Stress Corrosion Cracking Case Study

2470 Words 10 Pages
Study of Stress Corrosion Cracking (SCC) in

Alloys

Jiang Jingxing

1. Introduction of Stress Corrosion Cracking
Stress Corrosion Cracking (SCC) is the growth of cracks with a under the combined

effects of a static stress and a specific chemical environment. All SCC failures

involve a macroscopic brittle performance, and the ductility of the material is reduced.

It is universally acknowledged that there is no single mechanism for stress corrosion

cracking (Ford, 1984). Stress corrosion spectrum (Parkins, 1972) demonstrates the

significant alloy-environment the mechanism involved. Stress corrosion spectrum is

shown as Figure 1.

Figure.1 Stress corrosion spectrum

Stress Corrosion Cracking contains two main stages:
…show more content…
Figure 2 Dislocation movement in SCC

As shown in Figure 2 the “step” formed after slip is an active site for the initiation of a stress-concentration pit. And pits are important for cracks generation. The pit formation is

precursor to stress corrosion cracking. If some pits are formed in proper condition,

growth of crack takes place and propagation happens.

Chu et al. (2004) had demonstrated microcracks during initiation. In Chu’s experiment
X-65 pipeline steel samples were exposed to synthetic soil environment for 61 days and

111 days, and the morphology of two samples are shown in Figure 3. It was concluded

that majority of initiated microcracks were generated from pits at metallurgical

discontinuous points like grain boundaries and banded phases.

Figure 3 SEM images for 61 days exposure sample (a) and 111 days exposure

sample (b)

In practice, initiation is negligible, and few efforts are made to inhibit initiation.

Because in fabrication defects are enough for initiation sites, which means in
…show more content…
Prevention of Stress Corrosion Cracking
There are three key factors for stress corrosion cracking: stress, environment and

susceptible material. So, the prevention of SCC includes elimination of one of three

factors: remove stress, avoid the susceptible environment or modify microstructure of

materials.

2.1 Remove stress
In most cases it is impossible to remove the working stress of components,

which components are designed to bear. But reducing the residual stress is an

effective method to control stress corrosion cracking. Usually annealing is

adapted to relieve residual stress inside, but annealing is not effective for large

components to relive residual stress, sometimes unexpected phases form during

annealing.

Another way to remove stress is avoid stress concentrators like notches, which

can prevent stress concentration and reduce stress inside.

2.2 Avoid the susceptible environment Controlling environment is usually not possible to prevent SCC. The methods to

control the environment in service includes mixing inhibitors, modifying the

electrode potential of the alloy, and isolating the alloy by coatings.

2.3 Modify microstructure of

Related Documents