4. Forms of Corrosion

4.6 Stress Corrosion Cracking [2/4]

Factors Affecting SCC

Nearly all materials are susceptible to stress corrosion cracking in specific environment. For instance, austenitic steels are susceptible to SCC in hot chloride solution., whereas carbon steels are susceptible in hot nitrate and carbonate/bicarbonate solutions. Copper alloys are susceptible to SCC in ammoniacal solutions. Table below shows the alloys and the environments for SCC.

Table 4.1: Alloy and Species Conducive to Environmental Cracking

ALLOY

ENVIRONMENT
Carbon steels NAOH, NH3, NO2-, CN-, HCO4, CO/CO2/H2O
Austenitic steels Cl-, Cl-/H2S, NaOH
Nickel 2200 HF + O2, H2SiF6
Ferritic steels NaOH, marine atmosphere
Magnesium Aqueous Cl- plus oxidisers
Aluminum Aqueous Cl- plus oxidisers
Copper alloys NH3, ammonia vapors, amines
High nickel alloys High purity steam

The factors affecting SCC are:

1. Tensile Stress

The magnitude of SCC increases with an increase in tensile stress in the material. The stresses must be tensile and may result from loads applied in service or stress set up by the type of assembly or stresses set up by the assembly or from residual stresses resulting from the fabrication.  In many cases, a threshold stress for SCC may exist. The net stress at the metal surface in contact with the environmental species is the critical factor leading to SCC. The net stress includes all types of stresses such as residual stresses, thermal stresses and tensile stresses built in service.
2. Alloy Composition

The list of specific environments for different metals and alloys is given in Table 4.1 above. Each metal alloy system needs a specific environment.
3. Corrosive Environment

SCC occurs only in a particular environment. For example, austenitic steels undergo SCC only in chlorides. Copper alloys become susceptible to SCC only in ammoniacal solutions. It may be possible to control SCC by eliminating the damaging species from the environment.
4. Oxidizing Potential

SCC occurs only in a limited range of oxidizing potentials. SCC can be controlled by shifting the oxidizing potentials in a range where SCC does not occur. SCC occurs within a limited range of oxidizing potentials and these potentials can be shifted outside the range of SCC by altering the environment or by changing the composition of metal and alloys and by polarizing the metal/alloys.
5. Temperature

Generally the magnitude of stress corrosion cracking increases with increased temperature.