After over a century of research, the China Aluminium Network still faces debates regarding the exact mechanism of stress corrosion cracking (SCC). The two most widely accepted theories are hydrogen-induced cracking and anodic dissolution. These mechanisms often overlap in practice, making it challenging to distinguish between them.
Hydrogen-induced cracking has gained significant attention since the 1970s, especially in high-strength 7××× aluminum alloys. This theory suggests that hydrogen diffuses along dislocations and accumulates near precipitates, weakening grain boundaries and promoting intergranular fracture. Additionally, hydrogen buildup within cracks can create internal pressure, further accelerating crack propagation. Some models also suggest that hydrogen reduces atomic bonding strength or lowers surface energy, making the material more susceptible to cracking.
Several sub-theories support hydrogen-induced cracking. The hydrogen pressure theory states that when hydrogen becomes supersaturated, it forms H₂ molecules at defects, leading to increased pressure and local plastic deformation. The weak bond theory claims that hydrogen weakens atomic bonds, causing microcracks to form under stress. The surface energy theory argues that hydrogen adsorption on cracks reduces the critical stress needed for crack propagation. Lastly, the hydrogen-induced cracking mechanism integrates these factors, considering both hydrogen’s effect on plasticity and its role in reducing bonding forces.
On the other hand, the anodic dissolution theory proposes that SCC occurs due to the continuous dissolution of the metal at the anode. This leads to crack initiation and propagation. Key concepts include the anode channel theory, where corrosion follows grain boundary channels, and the slip dissolution theory, where stress causes the oxide film to rupture, exposing fresh metal to corrosion. The membrane fracture theory explains how protective films break under stress, creating localized anodes that dissolve rapidly.
In many cases, both anodic dissolution and hydrogen-induced cracking contribute to SCC. While cathodic protection can prevent anodic dissolution, it may actually promote hydrogen-induced cracking by increasing hydrogen concentration. Studies, such as those by Najjar et al., have shown that in 3% NaCl solutions, the SCC of 7050 aluminum alloy results from a combination of both mechanisms. Initially, anodic dissolution breaks the passive film, creating defects that allow hydrogen to penetrate and interact with the crack tip, worsening the damage.
Beyond these main theories, researchers have explored other perspectives, including surface migration, dislocation-free regions, and semi-empirical crack growth models. These alternative approaches provide additional insights into the complex nature of SCC in aluminum alloys. Despite extensive research, the precise mechanisms remain a topic of ongoing investigation.
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