Let’s talk about stress corrosion in metal materials

Thread Content

Stress corrosion cracking of metal materials refers to the phenomenon of corrosion-induced cracking that occurs under the combined action of static tensile force and a corrosive environment. It differs from damage caused solely by stress; this type of corrosion can occur even under extremely low stress levels ； It is also different from damage caused solely by corrosion; even media with very low corrosivity can lead to corrosion cracking. It is often a sudden break that occurs rapidly without any warning, prone to causing serious accidents. Therefore, it is a highly destructive form of damage. Based on the electrochemical reactions that occur during crack propagation, stress corrosion can be divided into two basic categories: anode reaction-sensitive and cathode reaction-sensitive. Anode reaction-sensitive stress corrosion refers to a type of stress corrosion cracking in which the formation and progression of such cracks are based on the anodic dissolution of the metal at the crack site, and the growth rate of the cracks is also determined by the rate of anodic dissolution of the metal. Cathode reaction-sensitive stress corrosion refers to the brittle failure that occurs during such stress corrosion processes due to cathodic hydrogen absorption; it is also known as hydrogen-induced stress corrosion or simply hydrogen embrittlement. Stress corrosion as commonly referred to denotes anode reaction-sensitive stress corrosion. The characteristics of stress corrosion in metal materials can be explained from four aspects. 1. Stress: The stress considered from an overhead perspective is mainly the static component of it; it can be stress caused by external loads or assembly forces (such as the force of tightening bolts, expansion forces, etc.), or it can also be internal stresses generated during processes such as machining, heat treatment, and welding. Regardless of the source, the stress that causes stress corrosion cracking must contain a tensile component; compressive stress does not cause stress corrosion cracking. Furthermore, this stress is usually relatively mild. In an environment free from corrosion, such low stresses will not cause mechanical failure of the component. The stress value that causes failure must be determined based on specific factors such as the material and the corrosive medium. 2. Corrosive media: The materials and media that cause stress corrosion are not arbitrary; stress corrosion occurs only when there is a certain combination between them. The corrosive agents that cause stress corrosion in ordinary steel include: hydroxide solutions ； Aqueous solution containing nitrates, carbonates, and hydrogen sulfide ； Seawater, sulfuric acid-nitric acid mixture ； Melted zinc, lithium ； Hot ferric chloride solution ； Liquid ammonia. The media that cause stress corrosion in austenitic stainless steels include acidic and neutral chloride solutions ； seawater ； molten chloride ； Hot fluoride solution ； A hydroxide solution of sodium. 3. Materials: It is generally believed that extremely pure metals do not suffer from stress corrosion failure; it occurs only in alloys or metals containing impurities. 4. Destruction process a. Incubation stage. This is a period prior to the formation of stress corrosion cracks, preparing for crack nucleation. b. Stage of stable crack propagation. Under the combined action of stress and corrosive media, the crack expands slowly. c. Crack instability expansion stage. This is the final mechanical damage. Furthermore, stress corrosion cracking in metallic materials is characterized by the fact that the cracking of the metal is independent of its thickness. The common scenario of large thicknesses resulting in slow corrosion (uniform corrosion) does not apply here. Therefore, delaying stress corrosion cracking by increasing the metal thickness is almost ineffective.