Intrusion kinetics and interfacial degradation mechanism of PU-steel system in chloride ion environment: a multiscale study

Abstract

This study utilised density functional theory (DFT) to predict the movement pathways and rates of chloride ions within polyurethane (PU), as well as their molecular dynamics and charge distribution during the erosion process. Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and electrochemical tests revealed chemical and electrochemical changes at the PU/steel interface after immersion in sodium chloride solution. Shear strength and nanoindentation tests were used to examine the impact of microscale erosion on macroscale mechanical properties. The results showed that chloride-ion penetration into PU requires overcoming a substantial energy barrier driven by the concentration gradient between the saline solution and PU. After immersion in sodium chloride solution, the −N=C=O functional groups in PU were disrupted, reducing chemical stability and molecular elasticity. This process accelerates the hydrolysis of ester bonds, leading to a decline in interfacial mechanical properties. Electrochemical results showed that the resistance of the PU coating decreased over time, indicating a transition in the electrochemical reactions that eventually led to interfacial corrosion. After prolonged immersion, both the interfacial shear strength and modulus significantly decreased, whereas the shear ductility initially increased but later decreased.