Understanding corrosion inhibition: A surface science study of thiophene derivatives on iron surfaces in gaseous and liquid systems

The interactions of thiophene and selected derivatives on iron surfaces have been investigated in ultra high vacuum (UHV), H2S saturated aqueous, and liquid hydrocarbon environments. The goal was to find quantitative structure-activity relationships (QSARs) to advance the understanding of interactions between organic molecules and metal surfaces, with a potential application towards the design and development of organic corrosion inhibitors. Thiophene was selected based on previous work in UHV indicating a high thermal stability (∼520K) on the Fe(100) surface. Derivatives with functional groups with electron donating character (3-methylthiophene, 3-methoxythiophene) and electron withdrawing character (3-thiophenecarboxylic acid, 3-nitrothiophene) were chosen with the intent to probe how thermal stability and potential inhibition properties would be affected.; The systems were characterized using temperature programmed desorption (TPD) and Auger spectroscopy (AES) in ultra high vacuum, Tafel corrosion experiments and electrochemical impedance spectroscopy (EIS) in aqueous phase, and corrosion rate measurements by weight loss in hydrocarbon liquids.; In the ultra high vacuum study, thiophene, the reference molecule, was confirmed to be thermally stable up to 520K. 3-methylthiophene was thermally stable up to 718K and 3-methoxythiophene to 486K. The 3-thiophenecarboxylic acid experienced significant decomposition upon adsorption. No intact molecules were observed at temperatures >300K. 3-nitrothiophene exhibited decomposition and a complex surface chemistry on the Fe(100) surface. Thermally stable fragments and/or surface reaction products were observed up to 683K.; In the electrochemical study, the addition of the investigated molecules either increased (3-methoxythiophene, 3-thiophenecarboxylic acid, and 3-nitrothiophene) or did not affect (thiophene, 3-methoxythiophene) the corrosion rate of iron in H2S saturated brine at 20°C. The molecules either did not reach, or interact, with the iron surface at all. Observed increases in corrosion rates were ascribed to changing solution properties, and classified according to their effect on the corrosion process as due to: enhanced cathodic activity (3-methoxythiophene), lowering of pH effect (3-thiophenecarboxylic acid), or elemental sulfur formation (3-nitrothiophene).; In the liquid hydrocarbon weight loss experiments, no discernable effects were observed upon addition of the investigated molecules. Corrosion rates equal to or higher than the reference case were observed at both 250°C and 200°C, most likely due to the decomposition of the molecules.