The Research Status of Aluminum Alloy Stress Corrosion Theory

After more than a century of research, there are still disagreements in the academic circles on the mechanism that causes SCC. The currently accepted mechanisms are hydrogen-induced cracking and anodic dissolution.

1. Hydrogen-induced cracking

Since the mid-1970s, many experiments have shown that the SCC of the 7 series high-strength aluminum alloy belongs to the hydrogen-induced cracking mechanism. The theory believes that: (1) hydrogen migrates to the grain boundary through dislocations and accumulates near the precipitated phase, which greatly reduces the bonding strength of the grain boundary, weakens the grain boundary, and causes intergranular fracture; (2) due to the accumulation of hydrogen in the crack, The formed hydrogen pressure promotes the fracture of the alloy; (3) hydrogen promotes the deformation of the alloy and causes the fracture; (4) the formed hydride promotes the fracture of the alloy. The hydrogen-induced cracking mechanism currently proposed mainly has the following theories:

(a) Hydrogen pressure theory: When there is supersaturated H in the metal, it will combine into H2 at various microscopic defects. It is an irreversible reaction at room temperature, that is, H2 will no longer decompose into H. With the defect, H2 As the concentration increases, the hydrogen pressure also increases. When the hydrogen pressure is greater than the yield strength, local plastic deformation will occur, which will swell the surface and form hydrogen bubbles.

(b) Weak bond theory: The hydrogen in the metal reduces the bonding force of the atomic bond. When the local stress concentration is equal to the bonding force of the atomic bond, the atomic bond breaks and microcracks nucleate.

(c) Hydrogen reduces the surface energy theory: hydrogen reduces the bonding force while inevitably reducing the surface energy, and vice versa. Hydrogen is adsorbed on the inner surface of the metal crack, reducing the surface energy, leading to the critical stress required for the instability and propagation of the crack Decline. Since plastic deformation work is not considered, it is not applicable to metal materials.

(d) Comprehensive hydrogen-induced cracking mechanism: This mechanism comprehensively considers the role of hydrogen to promote local plastic deformation, hydrogen to reduce atomic bonding force, and hydrogen pressure.

2, anode dissolution

The anodic dissolution theory [7~9] believes that the continuous dissolution of anode metal leads to the nucleation and propagation of SCC cracks, resulting in the fracture of the alloy structure. The main points of the anodic dissolution theory of aluminum alloy SCC are as follows:

(1) Anode channel theory: Corrosion occurs along the local channel and cracks are generated. The tensile stress is perpendicular to the channel, and stress concentration is generated at the tip of the local crack. The pre-existing anode channel in aluminum alloy is separated from the grain boundary precipitated phase and the substrate potential The difference is caused by the difference, and the stress causes the crack to open and expose the fresh surface. In this case, the corrosion accelerates along the grain boundary.

(2) Slip dissolution theory: There are local weak points in the surface oxide film of the aluminum alloy where SCC occurs. Under the action of stress, the part of the alloy matrix will move along the slip and form a slip ladder. When the surface film is large and cannot deform correspondingly with the formation of the sliding ladder, the film will rupture and expose the fresh surface, contact with corrosive media, and rapid anodic dissolution occurs.

(3) Film rupture theory: There is a protective film on the metal surface in the corrosive medium, which is caused by stress or active ions. The exposed fresh surface and the remaining surface film form a small anode and large cathode corrosion battery, resulting in fresh Anodic dissolution occurs on the surface.

3. Co-action of anode dissolution and hydrogen-induced cracking

Anodic dissolution and hydrogen-induced cracking are two different concepts. Pure anodic dissolution can be prevented by cathodic protection. For hydrogen-induced cracking, cathodic polarization tends to promote cracking. Some systems are based on anodic dissolution, and some Hydrogen-induced cracking is the main one. The SCC of aluminum alloys often includes these two processes at the same time, and it is actually difficult to clearly 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 combined effect of anodic dissolution and hydrogen-induced cracking. At the beginning, due to the potential difference of the particles at the grain boundary of the alloy, localized The anode dissolves, causing the passivation film to rupture, forming critical defects, and microcrack initiation. With the increase of local anodic dissolution at the grain boundary, reducing H atoms diffuse into the process zone and interact with the microscopic characteristic structure, crack tip stress and plastic strain , Causing damage.

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