STRUCTURES OF TESTED GEMINI SURFACTANTS Cationic gemini surfactants as corrosion inhibitors of stainless steel (AISI 304) in 3M hydrochloric acid O. Kaczerewska1,2, B. Brycki1, R. Leiva-Garcia2, R. Akid2 1Laboratory of Microbiocides Chemistry, Faculty of Chemistry, Adam Mickiewicz University in Poznań, Poznań, Poland 2School of Materials, University of Manchester, Manchester, M139PL, UK INTRODUCTION Corrosion is the degradation of materials, usually metals and alloys, which includes the chemical, biochemical and electrochemical interaction between metals and alloys and the environment. Corrosion is a major cost in both capital and production expenditure within the petrochemical, chemical, mining and food industries [1]. Stainless steel is used as a material for manufacturing equipment, which is exposed to corrosive environment. In particular, acid solutions, e.g., hydrochloric acid, which are used for industrial cleaning, pickling and acidic in petrochemical processes environments, increase the risk of corrosion to stainless steel. There are several strategies used to prevent corrosion, namely: electrochemical protection (anodic, cathodic), protective layers (metallic, non-metallic, organic) and the use of corrosion inhibitors (organic, inorganic) [2]. The use of corrosion inhibitors (CIs) is one of the simplest and most effective ways of preventing corrosion. Among the organic corrosion inhibitors, cationic surfactants show high inhibition efficacy because of their ability to adsorb onto metal surfaces. The adsorption process depends on the structure of the compounds, for example, the presence of heteroatoms (nitrogen, sulphur, oxygen, phosphorus), multiple bonds and aromatic rings. Unsaturated and aromatic rings favour adsorption due to the interaction of π-electrons with the metal surface. Gemini surfactants - dimeric quaternary ammonium salts - due to their low critical micelle concentration (CMC), high surface activity and good wetting properties are very effective corrosion inhibitors at CMC’s values [3]. In this study, three cationic gemini surfactants with a flexible spacer: 1,6-hexamethylene-bis(N-dodecyl-N,N-dimethylammonium) dibromide (12-6-12), 1,6-hexamethylene-bis(N-dodecyl-N-hydroxyethyl-N-methylammonium) dibromide (G6-MOH-12) and 3-oxa-1,5-pentamethyleno-bis(N-dodecyl-N-hydroxyethyl-N-methylammonium chloride) (12-MOH-O-MOH-12) were synthesised and characterised. Their corrosion inhibition efficiency, up to 7 days of immersion, to stainless steel AISI 304, in 3M hydrochloric acid, was investigated by weight loss and electrochemical impedance spectroscopy (EIS). Thermodynamic parameters have been obtained by adsorption theory. Following corrosion testing the surface of the stainless steel samples were studied by confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM). STRUCTURES OF TESTED GEMINI SURFACTANTS WEIGHT LOSS AND ELECTROCHEMICAL IMPEDANCE MEASUREMENTS (EIS) – AFTER 7 DAYS OF IMMERSION SURFACE MORPHOLOGY Table 1. Weight loss data of stainless steel in 3 M HCl in the absence and presence of the inhibitors after 7 days of immersion. Figure 1. Nyquist plots for stainless steel in 3 M HCl in the presence and absence of the synthesised inhibitors (naturally aerated, 7 days of immersion). Figure 2. SEM images of stainless steel surface (a) before immersion, (b) after 7 days of immersion in 3 M HCl without inhibitors and (c) after 7 days of immersion in 3 M HCl with addition of 12-MOH-O-MOH-12. ΔW [g] CR [mm/year] IE [%] 3M HCl 0.605 6.36 - 12-6-12 0.0153 0.16 75 G6-MOH-12 0.0140 0.15 77 12-MOH-O-MOH-12 0.0139 (a) (b) (c) Table 2. EIS parameters after 7 days of immersion in 3M HCl with absence and presence of the inhibitors. Table 3. The roughness (Ra) of the stainless steel samples before and after (7 days) immersion in 3M HCl without and with tested corrosion inhibitors. Cdl [μF/cm2] Rct [Ωcm2] CR [mm/year] IE [%] 3 M HCl 9613 58.31 6.27 - 12-6-12 2488 492.00 0.63 88 G6-MOH-12 1527 532.00 0.74 89 12-MOH-O-MOH-12 1653 638.10 0.62 91 Ra [μm] Before immersion 3M HCl 1 12-6-12 G6-MOH-12 12-MOH-O-MOH-12 CONCLUSIONS The gemini surfactants tested in this study inhibit the corrosion of stainless steel AISI 304 in 3 M HCl, after 7 days of immersion, therefore reducing the corrosion rate. The inhibition efficiency of the tested inhibitors is related to adsorption of cationic surfactant molecules onto the stainless steel surface and formation of a protective layer. The presence of oxygen atoms in the molecule increases the protection by favouring the adsorption process. References: K. S. Sanjay, E. O. Nabuk; Green Corrosion Chemistry and Engineering: Opportunities and Challenges, Wiley-VCH Verlag GmbH & Co. KGaA; 2012, England E. McCafferty, Introduction to Corrosion Science, Springer, London, 2010. B. Brycki, I. Kowalczyk, A. Szulc, O. Kaczerewska ,M.Pakiet (2017). Multifunctional Gemini Surfactants: Structure, Synthesis, Properties and Applications, Application and Characterization of Surfactants, InTech This work has been supported by the National Centre for Research and Development (Poland; TANGO1/266340/NCBR/2015).