Fatigue tests on compact tension specimens of 90-10 copper nickel were conducted in 3.5% NaCl solutions. Anodic or cathodic currents were applied during testing. Anodic currents decreased and cathodic currents increased the fatigue life. Both anodic and cathodic currents changed the fracture mode from predominantly transgranular to intergranular. Constant extension rate tests were performed on similar CT specimens in environments of 3.5% NaCl solution and 3.5% NaCl solution titrated to pH 1.0 with various levels of applied current. The environment had little influence on the monotonic failure of 90-10 copper nickel. Polarization studies were conducted to determine the effects of welding and pH on the corrosion behavior of 90-10 copper nickel. The rate of corrosion was less in the weld and the heat affected zone than in the base metal. As the pH of the environment was lowered, the corrosion rate of 90-10 copper nickel increased due to the retardation of film formation and repassivation. These studies showed that three different mechanisms of corrosion fatigue were likely: localized anodic dissolution, surface film rupture and hydrogen embrittlement. The dominance of one mechanism over the other two depends on the applied current. No evidence of susceptibility to stress corrosion cracking was found, therefore, a true corrosion fatigue process is operative in 90-10 copper nickel. / Master of Science / incomplete_metadata

Mode I and Mode III fatigue tests were performed on copper nickel alloys in helium, salt water environments. The hydrogen, oxygen, two alloys used air and in this investigation were 90-10 and 70-30 copper nickel. Both alloys contained iron which was added to improve the erosion corrosion resistance. The extent of cracking varied with the test environment. Tests showed that oxygen and humid air promoted cracking while salt water helium was used as the baseline retarded cracking when environment. Hydrogen promoted cracking when compared to helium but retarded cracking if comparisons were made with oxygen or humid air. The environmental effects (helium as the base case} in the Mode I tests in gaseous environments were manifested in the form of shorter fatigue lives, easier crack initiation, marginally higher crack growth rates and the development of intergranular fracture at the surface. These effects were accompanied by a change in the near surface deformation characteristics. The increases in fatigue life induced by testing in aqueous environments were greatly extended if the copper nickel was galvanically coupled to steel. Mode III tests showed the same ranking of environmental effects as Mode I tests and also showed multiple initiation, brittle fracture and secondary cracking. Two models were proposed to explain the observed results. One was based on the dilation-aided diffusion of oxygen ahead of the crack tip and subsequent oxidation of internal iron particles. The oxidation caused a volume expansion which produced internal tensile strains and facilitated fracture. The other mechanism was based on dilation-aided transport of hydrogen with subsequent accumulation of hydrogen at interfaces, resulting in a lowering of the interfacial strength and promoting intergranular fracture. The observed increases in life in the aqueous environments were rationalized by the reduced oxygen content available in the stagnant solutions. These observations suggest that the presence of iron accelarates fatigue in copper nickel alloys exposed to aggressive environments. Thus, any application involving fatigue loading with simultaneous exposure to aggressive environments should attempt to ensure that the iron content of the copper nickel alloys is minimized. / Doctor of Philosophy