Abstract

Organic surfactant-type corrosion inhibitors are widely applied in the oil and gas industry considering their high efficiency at low ppm concentrations. The investigation of organic inhibitor adsorption and inhibition mechanisms on carbon steel has been limited by the difficulties with surface characterization techniques at a molecular level. Atomic force microscopy (AFM) can provide localized visual observation, and can also achieve characterization of mechanical properties of an inhibitor film, through friction and surface stiffness measurements in multiple operational modes. Reported research has systematically studied the frictional properties of self-assembled surfactant monolayers on mica using lateral force microscopy. However, there has been no such studies done on carbon steels. In the work reported herein, in situ AFM topography measurements in contact mode, in situ AFM friction imaging and in situ AFM phase imaging techniques have been applied to investigate the influence of different microstructures present in a ferritic-pearlitic carbon steel on inhibitor adsorption mechanisms as well as corrosion inhibition of CO2 corrosion. AFM c friction images show a large friction contrast between inhibitor covered cementite structures and ferrite structures, while in the absence of inhibitor this friction contrast almost disappears, indicating the inhibitor adsorption induced this difference. AFM phase images indicate no preferential adsorption of inhibitor on cementite or ferrite structures. These results indicate that either the adhesion force of inhibitor molecules on the cementite structures could be much smaller than on ferrite structures, or the molecular orientations of inhibitor molecules adsorbed on the cementite and ferrite structures could be different. In either case, it is hypothesized that the carbide component of the steel microstructure directly influences inhibitor adsorption, which could decrease inhibitor efficiency in ferritic/pearlitic regions and areas where iron carbide is more prevalent.

Introduction

Corrosion resistant alloys (CRAs) are used for many pipeline and wellhead components associated with oil and gas production environments but may be considered too costly for longer crude oil and natural gas production lines. Mitigation of internal corrosion for these types of pipelines is normally carried out by batch treatment or continuous injection of corrosion inhibitors, especially the surfactant type of organic inhibitors, which are more economical than using a CRA.1, 2

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