An analysis of stress corrosion crack growth by anodic dissolution
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THE widely reported and
discussed catastrophic failure of an L. P. turbine disc at Hinkley Point Power Station ~and subsequent examination of other similar plant 2 have highlighted the susceptibility of low alloy ferritic steels to stress corrosion cracking in condensing steam environments. In attempting to predict the growth kinetics and allow reliable extrapolation of experimental cracking data to service conditions, much effort has been directed towards establishing the basic mechansims of cracking. Perhaps the most widely accepted mechanism for the growth of stress corrosion cracks in aqueous environments is that of 'slip-dissolution'. 3-7 The mechanism has been variously developed from the original proposals of Logan 8 and includes an enhanced anodic dissolution at the tip of a growing crack which is effected by the mechanical rupture of the protective oxide film. Such enhanced anodic dissolution at a 'bare surface' results in a rapid crack advance by a metal loss process. These models for stress corrosion crack growth are based on calculation of a maximum growth rate which is faradaically related to the maximum anodic current density at the crack tip. However, these predicted growth rates are often several orders of magnitude greater than those measured experimentally or experienced in service components. 7,1~Consequently, this has led to the introduction of a variety of rate controlling parameters, other than maximum dissolution current density, in order to reduce the maximum theoretical crack growth rate to more realistic values. These parameters include diffusional processes, repassivation rate, micro-creep, solution renewal and critical charge concepts. 7,~l-a6 In general, the various electrochemical parameters included in the various growth models have been measured under laboratory conditions using bulk specimens. The potential dependence on these parameters has then been investigated in an attempt to evaluate their contribution to stress corrosion crack growth rates measured at the same potentials. Implicit in such a P. DOIG is Research Officer and P. E. J. FLEWITT is Section Head, Central Electricity Generating Board, South Eastern Region Scientific ServicesDept., Gravesend, Kent, U.K. Manuscript submitted April 28, 1980. METALLURGICALTRANSACTIONSA
correlation is the assumption that the electrode potential which is determined for the bulk stress corrosion specimen is the potential prevailing at the tip of a growing crack. Recently, however, it has been shown ~7,18 that such an assumption is not generally valid and therefore any direct correlation of growth rate with other electrochemical parameters may be ill-founded. Stress corrosion crack growth rates in service environments are often sufficiently low that the experimental determination of relative susceptibility of a range of materials is obtained using accelerated test methods. Experimental stress corrosion growth rates are often measured from tests in concentrated solutions which are chosen to accelerate the assumed controlling electrode kineti
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