Test Methodology for the Investigation of Environmental Effects on Corrosion Fatigue

The objective of this work was to develop a test methodology for corrosion fatigue that allows researchers to investigate the effects of environmental parameters on fatigue crack growth rates of cracks nucleating from corrosion pits. Environmental factors are important as small changes in crack growth rates while the crack is small can have large effects on the overall life of structural components due to the exponential nature of fatigue crack growth. The effect of changing environments on the crack growth rate is important to the corrosion fatigue community as chromate containing primers and protective coatings are thought to help protect components during this mechanically small crack growth regime by chemically altering the local environment around the crack tip. The chromate protection offered in the early stages of crack growth can become very significant to structural part life as the effect of chromate is potentially lessened as the crack becomes larger. This is due to the environment having less of an influence on crack growth as the mechanical crack driving force overcomes the environmental effect as crack length increases. The current push for reduction in chromate containing coatings due to health and environmental concerns could have an unintended consequence that component lives will suffer due to the removal of the chromate corrosion protection. The test methodology developed herein was designed to address the potential reduction in life by providing a test platform where the effects of varying environmental factors on the crack growth of small cracks could be fully investigated.