corrosion-fatigue

name	corrosion-fatigue
description	Corrosion fatigue — synergistic interaction of cyclic stress and corrosive environment, da/dN vs. ΔK in corrosive media (corrosion-fatigue crack growth rate), S-N curves in corrosive environments (no endurance limit), anodic dissolution and hydrogen embrittlement mechanisms, pitting as crack initiation, corrosion fatigue of steel and aluminum in seawater, life prediction models (NASGRO with environment factor), cathodic protection effect, and ISO/ASTM standards for corrosion fatigue testing.
metadata	{"priority":7,"promptSignals":{"phrases":["corrosion fatigue","corrosion fatigue crack","fatigue in corrosive environment","seawater fatigue","environmental crack growth","pitting fatigue"],"minScore":3}}

Corrosion Fatigue — Complete Skill

Mechanism and Definition

Synergistic Damage

Corrosion fatigue: combined action of cyclic stress + corrosive environment → fatigue life significantly shorter than either mechanism alone Synergy: total damage > (mechanical fatigue damage) + (corrosion damage alone) Mechanisms:

1. Anodic dissolution: crack tip exposed to electrolyte → dissolution → crack extension before mechanical re-closure
2. Hydrogen embrittlement (HE): cathodic reaction at crack tip produces H → H diffuses into metal → enhances crack growth; especially in high-strength steels
3. Passive film fracture: oxide film at crack tip fractured by strain → fresh metal exposed → dissolution spike → net crack advance per cycle

Effect on S-N curve: Air: ferrous metals have endurance limit (σ_e at ~10⁷ cycles) Corrosive environment (seawater, acid, humid): NO endurance limit → continual degradation at any stress level Reduction factor: σ_e_corrosion / σ_e_air = 0.3–0.5 (50–70% reduction for steel in seawater)

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Effect of Environment on Crack Growth

da/dN vs. ΔK in Corrosive Media

Standard Paris law (air): da/dN = C × ΔK^m [C, m = material constants; ΔK = stress intensity range]

Corrosion fatigue crack growth rate (CFCGR): CFCGR > da/dN_air for same ΔK: environment accelerates growth Mechanisms appear as: lower threshold ΔK_th (crack grows at lower ΔK), plateau region at intermediate ΔK, steeper slope m

Cycle-time dependence (frequency effect): Slower cycling → more time per cycle for corrosive attack → higher da/dN_CF da/dN_CF / da/dN_air increases as: f ↓ (lower frequency) or R ↑ (higher stress ratio → more time at high K) For steel in seawater at 0.1 Hz vs. 10 Hz: CFCGR up to 10× higher at same ΔK

R-ratio effect in corrosive media: Higher R (= K_min/K_max) → crack open longer per cycle → more environmental access da/dN_CF particularly sensitive to R in corrosive environments (moreso than in air)

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Pitting as Initiation Site

Pit-to-Crack Transition

Pitting corrosion: localized dissolution creates surface pits → stress concentration → fatigue crack nucleation Stress concentration at pit: K_t ≈ 1 + 2√(d_pit / ρ_pit) [d_pit = depth; ρ_pit = root radius; similar to elliptical notch] Pit acts as equivalent notch → reduces S_e by K_f factor → Miner's damage accelerated

Pit-to-crack transition criterion: Crack initiates when K_pit_tip ≥ K_th,corrosion [K from pit geometry; K_th,corrosion = threshold in environment] ΔK_pit = Y × Δσ × √(π × a_pit) [Y ≈ 0.63–0.73 for hemispherical pit] Transition pit size: a_pit,crit = (K_th,corr / (Y × Δσ))² / π

Example (7075-T651, seawater): K_th,corr ≈ 3 MPa√m (vs. 5 MPa√m in air); Δσ = 100 MPa; Y = 0.65 a_pit,crit = (3/(0.65×100))² / π = (0.0462)² / π = 2.13×10⁻³ / π = 6.8×10⁻⁴ m = 0.68 mm → pits deeper than 0.68 mm initiate cracks at this stress range

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Corrosion Fatigue of Specific Materials

Steel in Seawater

Structural steel (S355) in seawater: Air S-N: σ_e ≈ 200 MPa (10⁷ cycles); Seawater S-N (free corrosion): no endurance limit; at 10⁸ cycles: σ ≈ 80 MPa Cathodic protection (CP at −800 mV Ag/AgCl): restores about 60–70% of air endurance limit (eliminates anodic dissolution but HE risk from overprotection) Offshore structural steel: -850 mV SCE (sweet spot for CP vs. HE balance per NACE RP0176)

