Influence of cyclic loads on epoxy coated carbon steel performance

Marine pipelines at the sea bottom are protected by a coating to prevent corrosion. Lifetime of coating are limited by coatings delamination. Delamination is caused by ingression of seawater through the coating and later attack of the metal. A zirconium conversion pre-treatment applied on the metal is used to improve the adhesion between the coating and metal substrate. During in-service lifetime, pipelines encounter cyclic loadings due to vortex induced vibrations or movements of fluid within the pipe which might affect the mechanical properties of the coating. The goal of this work is to investigate the effect of these cyclic loadings combined with immersion period on the epoxy coatings on the steel with and without a zirconium conversion layer. Samples are stressed in the elastic, and plastic region (of the steel) in uniaxial tensile direction, and also they strained up to 5% to investigate the effect of mechanical damage. After these tests, samples are immersed in NaCl solution for 30 days to evaluate the barrier properties of the coatings and compared their performance with the unstressed immersed samples. Electrochemical impedance spectroscopy (EIS) is used to monitor the protective properties of the coatings. Finally, adhesion tests are performed to see whether or not the adhesion strength of epoxy decreases by the combination of mechanical loading and immersion. Immersion of 30 days does not affect the coating barrier properties and Epoxy coated systems with and without Zr-pretreatment showed high barrier properties after 30 days of immersion in corrosive environment. Coated samples with and without Zr-pretreatment were not affected by cyclic loading in the elastic region. No cracks in the coatings were detected. Only a slightly increased of the roughness was noticed. EIS results showed that barrier properties of the coatings after elastic cyclic-loading are not affected and remained constant after the 30 days of immersion. Different behaviour after cyclic loading in the plastic region between treated and un-treated samples is observed. Samples without pre-treatment showed a decreasing of the barrier properties during the immersion test while treated-samples showed still high performance. An explanation is that cyclic loading in the plastic region affect significantly the polymer structure, creating a more open network. Then the penetration of solution is facilitated, affecting the protection properties of the coating. Additionally the roughness of the coatings increased significantly for (un)-treated samples. This is caused by a rougher steel substrate and the formation of shear bands in the epoxy coating. However the pre-treated samples showed no changes in the barrier properties. This may be associated to the better adhesion between the polymer and coating by the presence of zirconium layer. The adhesion strength is significantly affected by elastic/ plastic cyclic loading and 30 days of immersion for untreated and treated samples. More investigation is needed to discover if cyclic loading affects the adhesion strength or the combination of cyclic loading and immersion period. However it is believed that cyclic loading has an influence on the adhesion strength; the decrease of adhesion strength by only immersion is limited while the adhesion strength after cyclic loading and immersion has a much higher detrimental influence.

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