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In films forming in 2.4MH2SO4 normalat 5 normalmA/cm2 , many breakdown events including pit initiation were found to occur continually but to be followed by almost immediate repair, so that the stability of film growth was due not to the absence of breakdown but to the efficacy of repair. A relation between the sites of breakdown and substrate structure was not indicated. Film growth was interpreted as occurring through a compact film at pore bases, as in the classical mechanism, and also through breakdown—extension of a pore near to the metal interface—and repair by reanodization to form a hemisphere of compact film extending into the substrate. An interpretation of the geometrical structure of the film is proposed.

Single-pitted specimens of an HSLA steel, were tested in laboratory air and in 1 M NaCl solution to study the influence of a corrosive environment on its fatigue life.The growth of fatigue cracks and the partitioning of the fatigue life into fatigue crack initiation and fatigue crack propagation were studied by photographing the pit and the cracks developing on it periodically during testing. Non-propagating or dormant surface cracks were not observed in this study. Fractography using SEM showed the locations of fatigue crack initiation. The mechanisms of corrosion fatigue were studied by performing tests in 1 M NaCl at different test frequencies. Corrosion pits proved to be crack initiation sites. Hydrogen embrittlement was found to be unimportant in the corrosion fatigue of HSLA steel in this study. The 1 M NaCl corrosive environment appeared to reduce the fatigue life of this material by a dissolution mechanism. The effect of pit depth was studied by testing specimens having various pit depths. An effect of pit size was apparent. Fatigue life decreased with increasing pit depth. Pit depth, rather than the ratio of pit depth to pit diameter, influenced fatigue behaviour. A non-damaging pit depth was found. NOMENCLATURE a = material constant C = pit shape-related constant D = pit diameter E = elastic modulus E' =potential er = fracture strain h = pit depth i = current density K = strength coefficient Kt = elastic stress concentration factor Kr = experimental fatigue notch factor Kr = calculated fatigue notch factor L, = left-side crack length L2 = right-side crack length NT = total fatigue life Ni = surface crack initiation life Ni,T = fraction of fatigue life devoted to surface crack initiation life N p = surface crack propagation life n = strain hardening exponent R = stress range r = pit radius R.A. = reduction of area S, = ultimate tensile stress S, = 0.2% proof stress S,, = upper yield stress AK = stress intensity factor AS = applied stress range a, = fracture stress "Deceased.625