LSAW Steel Pipe: Hydrogen Induced Cracking Mechanism
1. Sources of hydrogen
Hydrogen came from the LSAW Steel Pipe micro crack, inside the battery cathode process. Atomic hydrogen is a product of the cathodic reaction, they would diffuse inside the metal pipe. Hydrogen Induced Cracking (HIC) pipe corrosion reactions occur far beyond the rolling offered, welding, cathodic hydrogen charging process which leads to excessive amounts of hydrogen within the metal. The second is the environment of hydrogen which containing media or in the corrosive environments, and industrial humid atmosphere of hydrogen induced cracking.
2. Dislocation — Hydrogen Induced Cracking
In the LSAW Steel Pipe, hydrogen exist as a form of H, H+, H, H2, metal hydrides, solid solution, hydrocarbons, and air mass. The distribution of hydrogen is not uniform, but also irregular. Its main enrichment in the pipe stress concentration dislocations, micro-cracks and other cutting edge defects, and to tensile stress concentration at the diffusion. In the actual use, the pipe base material can exist a large number of crystal dislocation, with the external force is generated or moved, and with the change of external conditions change. Dislocation and hydrogen induced cracking are closely linked, most common dissolved hydrogen interference to dislocation glide described hydrogen induced cracking phenomenon. Under certain stress conditions, in-service pipe crystal dislocation slip. When the rate of diffusion within a certain range of temperature and strain rate of a hydrogen atom and the dislocation velocity adapt hydrogen group with dislocation motion and movement, but some distance behind.
Only under certain conditions, environment factors, and the role of external forces, can create continuous generation of new dislocations. Therefore, the plastic deformation continuously carried out in the new generation of new dislocations hydrogen groups. When moving dislocation with obstacles, they formed dislocations and hydrogen buildup phenomenon under enough stress which lead to pipe cracks and pipe cracks extended. When the crack grew into a hydrogen-depleted region, the movement of dislocations and restore freedom to make pipe parent or matrix. Therefore, the stress relaxation by plastic deformation, crack stop expansion at this time. Until the hydrogen atoms under stress again after this build up, the crack extended only continue to grow. This cycle, can cause a section of the crack extended forward, finally resulting in brittle fracture.
3. Hydride — Hydrogen Induced Cracking
In the LSAW Steel Pipe Hydrogen Induced Cracking, the mainly reason is because the post-atomic hydrogen diffuses into the pipe to form the corresponding cause of brittle hydrides. Form Hydrides, hydrogen induced cracking impact will be as flaky hydride has greater sensitivity, since such hydrides prone to stress concentration.
One feature of pipe Hydrogen Induced Cracking is having hydrogen-induced cracking susceptibility. When the pipes under certain stress, the cracks will appeared a lag time. Therefore, the LSAW Steel Pipe hydrogen induced cracking will appear after a period of time after the occurrence. The lag time is not relevant to the stress applied on the pipe, but with the increase of hydrogen concentration in the pipe, the lag time will be shorter. Lag time indicates hydrogen diffusion into the core region near the crack, and the time to reach the desired concentration of hydrogen cracking required.
4. Sulfide Rupture
During the application of LSAW Steel Pipe, some pipes will be in contact with water which containing H2S often. In such conditions, when the initial crack formed under the stress and the media together, the crack will continue to expand. In practice, it is very difficult to distinguish hydrogen induced cracking and stress corrosion cracking. The solution The solution of hydrogen sulfide in pipe cracking, hydrogen induced cracking is a major cause. As in a relatively short period of time, it occurred hours or days by hydrogen induced cracking. Just a little slight corrosion, will produce hydrogen and formation, prompting a large number of hydrogen atoms adsorbed on the surface of the pipe so that the pipe is formed on the surface of a larger hydrogen atom concentration gradient accelerating the diffusion of hydrogen into the interior of the pipe. H2S caused by pipe rupture often referred sulfide rupture.
From LSAW Steel Pipe hydrogen induced cracking mechanism of view, no matter how caused by hydrogen induced cracking, it has the same characteristics which is mostly
inter-granular fracture surface cracking. From the mouth of the fracture surface, the hydrogen induced cracking and stress corrosion cracking is very similar to the straight seam on LSAW Steel Pipe during the pipe application. However, it is able to distinguish the difference by certain method. Cathodic polarization control stress corrosion, but it makes the hydrogen evolution reaction is accelerated, promoting hydrogen damage. The anodic polarization, can promote the anodic dissolution process, the stress corrosion cracking acceleration, it HIC no significant impact.