing, cooling takes place in the furnace, and during normalization, it takes place in air.

Methods for protecting metals from erosion

The main direction in the protection of metallic materials from erosion is the development of special coatings. In particular, metallization in the form of applying an anti-corrosion alloy to the workpiece increases the chemical and mechanical qualities of the structure. As a result, wear is reduced and the part design can maintain its previous performance.

Non-metallic wear-resistant coatings for specific applications are also being developed. For example, the erosion of metals, formed in the conditions of rubbing surfaces, is often found in vehicle parts. For this kind of protection, diamond-like, ceramic and combined compounds with increased strength and hardness are used.

Featuresgas erosion protection

In this case, the emphasis is not on the mechanical protection of parts, but on chemical-physical insulation. Both special modes of keeping and storing materials, as well as special lubricants that prevent metal erosion can be used. Wear protection and prevention also rely on thermal insulation.

In this direction, materials such as pure chromium and NT brand nairite are used. The disadvantage of chromium is that it is characterized by a lack of toughness and ductility. For this reason, it is rarely used as an element of structural insulation. As for nairite, gumming liquid mixtures are made on its basis, through which monolithic wear-resistant seals are formed.

Method of protection by thermal spraying

This is a versatile protection technology that is suitable for both corrosion prevention and mechanical wear isolation. The technique of its application lies in the fact that zinc particles are applied to the surface of the part with a gas jet. Unlike other metallization methods, this method forms a protective layer up to tens of microns thick. Thus, erosion processes that occur in the nodes of engineering equipment, as well as in transport networks and major oil pipelines are prevented.

Conclusion

The processes of negative impact on metal structures force operating companies to spendlarge sums for their maintenance. At the same time, the most effective means of protection, as a rule, are more expensive. On the other hand, preliminary studies of the conditions of use of products for the risks of rust formation or metal erosion can minimize such costs. The fact is that many technical and protective properties of critical structures are laid down at the stage of alloy selection. By alloying and introducing modifying additives at the stage of manufacturing the part, it is possible to provide it with optimal protective qualities.