Abstract
Development of service cracks in epoxy based corrosion protective coatings limits the life of the substrate structure. If cracks develop, corrosion protection is lost and costs of repair and re-protection of large marine structures can be crippling. Factors controlling development of cracks in the coating are poorly understood, and predictions of coating lifetime approximate. For bulk tanker applications service strains imposed on coatings arise from both low frequency thermal mismatch strains and mechanical strains from wave action. In this work the number of cycles to crack initiation has been measured during strain controlled fatigue of two selected 300 μm thick coatings applied to a 5.5 mm thick steel substrate. Cycling was performed at frequencies of 1 Hz, and strain amplitudes between ±0.16% and ±0.5%. Coating crack development was monitored optically. It was found that cycles to crack initiation depended on both strain amplitude and on the static ductility of the coating. After initiation crack growth rates in the coating reduced with increasing surface crack length. The significance of the results is considered in the light of requirements for quantitative models for service life prediction of coatings on metallic substrates.
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