The relative severity of natural and synthetic seawaters on fatigue crack growth in cathodically protected steel

Monahan, Craig C. (1988) The relative severity of natural and synthetic seawaters on fatigue crack growth in cathodically protected steel. Masters thesis, Memorial University of Newfoundland.

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Abstract

Two solutions, ASTM substitute ocean water and natural seawater, are used more or less interchangeably to evaluate the fatigue crack growth properties of steels. There is evidence in the literature to suggest that the mechanism responsible for accelerated fatigue crack growth in cathodically protected steel (i.e. water reduction) can be significantly influenced by bulk solution chemistry. Experiments were designed to investigate the relative severity of these two solutions when used with cathodically protected steel samples. The investigation was conducted using three types of specimens: welded T-plates, compact type (CT) specimens and artificial crevices. Welded T-plates provided data on the initiation and propagation behavior of semi-elliptical cracks. CT specimens were tested to obtain crack growth rate versus crack tip stress intensity range data, since it is difficult to derive similar curves from welded T-plate data. The artificial crevice experiments were combined with polarographic and scanning electron microprobe (SEM) analyses to study the chemistry of calcareous deposits and their influence on water reduction kinetics. -- The extent to which calcareous deposits affected the rate for water reduction within the artificial crevices depended on the bulk solution chemistry and electrochemical potential under which they formed. Magnesium rich deposits were less effective in reducing the rate for water reduction than were calcium rich deposits. Regions of localized corrosion were evident on several of the specimens. A theory is presented which explains how localized corrosion is possible within crevices and cracks cathodically protected to potentials more negative than -780 mV SCE, the generally accepted potential for complete protection against corrosion. -- For the intermediate levels of crack tip stress intensity range examined using CT specimens, the two solutions produced similar crack growth rate data. Crack propagation rates were generally between 2 and 2.5 times faster under cathodic protection in seawater than in air. Significantly different fatigue behaviors were observed for the welded T-plates. Fatigue cracks initiated early within the specimens tested in natural seawater, but propagated at rates close to those observed in air. Synthetic seawater prolonged the initiation phase but gave rise to crack propagation rates that were about 2 times higher than those observed in air. These results have been explained in terms of the properties and precipitation kinetics of calcareous deposits. -- Key Words: calcareous deposits; cathodic protection; corrosion; fatigue; hydrogen embrittlement; seawater chemistry; steel; water reduction

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