The Evaluation of Anticorrosive Behavior of Rigid Solvent Free Polyurethane Coatings

Abstract

Solvent-free rigid Polyurethane (PU) coatings are widely applied for pipeline corrosion protection due to their high-build capability, the elimination of Volatile Organic Compounds (VOCs), and suitability for structurally demanding and hygienic water-contact applications. Despite their extensive industrial use, the electrochemical degradation behavior of thick, highly resistive solvent-free PU systems-particularly under accelerated electrochemical stress-remains insufficiently clarified. Anticorrosive performance of a rigid solvent-free PU coating formulated for pipeline steel was investigated using a combined evaluation framework involving long-term Electrochemical Impedance Spectroscopy (EIS), Alternating Current-Direct Current-Alternating Current (AC-DC-AC) accelerated electrochemical cycling, and conventional durability tests, including Cathodic Disbondment (CD) and Neutral Salt Spray (NSS) exposure. Immersion EIS measurements in 5 wt% NaCl exhibited a single dominant time constant throughout the exposure period, with low-frequency impedance values exceeding 10⁸ Ω·cm² and coating capacitance on the order of 10^-9 F·cm^-2. This behavior indicates a highly resistive dielectric barrier governed by restricted ionic transport within a predominantly hydrophobic polyurethane network. During AC-DC-AC cycling, the impedance response evolved from an intact-barrier regime toward diffusion-controlled behavior, as evidenced by progressive reductions in coating resistance, changes in constant phase element parameters, and the emergence of Warburg-type diffusion features. At extended cycling times, an apparent, diffusion-limited stabilization of impedance was observed, suggesting transport limitation within confined interfacial regions rather than true barrier recovery. Notably, similar stabilization trends were detected in EIS measurements conducted after 2,500 h of NSS exposure, demonstrating a strong mechanistic correlation between accelerated electrochemical cycling and long-term environmental degradation. Cathodic disbondment testing confirmed excellent compatibility with cathodic protection systems, with disbondment values remaining well within industry acceptance limits. Overall, this work provides a mechanistically grounded electrochemical assessment of rigid solvent-free PU coatings and establishes AC-DC-AC testing as a resource-efficient and predictive approach for evaluating long-term pipeline coating performance.