【Q&A Question 183】2018.07.07

1. Reduce or eliminate stress (1) Improve structural design to avoid or reduce local stress concentration. Analysis of stress corrosion accidents shows that residual stress accounts for the largest proportion; therefore, large residual stresses should be avoided as much as possible during processing, manufacturing, and assembly. Structural design should minimize gaps and dead corners that could lead to the accumulation of corrosive liquids, in order to prevent the concentration of harmful substances such as Cl‑ and OH‑. (2) Stress relief treatment: Methods such as heat treatment annealing, over-deformation, and shot peening can be used to reduce residual stresses. Among them, stress-relief annealing is the most important method for reducing residual stresses, and it is particularly crucial for welded parts. (3) Structural design based on fracture mechanics: Since macroscopic or microscopic cracks and defects are inevitably present in components, designing using fracture mechanics provides higher reliability compared to traditional mechanical methods. In corrosive environments, it is of great practical significance to determine parameters such as the Kiscc of materials in advance, and to establish the allowable critical crack size for components based on the operating conditions. 2. Control environment (1) Improve operating conditions: Each alloy has its own stress-corrosion sensitive media. It is essential to reduce and control the amount of these harmful substances. Since stress corrosion is highly dependent on temperature, the ambient temperature should be controlled, and the temperature should be reduced whenever possible. Reducing the temperature difference between the inside and outside, and avoiding repeated heating and cooling, can prevent the damage caused by thermal stress. The pH value and oxygen content of the medium have a certain impact on various material-environment systems. Generally speaking, reducing oxygen content and increasing pH is beneficial. (2) Addition of corrosion inhibitors: Every material-environment system contains certain substances that can inhibit or slow down stress corrosion. These substances exert a corrosion-inhibiting effect by altering the potential, promoting film formation, preventing the intrusion of hydrogen or the adsorption of harmful substances, and affecting the kinetics of electrochemical reactions, thereby preventing or reducing stress corrosion. (3) Protective coating: Using organic coatings can isolate the material surface from the environment, or using metals that are not sensitive to the environment as coatings for sensitive materials can both reduce the material’s susceptibility to stress corrosion. (4) Electrochemical protection: Since stress corrosion occurs within a specific sensitive potential range, it is theoretically possible to prevent stress corrosion through cathodic or anodic protection by controlling the potential. However, different systems need to be analyzed specifically and treated separately. 3. Improve the material quality: (1) Select the appropriate material: Try to choose materials that have not experienced stress corrosion cracking in the given environment, or test and screen existing available materials to select the best ones for use. (2) Develop new materials resistant to stress corrosion. (3) Smelting and heat treatment processes: New metallurgical processes are employed to reduce impurities in the material and increase its purity. Heat treatment is used to modify the material’s structure, eliminate the segregation of harmful substances, and refine the grain structure, thereby reducing the material’s susceptibility to stress corrosion.

①Use heat treatment to eliminate residual stresses from welding and cold working, as well as for stabilization and solution treatment； ②Ultra-low carbon (less than 0.03%) stainless steel or stainless steel stabilized with niobium and titanium is used, and ultra-low carbon or niobium-containing welding electrodes are employed for welding.

Methods to prevent stress corrosion can be divided into three types: the first is to reduce tensile stress； The second is to reduce the effect of corrosive factors ； The third is to change the phase composition of the alloy.

(1) Use heat treatment to eliminate residual stresses from welding and cold working, as well as to carry out stabilization and solution treatment；

Use heat treatment to eliminate residual stresses from welding and cold working, as well as for stabilization and solution treatment

Measures to prevent stress corrosion: A. Proper selection of materials； B. Reduce or eliminate residual tensile stress ； C. Improve medium conditions ； D. Electrochemical protection ； E. Coating protection

Three methods: reducing tensile stress, mitigating the effects of corrosion factors, and altering the phase composition of the alloy.

Reduce tensile stress； Change the alloying element composition ； Reduce the impact of corrosive factors.

Methods to prevent stress corrosion can be divided into three types: the first is to reduce tensile stress； The second is to reduce the effect of corrosive factors ； The third is to change the phase composition of the alloy.

Methods to prevent stress corrosion can be divided into three types: ① First, it is necessary to reduce tensile stress； ②Then reduce the impact of corrosive factors ； ③Finally, there is the alteration of the phase composition of the alloy.