What is Stress Corrosion Cracking of Pressure Vessels

 
Pressure vessels play a critical role in industrial operations, with their safety and reliability directly impacting production stability and the safety of personnel. However, stress corrosion cracking represents one of the primary risks faced by pressure vessels. This form of cracking occurs when the vessel's shell is corroded by corrosive media. Stress corrosion cracking, categorized into uniform corrosion, pitting corrosion, intergranular corrosion, stress corrosion, and fatigue corrosion based on metal failure phenomena, typically progresses through three stages: initiation, propagation, and ultimate failure. This article aims to delve into each stage of stress corrosion cracking in pressure vessels, elucidating the factors influencing them.
 
Initiation Stage
 
1. Combined Effects of Stress Concentration and Corrosive Media
 
During the initiation stage, the surface of the metal gradually develops initial corrosion-mechanical cracks due to the combined action of the corrosion process and stress concentration. Factors contributing to stress concentration include uneven internal stresses, surface defects (such as scratches, machining marks, cracks, laminations, etc.), and structural shape irregularities. Stress concentration zones are the first areas to be attacked by the corrosive media.
 
2. Development of Corrosion Pits
 
If the local stress is initially insufficient to form cracks, this stage may prolong. Over time, certain areas of the metal undergo local corrosion, forming weak zones. These zones experience an increase in local stress, ultimately leading to the initiation of the initial corrosion-mechanical cracks. For instance, local stress on corrosion pits can evolve circular pits into corrosion cracks through the progression of electrochemical processes. The area of maximum stress in the pits usually acts as the anode of the corrosion cell, initiating electrochemical degradation.
 
Propagation Stage
 
1. Electrochemical and Stress Effects
 
During the propagation stage, the initial corrosion-mechanical cracks further expand under the combined action of electrochemical processes of the corrosive media and the predominant tensile stress within the metal. The overall direction of crack propagation typically remains perpendicular to the principal tensile stress direction.
 
2. Stress Concentration at Crack Tips
 
Highly concentrated local stresses exist at the tips of cracks, where the combination of large-area cathodic regions (crack sidewalls) and small-area anodic regions (crack tip) accelerates crack propagation rates significantly. Crack propagation during this stage is influenced not only by mechanical stresses but also by electrochemical reactions, leading to rapid crack extension.
 
Ultimate Failure Stage
 
1. Crack Growth and Stress Concentration
 
As cracks further extend during the propagation stage, one particular crack, due to increasing tensile stress, grows faster than others. It eventually dominates the extension process, redistributing the principal tensile stress to this dominant crack. Rapid crack propagation leads to a sharp decrease in the load-bearing capacity of the metal component.
 
2. Dominance of Mechanical Factors
 
In the ultimate failure stage, fracture primarily occurs due to mechanical factors, with the influence of mechanical forces becoming increasingly dominant over time. When the dominant crack reaches a critical size, the remaining strength of the metal is insufficient to withstand the accumulating stress, resulting in catastrophic fracture. The speed of crack propagation and the abruptness of fracture pose significant threats to the safety of pressure vessels.
 
Stress corrosion cracking in pressure vessels is a complex, multi-stage process. Understanding the characteristics and influencing factors of each stage is crucial for effectively preventing and controlling stress corrosion cracking. Employing appropriate protective measures tailored to different corrosion forms, such as material selection, corrosion-resistant coatings, and optimization of stress distribution, can significantly enhance the lifespan and safety of pressure vessels. In-depth research into the mechanisms of corrosion cracking and the development of scientifically sound maintenance and inspection protocols will help ensure the reliable operation of pressure vessels in diverse and challenging environments.