STRESS CORROSION BY ABHINAV GUPTA(12108030)

WHAT IS STRESS CORROSION ? Stress corrosion refers to cracking

caused by the simultaneous presence of tensile stress and a corrosive medium.

It is thus also known as “STRESS CORROSION CRACKING”(SCC).

The two prominent examples of stress-corrosion cracking are season cracking of brass and the caustic embrittlement of steel

Requirements for SCC

Environment

StressMetallurgy

Environmental composition

Stress Metal Temperature;

Examples : Brasses crack in NH3, not in Cl-; SSs crack in Cl-, not in NH3; SSs crack in caustic, not in H2SO4, HNO3, CH3COOH, . . . etc.

During stress-corrosion cracking, the metal or alloy is virtually unattacked over most of its surface, while fine cracks progress through it.

Here stress corrosion cracking of brass is shown.

Season cracking Occurs where brass

case is crimped onto bullet, i.e., in area of high residual stress.

Common in warm, wet environments (e.g., tropics).

Ammonia (from decomposition of organic matter, etc.) must be present.

Season cracking of German ammunition.

SCC: important features Materials breakdown happens at macroscopic

scale, without deformation and perpendicular to the stress direction.

No measurable material removal No visible corrosion products In most of the SCC failure, corrosion initiation

is difficult to detect. Major damage during SCC occurs in late

stages as cracks progress until final failure occurs by mechanical rupture.

Mechanism of SCC SCC is very complex. There is

probably no single mechanism, but several operating at the same time. Models (scientific descriptions) of mechanisms of two types:

• dissolution• mechanical fracture

Mechanical rupture model of crack propagation

Environments causing SCC of particular metals

Prevention of SCC1. Lowering the stress below the threshold value if

one exists :This may be done by annealing in the case of residual stresses, thickening the section, or reducing the load. For example : Plain carbon steels may be stress-relief annealed at 590 to 650C.

2. Eliminating the critical environmental species by:

1. de-gasification2. Demineralization3. distillation.

Prevention of SCC continued…3. Changing the alloy is one possible recourse if neither

the environment nor stress can be changed. For example, it is common practice to use Inconel (raising the nickel content) when type 304 stainless steel is not satisfactory. Although carbon steel is less resistant to general corrosion, it is more resistant to stress-corrosion cracking than are the stainless steels. Thus, under conditions which tend to produce stress-corrosion cracking, carbon steels are often found to be more satisfactory than the stainless steels. For example, heat exchangers used in contact with seawater or brackish waters are often constructed of ordinary mild steel.

Prevention of SCC continued… Other methods to prevent SCC are:-

4. Cathodic protection

5. Adding inhibitors

6. Coatings

7. Shot-peening