Corrosion
Heat exchangers failure because of stress corrosion cracking : a mechanistic , diagnostic and control perspective
Heat exchangers , integral to diverse industrial processes , serve as linchpins in the seamless transfer of thermal energy between fluids . Their role is pivotal , optimizing the efficiency of operations spanning from power generation to chemical production . Despite their critical significance , these components are susceptible to failure , and stress corrosion cracking ( SCC ) stands out as a formidable challenge contributing to their degradation .
By Omari Hussein Sabuni , Mechanical Engineer – Kinyerezi Power Plant
Concept of stress corrosion cracking ( SCC ) Definition : Stress Corrosion Cracking ( SCC ) is a highly specialized and insidious form of corrosion that manifests under the combined influence of tensile stress and a corrosive environment . It stands out as a unique corrosion phenomenon due to the simultaneous presence of mechanical stress and a corrosive medium , both of which synergistically contribute to the degradation of materials . Mechanism : The mechanism of stress corrosion cracking involves a complex interplay of mechanical and chemical factors . SCC typically begins at localized points on the material ’ s surface where there is a concentration of tensile stress . This stress can result from external loads , residual stresses from manufacturing processes , or a combination of both . The corrosive environment , which may include factors like moisture , chemicals , or specific gases , accelerates the corrosion process . The synergistic effect of tensile stress and corrosion acts as a catalyst for the initiation and propagation of cracks . The cracks tend to initiate at microscopic defects or points of stress concentration , creating a pathway for the corrosive medium to penetrate the material . Once initiated , these cracks can propagate rapidly , leading to a decrease in the material ’ s structural integrity .
Types of stress corrosion cracking 1 . Transgranular SCC Definition : Transgranular SCC is characterized by cracking occurring along the grain boundaries of the material , traversing the individual grains . Mechanism : The mechanism of transgranular SCC involves the propagation of cracks through the internal structure of the material . These cracks initiate and grow through the grains , causing disruptions along the boundaries . Several key aspects define the mechanism :
I . Grain boundary attack
• Cracks initiate and propagate predominantly within the grains , leading to a localized attack on the material .
• The choice of specific crystallographic orientations can influence the susceptibility of a material to transgranular SCC . Certain crystallographic planes and directions may be more prone to cracking .
II . Crystallographic orientations
• The propensity for transgranular SCC is often associated with specific crystallographic orientations of the grains .
• The anisotropic nature of materials , where properties vary with direction , can lead to preferential crack propagation along certain crystallographic planes .
III . Reduction in mechanical strength
• As cracks propagate through individual grains , they disrupt the continuity of the material , leading to a reduction in mechanical strength .
• The accumulated effect of multiple cracks can significantly compromise the structural integrity of the material . www . heat-exchanger-world . com Heat Exchanger World October 2024
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