Intergranular corrosion: causes, tests and ways to prevent

Intergranular corrosion (MCC) is a dangerous type of metal destruction, in which corrosion processes occur along grain boundaries in the material structure. This type of corrosion poses a serious threat to the performance of metals, as it causes loss of strength, plasticity and, ultimately, product destruction. This problem is specific to corrosion-resistant steels, especially when used in corrosive environments or after heat treatment. Let's consider the causes of MCC, test methods and methods for protecting materials.

The causes of intergranular corrosion

ICC can be caused by several key factors:

1. Heterogeneity of the material structure. The inclusions of impurities and the presence of microscopic defects at the grain boundaries create favorable conditions for the formation of corrosion cracks.
2. Heat treatment. Different modes heat treatmentsuch as welding or hardening, can change the internal structure of steel, increasing its tendency to intergranular corrosion.
3. The influence of corrosive environments. Acids, bases and other corrosive substances can penetrate along the grain boundaries, destroying them. Sulfuric and nitric acid solutions, which are often used in industrial processes, are particularly dangerous.

Chrome in steels

When working with chromium steels, remember that when combined with carbon chromium can form carbides. These chemical compounds segregate to grain boundaries, which leads to the formation of corrosive zones and increases the risk of MCC.

To prevent interracial corrosion, it is important to take into account the characteristics of the selected materials, as well as the recommended heat treatment modes. When working with most steels and alloys, including corrosion-resistant ones, the material should be protected from the effects of aggressive media.

Intergranular corrosion tests

One of the main reasons for the MCC of corrosion-resistant alloys is prolonged heating during welding or pressure treatment, which leads to electrochemical heterogeneity (surface heterogeneity) and disruption relationships between grains. The thermal effect also causes the border areas to be deployed with elements that made the material resistant to aggressive environments.

To assess the tendency of steels to intergranular corrosion, tests are carried out in accordance with GOST 6032—2017. During the study, the material is exposed to high temperatures and aggressive chemical media to simulate the conditions under which MCC can develop.

1. Thermal effect. Chromium steels are heated to 1100°C for 30 hours, and austenitic chromium-nickel steels are heated to 700°C for up to 60 hours. This makes it possible to artificially induce conditions conductive to the development of intergranular corrosion.
2. Effects of chemical reagents. After heat treatment, the samples are kept in a boiling solution of sulfuric or nitric acid to create an aggressive environment. The duration of exposure and the choice of the corrosive medium are determined by the scope of application of a particular steel grade.
3. Mechanical tests. The tests include bending the samples at an angle of 90° and their subsequent metallographic examination. Pickling with special reagents is also possible. The absence of cracks on the surface of the samples after mechanical stress indicates the material's resistance to MCC.

This method makes it possible to accurately determine how resistant the material is to intergranular corrosion under real operating conditions, which is especially important for steels intended for use in aggressive environments.

The main types of corrosion-resistant steels

Corrosion-resistant steels have a wide range of applications due to their resistance to the atmosphere and other aggressive media. There are several main groups among corrosion-resistant materials.

Ferritic, martensitic-ferritic and martensitic steels

These steels, which contain a significant amount of chromium, are obtained by a ferritic, martensitic or martense-ferritic structure after cooling in air. They are resistant to corrosion at moderate temperatures (up to 300°C), including in the presence of nitric acid, humid atmosphere, tap water and organic compounds. However, in seawater, such steels are subject to stress corrosion cracking.

An example of steel: brand 20X13, produced by PZPS, is a representative of martensitic steels that have good mechanical properties and corrosion resistance.

Austenitic corrosion-resistant steels

These steels were developed at the beginning of the 20th century by German engineer Benno Strauss, who was then director of the Krupp Iron Works Research Institute. In 1912, B. Strauss and his college E. Maurer patented the first austenitic alloy, which contained 7% nickel and 21% chromium. Since then, these materials have become among the most popular due to their high performance characteristics. The most widely used steels are 03X18H12, 04X18H10, 12X18N9, 12X18H10T and 17X18H9.

Chromium-nickel alloys get the austenitic structure by cooling in air. Compared to chromium steels, chromium-nickel steels have a higher resistance to corrosion, which does not decrease when the material is heated.

After hardening, austenitic steels acquire the following properties:

• high plasticity and low hardness;
• pure austenitic structure, which makes them non-magnetic;
• excellent weldability and manufacturability, which expands their applications.

An example of steel: PZPS produces austenitic steels 12X18N9, 12X18H10T, 12X18N9SMR et 10X17N13M3T, which are used in the chemical and food industries due to their corrosion resistance and manufacturability. In addition, the company can develop and manufacture other corrosion-resistant alloys.

Methods for preventing intergranular corrosion

To minimize the risk of intergranular corrosion, the following factors should be considered:

1. Choosing the right materials. The use of steels with a low carbon content or with the addition of stabilizing elements (for example, titanium) can prevent the formation of chromium carbides at grain boundaries.
2. Heat treatment control. The correct choice of heat treatment temperature conditions makes it possible to preserve the structure of the material and minimize the risks of MCC development.
3. Protection from aggressive environments. Coating materials with protective layers or using corrosion inhibitors helps prevent metal contact with corrosive substances.

PZPS specializes in the production of high-quality steels of various classes. The company's products comply with international standards and are rigorous overseeing for resistance to intergranular corrosion. Due to high technology and a wide range of products PZPS products It is used in various industries — from mechanical engineering to the chemical industry.

Contact us for advice or checkout — we will help you choose the steel that will meet all your requirements.

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