Homologous alkyl side-chain diphosphonate inhibitors for the corrosion protection of carbon steels

The interactions of six homologous diphosphonate additives, methylamine-N,N-bis(methylenephosphonate) (MBMP, C1-D), ethylamine-N,N-bis(methylenephosphonate) (EBMP, C2-D), butylamine-N,N-bis(methylenephosphonate) (BBMP, C4-D), hexylamine-N,N-bis(methylenephosphonate) (HBMP, C6-D), octylamine-N,N-bis(methylenephosphonate) (OBMP, C8-D), dodecylamine-N,N-bis(methylenephosphonate) (DBMP, C12-D), with carbon steel surfaces are studied by XPS at pH = 3.0. Structurally, all additives possess two methylenephosphonate moieties connected to a single N atom. The third substituent on N is a non-polar, variable-length alkyl chain, –(CH2)xCH3, where x = 0 (C1-D), 1 (C2-D), 3 (C4-D), 5 (C6-D), 7 (C8-D), and 11 (C12-D). XPS studies (on pristine solid diphosphonate samples and also on carbon steel specimens, immersed in aqueous solutions), indicate significant deprotonation of the diphosphonic acid molecules upon interaction with the metallic surface. They also prove that this surface interaction results in adsorption on the surface via the deprotonated phosphonic acid moieties. The adsorption of inhibitors on the metal surface was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy. The changes detected in the charge transfer resistance (Rct) and constant phase element (CPE) independently confirm inhibitor adsorption on metal surface. Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) spectroscopy and Scanning Electron Microscopy was used to investigate the nature of deposited film. Small alkyl chain diphosphonates (C1-D, C2-D and C4-D) exhibited lower corrosion resistance due to the thin, porous and/or incomplete layer formed on carbon steel surface. Longer alkyl chain molecules (C6-D, C8-D and C12-D) were found to adsorb more efficiently and form a more organized and thicker layer. The best results were obtained in the case of C8-D (lower corrosion current, higher Rct and surface coverage). In the presence of C8-D the corrosion rate was reduced by a factor of 6.