How to prevent stress corrosion cracking of nickel alloys？

What is stress corrosion cracking?

Stress corrosion cracking is the cracking of metal parts or materials caused by the combined action of stress and corrosive environment, which is the result of the combined effect of stress and corrosion. If there is only one aspect, the damage will not occur, but when the two work together, cracking can occur quickly. Therefore, when stress corrosion occurs, the stress is usually very low, and the corrosiveness of the medium is also very weak, so sometimes stress corrosion is often overlooked, leading to the continuous occurrence of accidents.

The hazards of stress corrosion cracking

The fracture caused by stress corrosion occurs without obvious macroscopic deformation, and its damage is sudden. Cracks often penetrate deep into the metal. Once they occur, they are difficult to repair, and sometimes result in the entire equipment being scrapped. It will cause a significant reduction in mechanical strength. Generally, the damage is not obvious so that it is difficult to find by accidental inspection. Stress corrosion cracking may cause a rapid mechanical fracture, and even cause catastrophic failure of components and structures. Stress corrosion cracking usually has no precursors of deformation, and the phenomenon of corrosion is not obvious. Therefore, it is a form of corrosion failure with great harm and strong concealment.

Classification of stress corrosion cracking

1. Pitting corrosion

It is a form of localized corrosion that causes corrosion.

2. Intergranular corrosion

Intergranular corrosion is common localized corrosion. Corrosion develops along the grain boundary of the metal or alloy or its adjacent area, and the grain itself corrodes very slightly. Most metals and alloys may exhibit intergranular corrosion in specific corrosive media, but some alloys have outstanding intergranular corrosion resistance, such as Hastelloy C-276, Hastelloy C-2000, Inconel 600, Inconel 718, and so on.

3. Crevice corrosion

Metal parts in the medium, due to the formation of a particularly small gap between metal and metal or metal and non-metal (usually between 0.025~0.1mm), the medium in the gap is in a stagnant state, causing the acceleration of the metal in the gap Corrosion, this kind of local corrosion is called crevice corrosion.

4. Comprehensive corrosion

It is a relatively uniform corrosion phenomenon on the entire alloy surface. When full-scale corrosion occurs, the material gradually becomes thinner due to the corrosion, and even causes the material to fail. Stainless steel may undergo general corrosion in strong acids and alkalis. The failure problem caused by general corrosion is not very worrying, because this kind of corrosion is usually predictable.

Stress corrosion cracking characteristics

1. It is static stress that causes stress-corrosion damage, which is much lower than the yield strength of the material, and it is generally tensile stress.

2. The damage caused by stress corrosion is brittle fracture without obvious plastic deformation.

3. Stress corrosion can only be caused when a specific alloy composition is combined with a specific medium.

4. The crack growth rate of stress corrosion is a bit like fatigue, it is gradual and slow. This kind of expansion has reached a certain critical size, and when the remaining section cannot withstand the external load, it suddenly breaks.

5. Stress corrosion cracks mostly originate from surface pits, and the propagation path of cracks is often perpendicular to the tension axis.

6. Fractures damaged by stress corrosion are dark in color and often have corrosion products on the surface.

7. The main crack propagation of stress corrosion often has branches.

8. The fracture caused by stress corrosion can be a transgranular fracture or intergranular fracture.

Prevention of stress corrosion cracking

1. Reasonable selection of materials

It is a basic principle to select stress corrosion-resistant materials according to the stress and use conditions of the parts. For example, copper is very sensitive to ammonia stress corrosion, therefore, copper alloys should be avoided for parts that contact ammonia; and for high concentration chloride media, low-carbon high-chromium ferritic stainless steel (Hastelloy C2000, Hastelloy C2000, Hastelloy G30), or chromium-nickel stainless steel with high silicon content (Incoloy 800, Hastelloy G30), nickel-based (Hastelloy C276) and iron-nickel-based corrosion-resistant alloy (Incoloy 825) can also be used.

2. Reduce or eliminate residual tensile stress in parts

Residual tensile stress is an important condition for stress corrosion. For this reason, the design should minimize the stress concentration on the parts. In terms of process, heating and cooling should be uniform, and the annealing process should be used to eliminate internal stress if necessary. Or using shot peening or surface heat treatment to produce a certain residual compressive stress on the surface of the part is also effective to prevent stress corrosion.

3. Improve medium conditions

This can be considered from two aspects: On the one hand, try to reduce or eliminate harmful chemical ions that promote stress corrosion cracking. For example, through water purification treatment, reducing the chloride ion content in cooling water and steam is very effective in preventing chlorine embrittlement of austenitic stainless steel. On the other hand, corrosion inhibitors can also be added to the corrosive medium, such as adding phosphate in high-temperature water, which can greatly improve the stress corrosion resistance of chromium-nickel austenitic stainless steel.

4. Using electrochemical protection

Since metal in the medium can only produce stress corrosion within a certain electrode potential range, the method of applying potential is used to keep the potential of the metal in the medium away from the stress corrosion sensitive potential area, which is also a measure to prevent stress corrosion. Generally, the cathodic protection method is adopted. However, for high-strength steel and other materials sensitive to hydrogen embrittlement, this protection method cannot be used. Sometimes the sacrificial anode method for electrochemical protection is also very effective.

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