The coupled environment models for localized corrosions; Crevice corrosion and stress corrosion cracking.

摘要

This dissertation details my investigation of crevice corrosion and stress corrosion cracking based upon the differential aeration hypothesis, currently considered the physical basis of virtually all localized corrosion phenomena. The differential aeration hypothesis attributes localized corrosion to a spatial separation of local anodes and local cathodes. The former occurs in the region of a given system (e.g. within a crevice or crack) that has the least access to a cathodic depolarizer (e.g. O2). The latter occurs in the region (e.g. on an external surface) with the greatest access to a cathodic depolarizer. This hypothesis, combined with the natural law of charge conservation, yields the measurements of the electron coupling current that flows from local anodes to local cathodes, which contains valuable information concerning the processes that occur within local anodes.;A simple crevice corrosion monitor was developed to monitor crevice corrosion in 1018 mild steel, Type 304 stainless steel, and Type 410 stainless steel in deionized water and in NaCl solutions with and without the addition of a chemical corrosion inhibitor. The monitor, which measures the electron coupling current that flows from the crevice to the external surface, followed the evolution of crevice activity in a manner that can be understood in terms of the cathodic process that occurs on the external surface and the partial anodic process that develops within the crevice, due to the accumulation of H+ and Cl-. The crevice initiation time is typically very short, but varies depending upon chloride concentration and, possibly, inhibitor concentration. After initiation, the coupling current increases over time, passing through a maximum, then decreasing and eventually changing sign, from positive to negative, which indicates crevice inversion. This inversion is attributable to the gradual build-up of H+ within the crevice to the extent that proton reduction within the crevice becomes the principal cathodic reaction in the system, while the anodic reaction moves to the external surface. In addition, amines are effective corrosion inhibitors of crevice corrosion of mild steel and stainless steels in NaCl solutions by forming a protective inhibitor film, thereby inhibiting the cathodic reaction occurring on the external surface, provided that they are present in sufficiently high concentrations.;The shape evolution of sensitized Type 304 stainless steel surface cracks in boiling water reactor primary coolant circuit piping at 288 °C was explored as a function of environmental variables---such as electrochemical potential, solution conductivity, flow velocity, and multiplier of standard exchange current density for O2 reduction---using the coupled environment fracture model (CEFM). For more accurate prediction of crack growth rate, Shoji's approach for calculating crack tip strain rate, together with a precise treatment of the stress intensity factor for semi-elliptical surface cracks, has been integrated into the CEFM. This revised CEFM accurately predicted the dependence of crack growth rates on the stress intensity factor and offers an alternative explanation for the development of semi-elliptical cracks to those suggested by fracture mechanics alone. Moreover, the CEFM predicted that the minor axis of the ellipse should be oriented perpendicular to the surface, in agreement with observation. The development of the observed semi-elliptical cracks with the minor axis perpendicular to the surface is therefore attributed to the dependence of the crack growth rate on the electrochemical crack length. (Abstract shortened by UMI.).