Electrochemical Conditions at the Crack Tip and Crack Propagation Rates during Corrosion Fatigue of Steel in Deaerated Sea Water

Solute Transport in Corrosion Fatigue Cracks

“…Equation [1], which was derived considering the increased surface area of concentration profiles resulting from shear in the solution, was the basis of the calculation of Deff in our previous work (2).…”

“…The "flow enhanced diffusion" effect (2) may have far reaching consequences for the interpretation of experimentally observed electrochemical conditions in corrosion fatigue (C.F.) cracks, as illustrated by the following example from earlier work (1). Acidification (pH = 5.5) of the crack tip solution has been measured at an externally applied anodic potential of -550 mV vs. SCE in C.F.…”

“…Acidification (pH = 5.5) of the crack tip solution has been measured at an externally applied anodic potential of -550 mV vs. SCE in C.F. cracks in compact tension specimens of the pipeline steel Union 47 in a deaerated artificial seawater environment (1). According to the conventional interpretation this acidity is essentially the result of the cation hydrolysis reaction associated with metal dissolution at the crack tip.…”

“…When high IR drops resulting from strong currents on the crack walls result in a situation as described above, where the crack tip is shielded electrically, the crack tip area may be considered an "occluded cell" from the electrical point of view. It can be shown, however, that a certain degree of anodic polarization above the local corrosion potential of the steel is necessary in order to produce an acidified solution on a certain part of the steel surface (1), In particular, anodic polarization resulting in a ratio of anodic to cathodic current densities of about 40 would be required if the acidity of pH = 5.5, as measured at the crack tip in the above case of external polarization at -550 mV, is to be accounted for on the basis of hydrolysis of locally dissolving iron cations (1). If the local anodic polarization is not sufficient to maintain the experimentally observed pH, a remaining possibility to account for the observed crack tip acidity is that acidified solution is produced at the crack walls outside of the electrically occluded cell area near the crack tip, on those parts of the walls that are sufficiently polarized.…”

“…cracks. The crack tip acidification during strong anodic polarization can thus essentially be attributed to the transport of acidi-fled solution originating from strongly polarized areas of the crack walls away from the tip and nearer to the crack mouth, to the crack tip area which was concluded to be not significantly polarized (1,2),…”