Corrosion Fatigue
How is corrosion fatigue different from ordinary fatigue?
Ordinary fatigue is driven mainly by cyclic stress, while corrosion fatigue combines that mechanical process with electrochemical damage that destroys the protective film and accelerates crack initiation and growth. It can also eliminate the classic fatigue limit seen in some alloys in air.
What shop-floor controls are most effective?
Reduce surface roughness, avoid tensile residual stress, eliminate burrs and sharp transitions, control vibration, and keep coolant chemistry, rinsing, and drying under control. Coatings, inhibitors, and corrosion-resistant alloys can be used where the service environment justifies them.
Can machining alone improve fatigue life?
Yes. Finishing strategy, coolant control, and surface integrity can produce large fatigue-life differences. Post-machining compressive-surface treatments such as low-plasticity burnishing can further improve corrosion-fatigue resistance.
Corrosion fatigue is the progressive failure of a metal under combined cyclic stress and a corrosive environment. Corrosion accelerates crack initiation and growth compared with fatigue in air, often eliminating the familiar fatigue limit. Surface pits, damaged passive films, and stress concentrators shorten component life even at lower stress amplitudes.
On the shop floor, corrosion fatigue matters whenever a machined part carries repeated loads while exposed to moisture, coolant, humidity, washdown fluids, or process chemicals. The danger is not just visible rust: tool marks, burrs, EDM recast, tensile residual stress, and corrosion pits raise local stress intensity and become fatigue crack starters. Components that survive in air may fail under continued corrosive exposure because corrosion damages the passive film and roughens the surface, concentrating stress at microscopic notches. Control starts at the process window: reduce surface roughness, avoid sharp stress raisers, manage residual stress, and keep coolant chemistry, rinsing, and drying under control so the machined surface does not become a corrosion-initiation site. Deep surface treatments like low-plasticity burnishing can be run on CNC machines and improve corrosion-fatigue performance by introducing compressive residual stress; finishing strategy and cutting parameters can shift fatigue strength dramatically.
Rough or thermally damaged surface: Dull tools, chatter, or EDM recast leave micro-notches and tensile residual stress that accelerate corrosion-fatigue crack initiation. Pits and roughness magnify local stress, starting cracks sooner than a clean, compressively stressed surface.
Pitting under retained moisture: Poor rinsing, wet storage, or chloride exposure creates pits on stainless or high-strength steels with damaged passive films. A pit acts as a sharp crack-like defect and becomes the preferred crack-initiation site under cyclic load.
Residual tensile stress from fabrication: Welding, grinding burn, or aggressive machining leaves tensile residual stress that combines with corrosion to accelerate crack opening and propagation. Tensile stress reduces crack-closure margin and keeps the crack tip active.