Preventive measures against stress corrosion

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Cracking of materials or parts caused by the combined effect of stress and a corrosive environment is known as stress corrosion cracking, which is the result of the interaction between stress and corrosion. If there is only one factor, such as stress or the effect of the medium, failure will not occur; but when these two act together, cracking can happen very quickly. Therefore, when stress corrosion occurs, the stress level is very low and the corrosivity of the medium is also weak. It is for this reason that stress corrosion is often overlooked, leading to repeated \"accidental\" incidents that cause significant harm and losses. ◆Category 1: Pitting corrosion is a form of localized corrosion that causes degradation. 2. Intergranular corrosion: The grain boundaries are zones where grains with different crystallographic orientations are misaligned in a disordered manner; therefore, they serve as favorable areas for the segregation of various solute elements in steel or for the precipitation of metal compounds such as carbides and the δ-phase. Therefore, it is not surprising that in certain corrosive media, the grain boundaries may be corroded first. This type of corrosion is known as intergranular corrosion, and most metals and alloys can experience intergranular corrosion in certain corrosive environments. 3. Crevice corrosion is a form of local corrosion that can occur in gaps where solutions remain stagnant or within shielded surfaces. Such gaps can form at the joints between metals or between metals and non-metals; for example, they form where there is contact with rivets, bolts, gaskets, valve seats, loose surface deposits, and marine organisms. 4. General corrosion is a term used to describe corrosion that occurs in a relatively uniform manner across the entire surface of an alloy. When general corrosion occurs, the material gradually thins due to corrosion, and in extreme cases the material fails as a result of corrosion. Stainless steel may suffer from general corrosion in strong acids and strong bases. Failure issues caused by general corrosion are not particularly concerning, as this type of corrosion can usually be predicted through simple immersion tests or by consulting literature on corrosion. ◆Features: 1. The stress that causes stress corrosion failure is a static stress, which is much lower than the yield strength of the material, and it is generally a tensile stress. 2. Damage caused by stress corrosion is a brittle fracture, with no significant plastic deformation. 3. Stress corrosion occurs only when specific alloy compositions are combined with specific media. 4. The crack growth rate in stress corrosion is generally between 10-9 and 10-6 m/s; it is somewhat similar to fatigue, being a gradual and slow process. This subcritical growth continues until a certain critical size is reached, at which point the remaining cross-section can no longer withstand external loads, leading to sudden fracture. 5. Cracks caused by stress corrosion generally originate from surface pits, and the propagation direction of these cracks is often perpendicular to the tensile axis. 6. The fracture surface of stress corrosion failure is dull in color, with corrosion products often present on its surface. 7. The main crack propagation in stress corrosion often involves branching. 8. Fracture caused by stress corrosion can be transgranular or intergranular. ◆Measure 1: Select materials appropriately. Choosing stress-corrosion resistant materials based on the stresses to which the parts are subjected and the operating conditions is a fundamental principle. Since copper is highly sensitive to stress corrosion in the presence of ammonia, copper alloys should be avoided for components that come into contact with ammonia ； For example, in high-concentration chloride media, low-carbon high-chromium ferritic stainless steels that contain no nickel or copper, or only trace amounts of these metals, or chromium-nickel stainless steels with a high silicon content can be used; nickel-based and iron-nickel-based corrosion-resistant alloys are also options. In pressure pipeline engineering, the following combinations of media are prone to stress corrosion and should be avoided: Figure 2. Reduce or eliminate residual tensile stress in components; residual tensile stress is an important factor that leads to stress corrosion. To this end, the design should minimize stress concentration in the components as much as possible. From a technical standpoint, heating and cooling must be uniform, and annealing processes should be employed if necessary to eliminate internal stresses. Alternatively, shot peening or surface heat treatment can be used to generate a certain amount of residual compressive stress in the surface layer of the part, which is also effective in preventing stress corrosion. 3. Improving the medium conditions can be approached from two aspects: on one hand, efforts should be made to reduce or eliminate harmful chemical ions that promote stress corrosion cracking; for example, water purification processes can help lower the chloride ion content in cooling water and steam, which is very effective in preventing chloride-induced cracking in austenitic stainless steels ； On the other hand, corrosion inhibitors can also be added to the corrosive medium; for example, adding 300×10-6 mol/L of phosphates to high-temperature water can **improve** the stress corrosion resistance of chromium-nickel austenitic stainless steels. 4. Use of electrochemical protection: Since metals suffer from stress corrosion only within a certain range of electrode potentials in a medium, an external potential is applied to keep the metal’s potential away from the region sensitive to stress corrosion. This is also a measure to prevent stress corrosion, and cathodic protection is commonly used for this purpose. However, this protection method cannot be used for high-strength steels and other materials sensitive to hydrogen embrittlement. Sometimes, electrochemical protection using the sacrificial anode method is also very effective.