Simulation of water absorption and desorption behavior for anti-corrosion coatings in existing and new accelerated corrosion tests

Abstract

A cyclic corrosion test (CCT) that ensures compatibility with acceleration of corrosion and approximation of actual outdoor corrosion is in high demand, and many kinds of CCTs have been developed. We have proposed a new CCT for evaluating corrosion resistance of anti-corrosion coatings. In this work, we measured the water diffusion coefficients of three kinds of anti-corrosion urethane/epoxy coatings at 20–60 °C and calculated their activation energies. In addition, the water absorption/desorption behaviors of the coatings in several CCTs, including our new CCT, were simulated.It was found that the desorption diffusion coefficients of the coatings are much higher than their absorption diffusion coefficients. It was considered that water molecules might need to push out the mesh of the molecular network to penetrate the coatings during water absorption, but that they might not need to do so during water desorption. Therefore, the dry step in the new CCT might be sufficient even though the time is shorter than in existing CCTs. In addition, the temperature in the dry step in the new CCT is also higher than in the salt fog step and humidity step. Therefore, drying is much faster than wetting. In the new CCT, by minimizing the time in the dry step, the corrosion rate of steel and zinc is accelerated more than in existing CCTs without sacrificing the approximation of the water absorption and desorption behaviors of the coatings.It was also found that the desorption activation energies, 45 ∼ 55 kJ/mol, are lower than the absorption activation energies, 74 ∼ 96 kJ/mol. It was considered that water molecules widen the mesh of the polymer network more easily at higher temperatures in water absorption; however, in water desorption, water molecules need not expand the mesh. In addition, the results of the simulation of water absorption and desorption behaviors show that water absorption in three kinds of CCTs is insufficient or excessive and that two kinds of CCTs, including our new one, can reproduce the same level of water absorption and desorption as in the summer outdoor environment.