Research Status of Stress Corrosion of Aluminum Alloys

After more than a century of research, China Aluminium Network still has divergence on the mechanism of SCC. The currently accepted mechanism is hydrogen induced cracking and anodic dissolution mechanisms.

1. Hydrogen-induced cracking Since the mid-1970s, more experiments have shown that the SCC of 7××× high-strength aluminum alloy belongs to the hydrogen-induced cracking mechanism. The theory is that: (1) hydrogen migrates to the grain boundary through dislocations and accumulates in the vicinity of the precipitated phase, greatly reducing the bond strength at the grain boundary, weakening the grain boundary and causing intergranular fracture; (2) Hydrogen accumulates in the crack, The formed hydrogen pressure promotes the fracture of the alloy; (3) hydrogen promotes deformation of the alloy and causes fracture; (4) the formed hydride promotes fracture of the alloy. The currently proposed hydrogen induced cracking mechanism has the following theories:

(a) Hydrogen pressure theory: When supersaturated H is present in the metal, it will bind to H2 at various microscopic defects, and the room temperature is an irreversible reaction, ie, H2 will not be decomposed into H again. With increasing concentration of H2 at the defect, Hydrogen pressure also increases. When the hydrogen pressure is greater than the yield strength will produce local plastic deformation, so that the surface bulge, the formation of hydrogen bubbles.

(b) Weak Bond Theory: Hydrogen in metals reduces the bond strength of atomic bonds. When the local stress concentration is equal to the bonding strength of atomic bonds, the atomic bonds break and microcracks nucleate.

(c) Hydrogen lowers the surface energy theory: Hydrogen reduces the bonding force and at the same time necessarily reduces the surface energy, and vice versa. Hydrogen is adsorbed on the inner surface of the metal crack, causing the surface energy to decrease, leading to a drop in the critical stress required for crack instability expansion. Does not consider the plastic deformation work, it is not suitable for metal materials.

(d) Hydrogen-induced cracking mechanism: This mechanism takes into account hydrogen's contribution to local plastic deformation, hydrogen's reduction of atomic bonding forces, and hydrogen pressure.

2. Anodic Anodic Dissolution Theory [7-9] believes that the continuous dissolution of the anode metal leads to the nucleation and expansion of the SCC crack, resulting in the fracture of the alloy structure. The main points of view of the anodic dissolution theory of the aluminum alloy SCC are as follows:

(1) Anode channel theory: Corrosion occurs along local channels and causes cracks. Tensile stress is perpendicular to the channel, and stress concentration occurs at the local crack tip. The pre-existing anode channel in the aluminum alloy is caused by the potential difference between the grain boundary precipitation phase and the matrix. The stress causes the crack to open and expose the fresh surface. In this case, corrosion accelerates along the grain boundary.

(2) Slip Dissolution Theory: There are localized weak spots on the surface of the aluminum oxide film on the aluminum alloy where SCC occurs. Under the action of the stress, the internal faults of the alloy matrix will move along the slip and form a slip step. When the slip step is large, the surface When the film does not deform with the formation of the sliding step, the film will rupture and expose the fresh surface, and contact with the corrosive medium, resulting in rapid anodic dissolution.

(3) Membrane Fracture Theory: There is a protective film on the metal surface in the corrosive medium. Due to the stress or the action of active ions, the exposed fresh surface and the rest of the surface film constitute a corroded cell of a small anode and a large cathode, resulting in a fresh anode surface. Dissolved.

3. Anodic dissolution and hydrogen-induced cracking work together Anodic dissolution and hydrogen-induced cracking are two different concepts. Simple anodic dissolution can be prevented by cathodic protection, whereas for hydrogen induced cracking, cathodic polarization tends to promote cracking. Some systems Anodic dissolution is dominant, while others are dominated by hydrogen induced cracking. The SCC of aluminum alloys often includes both processes. It is actually difficult to distinguish these two phenomena.

Najjar et al. [10] found that the SCC of 7050 aluminum alloy in 3% NaCl solution is the result of the combination of anodic dissolution and hydrogen-induced cracking. At the beginning, local anodic dissolution occurred due to the potential difference between the particles at the grain boundaries of the alloy. The passivation film is broken, critical defects are formed, and microcracks are initiated. As the local anodic dissolution of grain boundaries increases, the reducing H atoms diffuse into the process zone and interact with the micro-characteristic structure, crack tip stress and plastic strain, causing damage. .

In addition to the above SCC mechanism, the researchers also studied the SCC mechanism from other perspectives, including the SCC surface migration theory, the SCC-free dislocation region theory and the semi-empirical model of crack growth.

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