Date of Award

Winter 1992

Project Type

Dissertation

Program or Major

Engineering

Degree Name

Doctor of Philosophy

First Advisor

Dale P Barkey

Abstract

Electrochemically prepared poly(3-methylthiophene) (P3MT) films were used as conductive polymer coatings to poise a substrate metal in the passive potential range in corrosive media. To firmly adhere P3MT coatings onto substrate metals, different methods were developed for each metal; stainless steel (430SS), titanium (TiGr2) and carbon steel (C1010). The samples were either pretreated by anodization in phosphate solution to form a uniform porous surface or pitted by chloride ions simultaneously with the formation of an adhesive thin film. Coatings were characterized for various galvanostatic electropolymerization conditions on both platinum and phosphated 430SS (Ph430SS) rotating disk electrodes (RDEs). Both applied current density and rotation rate affected subsequent redox capacity, polarization and self-discharge behavior of the coatings in 1 N sulfuric acid solution.

With Ph430SS as substrate, the stability of P3MT coatings was investigated in various corrosive media. It was observed that the reversibility of the coatings above $-$0.3 V in several aqueous acid solutions contributed to their stability. In the presence of chloride ions, the coatings failed to protect Ph430SS because of penetration of Cl$\sp-$ ions through the coating and resulting pit formation on the substrate.

Galvanic corrosion experiments were conducted to evaluate the effectiveness of the coatings in stabilizing Ph430SS and titanium in the passive state. In these experiments, a coated electrode was coupled to a bare electrode, and both electrodes were immersed. The coatings stabilized the exposed substrate in certain corrosive media at room temperature under either air or nitrogen purge.

Coatings on titanium were tested in 4 N sulfuric and hydrochloric acid solutions at various temperatures. The coatings stabilized the substrate at temperatures below 50$\sp\circ$C. Electrochemical Impedance Spectroscopy (EIS) was used to evaluate the short-term stability of P3MT coatings on titanium both in solution and in ambient air.

Although C1010 carbon steel can be passivated by anodic pretreatment, P3MT coatings did not sustain the passive state after immersion in corrosive media.

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