A form of fatigue cracking in which cracks develop under the combined effects of cyclic loading and corrosion.
Corrosion Fatigue | Materials And Corrosion Control
Damage Mechanism |
Corrosion Fatigue |
Damage Description |
· A form of fatigue cracking in which cracks develop under the combined effects of cyclic loading and corrosion.
· It is characterized by the premature failure of a cyclically-loaded part. This failure may occur at a lower stress, or in a fewer number of cycles in the corrosion environment than it would be in an inert environment. · Cracking often initiates at a stress concentration such as a pit in the surface. · Cracking can initiate at multiple sites. |
Affected Materials |
· All metals and alloys (Aluminum alloys, Copper alloys, Carbon and Low-Alloy Steels, Stainless Steels and Nickel alloys and Titanium Alloys) |
Control Methodology |
· Verify the following during design: 1) Lower the tensile stresses 2) Avoid geometries in which solution can become concentrated 3) Compatibility of materials · Modify the corrosive environment by using coatings and/or inhibitors. · Minimize galvanic couple effects. · Use more corrosion resistant materials. · Minimize residual welding and fabrication stresses through PWHT. · Minimize weld reinforcement by grinding weld contours smooth. |
Monitoring Techniques |
· Visual Inspection
· MT Survey · WFMT inspection · Acoustic emission |
Inspection Frequency |
· Annually |
KPIs |
· Corrosion Rate: < 5 mpy · WFMT: Absence of cracks |
Reference Resources (Standards/GIs/BPs) |
· API RP 571 (DM#43)
· NACE Basic Corrosion Course · ASME SEC V |
Thermal Fatigue | Materials And Corrosion Control
Hydrogen Embrittlement | Materials And Corrosion Control
Brittle Fracture | Materials And Corrosion Control
Notes:
- The critical factors are the material, corrosive environment, cyclic stresses and stress raisers.
- Cracking is more likely to occur in environments that promote pitting or localized corrosion under cyclic stress due to thermal stress, vibration or differential expansion.
- Contrary to a pure mechanical fatigue, there is no fatigue limit load in corrosion-assisted fatigue.
- Corrosion promotes failure at a lower stress and number of cycles than the materials’ normal endurance limit in the absence of corrosion and often results in propagation of multiple parallel cracks.
- Crack initiation sites include concentrators such as pits, notches, surface defects, changes in section or fillet welds.