Nondestructive and structural evaluation of steel coated with CFRTP composites for metallic pipeline systems

Carbon fiber-reinforced thermoplastic (CFRTP) composites represent a promising advanced coating solution for steel pipelines, offering superior chemical resistance, mechanical reinforcement, and durability compared to conventional polymer coatings. This study compares the performance of single-layer and multi-layer coatings made of carbon fiber-reinforced polyphenylene sulfide (CF/PPS) composites bonded to low-carbon steel substrates. Four composite configurations were investigated: 1-layer unidirectional (UD), 2-layer biaxial, 7-layer UD, and 7-layer biaxial samples. Both UD and biaxial ([0/90]) layups were bonded to low-carbon steel substrates using epoxy adhesive. The ultrasonic nondestructive evaluation was employed to characterize coating quality and detect subsurface defects. Then the mechanical integrity was assessed via three-point bending tests and the microstructural features were examined using optical microscopy. The experimental results demonstrated that while 1- layer coatings provided negligible structural reinforcement (flexural strength: 5.1-5.3 MPa), they induced detectable alterations in ultrasonic response characteristics, confirming the sensitivity of hybrid ultrasonic techniques. The 7-layer coatings significantly enhanced mechanical behavior, with the UD layup achieving a flexural strength of 908 MPa (approaching the 900 MPa design target) compared to 638 MPa for the 7-layer biaxial configuration. The hybrid ultrasonic indices (Sideband Peak Count Index - SPC-I, and Spectral Dissipation Index - SDI) increased significantly with ply count and were significantly higher for the 7-layer biaxial configuration (SPCI = 2.91 ± 0.51, SDI = 2.78 ± 0.33) compared to the 7-layer UD (SPC-I = 1.70 ± 0.21, SDI = 1.94 ± 0.22), reflecting greater microstructural heterogeneity and interfacial defects. The findings demonstrate the superior sensitivity of hybrid ultrasonic indices for detecting microstructural variations (interfaces, porosity) that influence the mechanical integrity of multi-layer composite coatings. This highlights their potential for nondestructive evaluation and quality control of composite pipeline coating systems. Prototype steel pipe sections coated with CFRTP composites demonstrated the feasibility of the coating system for real-world pipeline applications.