After over a century of research, the China Aluminium Network still lacks a unified understanding of the mechanism behind stress corrosion cracking (SCC). The two most widely accepted theories are hydrogen-induced cracking and anodic dissolution. These mechanisms often interact, making it challenging to distinguish between them in practice.
Hydrogen-induced cracking has been particularly associated with high-strength 7xxx aluminum alloys since the 1970s. This mechanism suggests that hydrogen atoms migrate through dislocations and accumulate near precipitates or grain boundaries. This accumulation weakens the interatomic bonds, reducing the strength of the grain boundary and promoting intergranular fracture. Additionally, hydrogen can build up within cracks, creating internal pressure that further contributes to crack propagation. Some theories also suggest that hydrogen promotes plastic deformation or forms hydrides, which can lead to failure.
One prominent theory is the hydrogen pressure model, which states that when hydrogen becomes supersaturated in the metal, it combines into H₂ molecules at defects. This process is irreversible at room temperature, leading to increasing hydrogen pressure. When this pressure exceeds the material’s yield strength, localized plastic deformation occurs, forming bubbles and contributing to crack growth.
Another theory is the weak bond theory, which proposes that hydrogen reduces the atomic bonding strength, making it easier for microcracks to nucleate under stress. The surface energy theory suggests that hydrogen adsorbs on crack surfaces, lowering the energy required for crack propagation. However, this theory doesn't account for plastic deformation and is less applicable to metals.
The hydrogen-induced cracking mechanism integrates multiple factors: hydrogen's role in local plastic deformation, its effect on atomic bonding, and the pressure it generates. This comprehensive view helps explain the complex nature of SCC in aluminum alloys.
On the other hand, the anodic dissolution theory posits that SCC arises from the continuous dissolution of the metal at the anode, leading to crack initiation and propagation. Three main sub-theories support this:
- The anode channel theory explains how corrosion progresses along grain boundaries, especially where there is a potential difference between precipitates and the matrix.
- The slip dissolution theory describes how stress causes slip steps on the oxide film, leading to rupture and exposure of fresh metal to corrosive environments.
- The membrane fracture theory involves the breakdown of protective films, creating anodic areas that dissolve rapidly.
These mechanisms often work together. While anodic dissolution can be mitigated by cathodic protection, hydrogen-induced cracking may actually be exacerbated by such protection. In many cases, both processes contribute to SCC in aluminum alloys, making it difficult to isolate one cause from the other.
For example, Najjar et al. [10] observed that SCC in 7050 aluminum alloy in 3% NaCl solution results from a combination of both mechanisms. Initially, anodic dissolution occurs due to differences in potential at grain boundary particles, breaking the passive film and initiating microcracks. As dissolution continues, hydrogen atoms diffuse into the crack tip, interacting with the local structure and enhancing damage through stress and plastic strain.
Beyond these primary theories, researchers have explored alternative explanations, such as surface migration, dislocation-free regions, and semi-empirical crack growth models. These additional perspectives continue to enrich our understanding of SCC in aluminum alloys, highlighting the complexity of the phenomenon.
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