Corrosion Crack Growth Models

Ford and Emig [20] proposed the use of the strain rate at the crack tip, ket, as fundamental parameter to describe the corrosion crack growth (CCG) process. By posing ket — Ejitf, with Ef the deformation at failure of the passive film at the crack tip, it follows that the characteristic time is = s/set- Considering that the oxidation density charge Q can be obtained by integrating Eqs. (13.25 and 13.27) becomes 

Average current density (A/cm Fig. 13.19 CCGR vs. average oxidation current density [21] 

Note that for incubation times much long and, above all, for high strain rate Eq. (13.28) can be approximated by 

When this happens, a contribution of hydrogen embrittlement penetration by cleavage, da, in addition to the dissolution penetration process must be taken into consideration. The crack tip average penetration rate, then, becomes 

The extent of this additional environmental-assisted cleavage a is of the order of 0.1-1 pm, depending on the metal-environment combination. This further 

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Krafft [4] and Krafft and Mulherin [5] later extended the TLI model to describe stress corrosion crack growth. Crack growth was viewed in terms of the instability of tensile ligaments where their lateral contraction was augmented by uniform chemical dissolution of the tensile ligaments. For sustained-load crack growth in an inert environment, on the other hand, the reduction in the cross-sectional area of the ligaments would be associated with the creep rate (Landes and Wei [2], Yin et aL... 

D. D. Macdonald, M. Urquidi-Macdonald, P.-C. Lu, The Coupled Environment fracture Model - A Deterministic Method for Calculating Crack Growth Rates, presented at CORROSION 94, Baltimore, MD, February 28-March 4, 1994, Paper 246. 

Figure 9.13. Comparison of normalized (corrosion) fatigue crack growth rates for solution-treated (ST) and solution-treated plus averages (STOA) Ti-6A1-4V alloy in water vapor with model predictions for pressure dependence at room temperatnre (i = 0.1, / = 5Hz)[6],...

Harlow, D. G., and Wei, R. P., A Mechanistically Based Approach to Probability Modeling for Corrosion Fatigue Crack Growth, Engr. Frac. Mech., 45, 1 (1993), 79-88. 

Thomas, J. P., and Wei, R. P., Corrosion Fatigue Crack Growth of Steels in Aqueous Solutions - I. Experimental Results Modeling the Effects of Frequency and Temperature, Matls. Sci. Engr., A159 (1992), 205-221. 

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