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 conditions ； 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 crack 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 include the following: 1. Stress – The stress that induces stress corrosion is mainly the static component of it; this stress can be caused by external loads or assembly forces (such as the force required to tighten bolts, expansion forces, etc.), or it can 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 components. 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 solution of hydroxide on a daily basis. 3. Materials: It is generally believed that extremely pure metals do not suffer from stress corrosion failure; this phenomenon occurs only in alloys or metals containing impurities. 4. Destruction process ⑴ Incubation stage. This is a period prior to the formation of stress corrosion cracks, preparing for crack nucleation. ⑵Stage of stable crack propagation. Under the combined action of stress and corrosive media, the crack expands slowly ⑶ Crack instability expansion stage. This is the final mechanical damage. Additionally, stress corrosion cracking in metallic materials is characterized by the fact that the cracking of the metal is independent of the metal’s thickness. The common situation of large thickness and slow corrosion (uniform corrosion) does not apply here. Therefore, delaying stress corrosion cracking by increasing the metal thickness is almost ineffective. Case Analysis: Case – A storage tank used by a chemical plant to store 30% sodium hydroxide, with a diameter of 1.2 m and a height of 1.4 m, made of carbon steel. A jacket is welded around the exterior of the storage tank, used to circulate hot water at 30–50°C in order to maintain the temperature of the sodium hydroxide solution. In the cold winters of the north, steam (0.2 Mpa) is introduced into the jacket instead. It was soon discovered that the bottom of the tank was leaking. Upon inspection, radial cracks were found around the alkali solution outlet pipe at the bottom of the tank. If steam is introduced through the pipe opening above, cracks will quickly form at the weld between the jacket and the cylinder. Analysis: This type of corrosion damage is typical alkali embrittlement. That is, the stress corrosion cracking that occurs in stressed metal materials in sodium hydroxide solution. Carbon steel is a type of metal material that is prone to alkali embrittlement. Based on literature, the sodium hydroxide concentration and temperature range at which carbon steel suffers from alkali embrittlement are presented. As can be seen from the graph, for a 30% NaOH solution, carbon steel may suffer from alkali embrittlement when the temperature exceeds 55°C. In this case, no corrosion issues occurred when hot water at 30–50°C was used. With steam flowing through, the temperature at the steam inlet is very high, far exceeding the 55°C threshold. And at the steam inlet, whether it is the upper or lower opening, it is located in an area where welding stress concentrations occur. It’s not surprising at all that the storage tank suffered from alkali embrittlement. The conditions under which alkaline embrittlement occurs are well known, yet for decades many carbon steel components have still suffered from alkaline embrittlement failure, notably in environments characterized by a \"fracture zone\". Corrective measures: In this case, to prevent alkali embrittlement in the storage tank, improvements should be made to the equipment’s structure to ensure as uniform a temperature distribution as possible and avoid localized overheating; in particular, areas with higher temperatures should be kept away from zones where welding stress concentrations occur. For example, some factories switched from jacket heating to coil heating, and alkali embrittlement failure no longer occurred. Of course, eliminating welding stress is also effective.