ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY OF STEEL CORROSION AND ITS INHIBITION BY ELECTROACTIVE POLYANILINE

Abstract

This project focuses on the characterization and inhibition of mild steel corrosion by · electrochemical methodologies. The anodic dissolution of mild steel and the inhibition by organic amines were first investigated by DC polarization, electrochemical impedance spectroscopy (EIS), optical microscopy and/or X-ray photoelectronic spectroscopy (XPS). In addition to deriving models that satisfactorily described the observed electrochemical behaviour, the results were also analyzed by grey system theories to predict inhibitor performance based on chemical structures of the inhibitors and application conditions. The use of the polyaniline (PANi) as a protective coating in acidic and saline corrosion environment was investigated and its compatibility with other corrosion protection techniques including anodic and cathodic protection, and the presence of a blocking overlayer were exploited. This thesis work also discovered a rather interesting relationship between the pitting susceptibility of mild steel, and the voltametric response in the electrochemical deposition of PANi on mild steel. This may be developed as a quick test for pitting. In the following chapters, the anodic dissolution of mild steel in O.SM H2SO4 with close reference to the active, prepassive, passive and transpassive regions investigated by classical electrochemical methods will be described first. EIS was the main technique used to characterize the generation, adsorption, accumulation and transportation of corrosion intermediates. The mathematical models were developed to relate the observed impedance behaviour to various corrosion processes taking place on the metal surface. The results were also used to obtain the charge transfer resistance Rt and an indicator of surface roughness n. These parameters when used in tandem are able to differentiate various forms of corrosion and could provide a basis for on-line monitoring of corrosion. Corrosion inhibition by organic amines was studied next. The inhibition performance was strongly dependent on the extent of adsorption, and hence on the nature of the metal surface under application conditions. In the active region adsorption is predominately chemisorptive whereas hydrogen bond formation is the major mode of adsorption in the passive region. The grey system theory was used to identify major factors in inhibitor performance for the different corrosion regimes. The advantages of the grey system theory is that it makes use of fewer data sets to produce relationships that are useful in the design and selection of corrosion inhibitors for a given service. The: application of the conductive polymer in corrosion protection was also studied in this project. Polyaniline (PANi) was applied as a protective coating for mild steel in acid and saline solutions. The results confirmed that PANi is able to promptly repassivate the underlying metal, through a mediated redox process, in the event of a mechanical damage in the native oxide film. The protection was much better in acids than in salines as a result of the improved electroactivity of PANi in an acid environment. The corrosion protection performance was further improved in the presence of a top-coat. The adhesion of PANi with the metal substrate however deteriorated with the increase in acid concentration and the decrease in saline concentration, but could be overcome by a phosphate treatment of the surface before PANi deposition. Electrodeposition of PANi was also carried out on mild steel surface oxidized to different extent. The voltammetric features of the deposition, which depended strongly on the nature of the depositing surface, corresponded very well with the pitting susceptibility of the substrate as measured by conventional mean. The finding suggests that PANi electrodeposition can be used as a potential method for the characterization of the nature of surface oxide as well as the pitting susceptibility of the substrate.