A Next-Generation Thin-Film Coating Meeting API 20E Requirements

Abstract For decades, fluoropolymer/ceramic coatings have been widely used as corrosion-resistant surface treatments for fasteners. As a next-generation alternative, a carbon nanotube (CNT) composite coating has been developed, offering superior mechanical properties to minimize damage from sand abrasion and fastening. In high-pressure environments specified by standards such as API 20E, oversizing of internal threads is not permitted, making it challenging to apply conventional thick coatings with high corrosion resistance. This study focuses on the development of an ultra-thin CNT composite film that achieves high corrosion resistance without requiring oversizing. Experimental evaluation determined the maximum coating thickness that enables proper assembly without oversizing. Dimensional mapping of M12 and M16 bolts and nuts indicated that coatings of 10–20 μm allow reliable fitting. Salt spray testing showed that 20 μm CNT composite coatings achieved approximately three times the corrosion resistance of 10 μm coatings, though still lower than 40 μm coatings applied with oversizing. These findings demonstrate the potential of CNT composite coatings as a thin-film solution for fastener corrosion protection in demanding environments. Future work will expand dimensional datasets for bolts and nuts, optimize coating formulations, establish precise thickness control for mass production, and explore environmentally friendly alternatives aligned with regulatory trends.