Current Theories of aluminum stress corrosion

After more than a century of research , the mechanism for causing SCC academia there are still differences . Now generally accepted mechanism of hydrogen-induced cracking and anodic dissolution mechanism .

1 , hydrogen-induced cracking

Since the mid-seventies , many experiments show , 7 × × × series high strength aluminum alloy SCC is hydrogen-induced cracking mechanism . The theory is that : ( 1 ) the migration of hydrogen through the grain boundary dislocations to accumulate around the precipitates , the bonding strength of the grain boundaries greatly reduced , weakening the grain boundary , resulting in intergranular fracture ; ( 2 ) the accumulation of hydrogen within the crack , promote the formation of hydrogen pressure alloy fracture ; ( 3 ) to promote hydrogen alloy deformation which results in fracture ; ( 4 ) to promote the formation of hydride alloy fracture. currently proposed mechanism of hydrogen induced cracking has the following theory :

(a) hydrogen pressure of theory : When the metal present in supersaturated H, the microscopic defects in the various combined into H2, room temperature, the reaction is not reversible , that will not decompose into H2 H. H2 concentration increases as 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 muster , forming hydrogen bubbles .

(b) a weak bond theory : reduction of hydrogen atoms in the metal bond strength , when the local stress concentration equal atomic bonding force of atomic bond rupture, micro- crack nucleation .

(c) reduce the surface energy of hydrogen theory : hydrogen bonding force at the same time reduce the inevitable lowering the surface energy , and vice versa . hydrogen adsorbed on the metal surface inside the crack , the surface can be reduced , resulting in crack instability critical stress required to decline due to did not consider the plastic deformation work , so the metal does not apply.

(d) Comprehensive hydrogen induced cracking mechanism : This mechanism considering promoting local plastic deformation of hydrogen , hydrogen atoms bond together and reduce the role of hydrogen pressure .

2 , anodic dissolution

Anodic dissolution theory [ 7-9 ] that the anodic dissolution of the metal continues to lead SCC crack nucleation and expansion , resulting in the fracture alloy Anodized Aluminum dissolution SCC main points of the theory are as follows:

( 1 ) Anode channel Theory : the corrosion along the local channels and cracks occur , the tensile stress perpendicular to the channel , the local concentration of stress on the crack tip . Aluminum alloy anode channel consists of a pre-existing grain boundary phase difference caused by the substrate potential , and then the crack opening stress exposing a fresh surface in this case , accelerated corrosion along the grain boundaries .

( 2 ) slip dissolution theory: the occurrence of SCC aluminum oxide film on the surface of the existence of local weak spots in the alloy matrix under stress along the sliding parts arising wrong move , the formation of slip ladder when sliding ladder , surface film can not slip ladder with the formation of the corresponding deformation occurs , the membrane will rupture and fresh bare surface in contact with the corrosive medium , undergoing rapid anodic dissolution .

( 3 ) membrane rupture theory: the presence of corrosive media, metal surface protection film , as reactive ion suffer from stress or rupture caused by the action of , exposed fresh surface with the rest of the surface film to form the small yang great cathode corrosion cell , resulting in fresh surface of the anode dissolved.

3 , anodic dissolution and hydrogen-induced cracking joint action

Anodic dissolution and hydrogen-induced cracking are two different concepts, simple anodic dissolution can be prevented by cathodic protection , and for the hydrogen-induced cracking, cathodic polarization tend to promote cracking and some system dominated by anodic dissolution , some places hydrogen mainly induced cracking of aluminum alloys SCC often include both process and to a sharp distinction between the two phenomena is actually difficult.

Najjar , etc. [ 10 ] found that 7050 aluminum alloy in 3% NaCl solution SCC is due to anodic dissolution and hydrogen-induced cracking result of joint action . Initially, the alloy grain boundaries particle potential difference exists , the occurrence of local anodic dissolution , resulting in rupture of the passive film formed critical defects , micro- crack initiation with partial anodic dissolution of grain boundary increases, reducing the H atoms diffuse into the process zone , and microscopic characteristics of the structure , the crack tip stress and plastic strain interactions , causing damage .

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