New approaches to enhancing hydrogen permeation inhibition in pipeline steel through cationic surfactant-assisted adsorption of organic inhibitors and mechanistic understanding

Hydrogen permeation inhibition using organic inhibitors is considered to be a promising approach to protecting pipeline steels from hydrogen embrittlement. However, the effectiveness of existing inhibitors remains insufficient for practical industrial applications. This study investigates the enhancement of hydrogen permeation inhibition in API X65 pipeline steel using new compounds synthesised through the combination of a cationic surfactant with environmentally friendly anionic organic inhibitors, specifically cetyltrimethylammonium (CTA⁺) with trans-4-hydroxycinnamate (4OHcinn^-) and trans-4-ethoxycinnamate (4EtOcinn^-). While the individual components, cetyltrimethylammonium bromide (CTAB) and sodium 4-hydroxycinnamate (Na-4OHcinn), exhibited low hydrogen permeation inhibition efficiency with values below 20 %, the synthesised compounds CTA-4OHcinn and CTA-4EtOcinn achieved significantly higher inhibition efficiencies of 65 % and 80 %, respectively. The inhibition efficiency enhancement mechanism was investigated using electrochemical and surface analytical techniques, including the advanced methods of local electrochemical impedance spectroscopy and atom probe tomography for probing inhibitor film coverage characteristics and nanometre scale three-dimensional element distribution. The addition of these inhibitors during hydrogen charging was found to result in the formation of compact surface inhibitor films, with the significantly improved hydrogen permeation inhibition efficiency attributed to micelle-assisted adsorption of anionic organic inhibitors. These results show the prospective of developing high efficiency hydrogen inhibition inhibitors by combining cationic surfactants with environmentally friendly anionic inhibitors to mitigate hydrogen ingress in steels.