Abstract
The combined effects of corrosion and fatigue are known to be hazardous threats to structural integrity of aluminium alloys that are being extensively used in marine applications. This work investigated the fatigue crack initiation and growth behaviour of AA6061-T6 alloys in 3.5 wt% NaCl simulated seawater using scanning electron microscope and electron backscatter diffraction characterisation techniques. It was found that the fatigue resistance of AA6061-T6 is drastically downgraded when subjected to the corrosive environment of 3.5 wt% NaCl solution. High stress concentration at both sides of a pit mouth in conjunction with attacked grain boundaries facilitates fatigue crack nucleation, while the presence of hydrogen formed by corrosion reactions causes crack tip embrittlement and thus increases crack growth rate. Fractographic analysis reveals that there is a change in fatigue crack growth mechanism of AA6061-T6 alloys tested in the NaCl solution. At short crack length, the crack develops transgranularly along crystallographic planes due to hydrogen-enhanced decohesion process. Further crack growth is dominated by adsorption induced dislocation emission process, resulting in the mixed mode of intergranular and transgranular crack growth.
Highlights
Corrosion fatigue is the response of a material to alternating stresses in a corrosive environment by the initiation and propagation of cracks
This study investigated the effect of pitting corrosion on fatigue behaviour of AA6061-T6 alloys with the aim of having a better understanding of fatigue crack initiation and propagation mechanisms of aluminium alloys in a corrosive environment
The fatigue lives of AA6061-T6 alloy specimens tested in the air atmosphere and in the 3.5 wt% NaCl solution were measured at different stress levels
Summary
Corrosion fatigue is the response of a material to alternating stresses in a corrosive environment by the initiation and propagation of cracks. It has been known as a dominant factor on the reduction of service life of marine structures and other equipment used in the offshore environment. The pitting process causes local discontinuities on the surface of a structure and enhances the nucleation as well as propagation processes of small fatigue cracks. Due to the presence of these local discontinuities, short cracks can initiate and develop even below the threshold range of stress intensity factor determined for long crack growth [5]. There is broad consensus that degradation of crack growth resistance is caused by crack tip embrittlement due to the presence of hydrogen species
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