Discuss stress corrosion and its prevention methods

Thread Content

This post was last edited by Wang Wei2 on 2024-10-20 08:44. Many devices are subject to certain external stresses, and local residual stresses of varying degrees are also generated during manufacturing, processing (especially welding), and heat treatment; stress corrosion cracking can occur in specific environments. According to the three-stage theory, the role of chemical media in stress corrosion can be divided into three types: ① Facilitating global passivation ； ②Disruption of local passivation ； ③Entering the crack (mainly anions) promotes corrosion-induced hydrogen evolution, taking the \"austenitic stainless steel-chloride ions\" system as an example. Oxygen in the solution promotes overall passivation, while Cl- destroys local passivation and enters the tips of cracks to form hydrochloric acid, thereby accelerating corrosion. There are two types of crack morphology: one develops along the grain boundaries, resulting in grain boundary fracture ； The other type is to pass through the grains, resulting in transgranular fracture ； There are also mixed types, such as intergranular for the main grain boundary, and transgranular for the single grain boundary. Methods to prevent stress corrosion include eliminating or reducing stress through heat treatment, as shown in Table 1 ； Stress levels below the critical stress corrosion cracking strength are selected in the design ； Improve the design structure to avoid stress concentration ； Apply compressive stress to the surface ； Electrochemical protection, coatings, or corrosion inhibitors are used. Table 1 Annealing procedures for stress relief of some important metals and alloys. Metal and alloy, Heating temperature (°C), Slow cooling time, hours: Carbon steel and cast iron, Stainless steel, Aluminum alloys, Magnesium alloys, Nickel alloys, Brass – 500–600, 730–870, 230–250, 130–300, 600–650; 200–300. Time ranges: 1/2–1, 1–2, 1, 1/4–1, 1/2–1, 1/2–1. The disadvantages of heat treatment are that metals may develop scale at high temperatures and may also deform. To prevent deformation, austenitic stainless steels are sometimes subjected to prolonged treatment at 550°C. For molybdenum-containing austenitic stainless steels, heat treatment cannot completely eliminate stress, and this method is troublesome and costly for large-scale equipment. A proper design can also prevent or reduce the risk of stress corrosion; for example, by ensuring a uniform stress distribution to minimize local stress concentrations, or by placing critical areas such as edges, corners, seams, and holes in regions of low stress or compressive stress. It is also necessary to use a sufficiently large radius and select nozzles of appropriate design. As well as increasing the wall thickness, etc.