High-strength steel (σ_UTS > 1200 MPa): More susceptible to HE component of CF → da/dN much higher than lower-strength steel ASTM Grade 150 offshore bolts: limit to σ_e_CF = 0.35 × σ_UTS in seawater

Aluminum in Seawater

5xxx and 7xxx aluminum: 7075-T651: excellent air fatigue (σ_e = 150 MPa); seawater: σ_CF ≈ 90–100 MPa at 10⁸ cycles (33% reduction) 5083-H111: more corrosion resistant; σ_CF ≈ 90 MPa (similar to 7075 in corrosion but less sensitive in crack growth) Al alloys: HE mechanism minor (not iron-based); anodic dissolution and passive film fracture dominate Pitting from chloride attack (Cl⁻ breaks down passive Al₂O₃) → critical initiation mechanism

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Life Prediction

NASGRO Equation with Environmental Factor

NASGRO crack growth model: da/dN = C × [(1−f)/(1−R)]^n × ΔK^p / [(1 − ΔK_th/ΔK)^q × (1 − K_max/K_Ic)^r] Where: f = Newman crack opening function; C, n, p, q, r = material constants (NASGRO database)

Environmental modification: For corrosion fatigue: replace ΔK_th with ΔK_th,env (lower); C with C_env (higher) Or: use separate CFCGR curves for specific environment (temperature, pH, salinity, frequency) Source: NASGRO material database includes limited environmental data; typically supplement with test data

Conservative life prediction:

1. Pit-to-crack transition at a_pit,crit
2. Crack growth from a_pit,crit to a_crit (from K_Ic)
3. N_total = N_pit_growth_to_crit + N_CF_growth
4. Inspection interval = N_total / SF [SF = 2 for critical offshore structures per ISO 19902]

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Protective Measures

Cathodic Protection (CP)

CP mechanism: Apply negative potential → suppress anodic dissolution at crack tip Cathodic reaction: O₂ + 2H₂O + 4e⁻ → 4OH⁻ (reduces oxidation at metal surface) Overcathodic protection risk: excessive H production → HE → cracks grow faster Optimal potential: −800 to −900 mV Ag/AgCl/seawater for steel; −750 to −850 mV for high-strength steel

Inhibitors and coatings: Corrosion inhibitors (anodic/cathodic type): reduce anodic dissolution → extend pit-to-crack transition Epoxy coating + CP: standard offshore protection system Surface treatment: shot peening → compressive residual stress → retards crack initiation and early growth

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Standards and References

Standard	Scope
ASTM E647	Fatigue crack growth testing (baseline; environmental chamber added)
ISO 11782-1	Corrosion of metals — CF testing part 1: cyclic
ISO 11782-2	CF testing part 2: pre-cracked specimens
NACE SP0176	Corrosion control of steel fixed offshore structures
ISO 19902	Petroleum — offshore steel structures (CF in design)
DNV-RP-C203	Fatigue design of offshore steel structures

Output

Provide: material and environment (alloy: steel/Al/Ti; strength level σ_UTS [MPa]; environment: seawater/acid/humid/salt fog; temperature [°C]; pH; Cl⁻ concentration [ppm]), loading (cyclic stress range Δσ [MPa]; stress ratio R = σ_min/σ_max; frequency f [Hz]; load spectrum if variable amplitude), S-N reduction (air σ_e [MPa]; CF σ_e_corr [MPa] = reduction_factor × σ_e_air; no endurance limit in corrosive media — extrapolate S-N slope to required life), crack growth (da/dN in air: C, m, ΔK_th [MPa√m]; in environment: ΔK_th,corr ≈ [0.5–0.8]×ΔK_th; da/dN_CF at representative ΔK [mm/cycle]; frequency effect factor), pitting analysis (pit depth from inspection [μm]; K_t at pit; ΔK_pit = Y×Δσ×√(π×a_pit); transition if ΔK_pit ≥ ΔK_th,corr), life prediction (N_pit [cycles]; N_crack_growth from a_pit to a_final [cycles]; N_total; inspection interval = N_total/2), CP specification (target potential [mV Ag/AgCl]; anode material: Al/Zn; design life [years]; overprotection risk assessment for high-strength steel), and applicable standard (ISO 11782 for CF testing; DNV-RP-C203 for offshore structures; NACE SP0176 for CP).