Abstract

Fretting in seawater between normally loaded orthogonally crossed cylinders of the high strength low alloy steel Cor-Ten A has been investigated at selected frequencies and amplitudes under controlled electrochemical conditions. Electrochemical and surface profiling techniques and electron microscopy have given a quantitative assessment of the degree and nature of the surface damage suffered by the steel fretted in aerated and nitrogen saturated seawater, and with the steel cathodically protected. When fretted in aerated seawater, Cor-Ten A exhibits a significant increase in corrosion rate compared with that measured under static contact conditions. There is a rise in the rate of corrosion during the early stages of fretting usually followed by a lower corrosion rate as fretting proceeds. The increase in rate has been associated with the breakdown of surface oxide films to expose underlying active metal, and a reduction in rate to a slowing in the growth rate of the damaged zone as fretting continues. The relative electrochemical and mechanical contributions to the amount of material lost have been estimated. In aerated seawater electro chemical dissolution processes were found to be responsible for the major proportion of the surface material removed; under cathodically protected conditions there remained a substantial degree of mechanical damage.

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