A comparison of fatigue properties of epoxy coatings in marine environments

Abstract

Three experimental marine epoxy coatings on steel substrates were tested under four-point bend cyclic-loading in both simulated sea water, at two temperatures, and in air. Freshly prepared and artificially aged coatings were examined. The samples were characterised by sectioning, scanning electron microscopy and EDX elemental analysis. The ageing processes used had multiple stages involving temperature cycles and/or repeated water soaking and drying. The coatings were loaded at strain amplitudes of 0.15% and 0.22% for 500,000 cycles at frequencies simulating wave action. The coatings showed high fatigue resistance, so slot defects of different depths and widths were introduced to simulate damage in service and to act as crack initiators. This permitted observation of crack propagation under cyclic loading. Additionally, plastic substrates were tried so that tests at higher coating strains of 1.33% could be performed. The results showed that the softest epoxy coating was the most fatigue resistant. An epoxy containing large filler particles (50gm diameter) was the next most resistant while a hard epoxy with small filler particles showed multiple fatigue cracking after 300,000 cycles when tested on steel substrates. No fatigue cracks were observed on aged coatings. Internal stress measurements were performed that showed the coating which was most resistant to fatigue had the lowest internal stress of around 2 MPa. The other coatings internal stress levels were over 15 MPa. Tensile tests on free-films indicated that the coating most resistant to fatigue showed the lowest tensile strength, approximately 10 MPa, and the highest strain to failure, the latter property being the more important one. Hardness measurements using micro and macro Vickers indentors were performed between stages of an ageing process, along with nano-indentation (to measure Young's modulus) showing that the softest coating exhibited low elasticity. Higher hardness values were observed for coatings after ageing.