Environmental cracking refers to a corrosion cracking caused by a combination of conditions that can specifically result in one of the following form of corrosion damage:
Stress Corrosion Cracking (SCC)
Stresses that cause environmental cracking arise from residual cold work, welding, grinding, thermal treatment, or may be externally applied during service and, to be effective, must be tensile (as opposed to compressive).
Stress definition or stress variables
- Mean stress
- Maximum stress
- Minimum stress
- Constant load/constant strain
- Strain rate
- Plane stress/plane strain
- Modes I, II, or III
- Cyclic frequency
- Wave shape
- Shearing, punching, cutting
- Bending, crimping, riveting
Produced by reacted products
- Thermal cycling
- Thermal expansion
- Dead load
The cracks form and propagate approximately at right angles to the direction of the tensile stresses at stress levels much lower than those required to fracture the material in the absence of the corrosive environment. As cracking penetrates further into the material, it eventually reduces the supporting cross section of the material to the point of structural failure from overload. SCC occurs in metals exposed to an environment where, if the stress was not present or was at much lower levels, there would be no damage. If the structure, subject to the same stresses, were in a different environment (noncorrosive for that material), there would be no failure. Examples of SCC in the nuclear industry are cracks in stainless steel piping systems and stainless steel valve stems.
Stress cells can exist in a single piece of metal where a portion of the metal's microstructure possesses more stored strain energy than the rest of the metal. Metal atoms are at their lowest strain energy state when situated in a regular crystal array. Deviations from this lowest-strain state follow reference:
Grain boundaries: By definition, metal atoms situated along grain boundaries are not located in a regular crystal array (i.e. a grain). Their increased strain energy translates into an electrode potential that is anodic to the metal in the grains proper. Thus, corrosion can selectively occur along grain boundaries.
High localized stress: Regions within a metal subject to a high local stress will contain metal atoms at a higher strain energy state. As a result, high-stress regions will be anodic to low-stress regions and can corrode selectively. For example, bolts under load are subject to more corrosion than similar bolts that are unloaded. A good rule of thumb is to select fasteners that are cathodic (i.e. higher on the Electrochemical Series) to the metal being fastened in order to prevent fastener corrosion.
Cold worked: Regions within a metal subjected to cold-work contain a higher concentration of dislocations, and as a result will be anodic to non-cold-worked regions. Thus, cold-worked sections of a metal will corrode faster. For example, nails that are bent will often corrode at the bend, or at their head where they were worked by the hammer.