Date of Award

1992

Degree Type

Thesis

Degree Name

Master of Science in Chemical Engineering (MSChE)

Department

Chemical Engineering

First Advisor

Richard Brown

Abstract

Electrochemical impedance spectroscopy (EIS) has been widely used to characterize rates and mechanisms during electrochemical processes. In this study, EIS was employed to probe the behavior for simulated galvanic coupling of a carbon fiber/polymer matrix composite in the marine environment. In a later study the effect of cathodic 10n plating and ion implantation of 304 stainless steel on its corrosion behavior was examined.

The electrochemical behavior of a carbon fiber/vinyl ester composite in 3.5% NaCl solution was investigated. Negative potentials were applied to the composite material to simulate galvanic coupling of metals. EIS enables time dependent data to be acquired non destructively from a single sample. The impedance of the material as a function of time and applied negative potential was measured. It was found that increased damage to the composite was induced by more applied negative potentials, as cathodic reactions were increased with decreasing potential. Experimental data from the measurement technique is also amenable to modelling by passive electrical circuit elements. The system was equivalently modelled by two interacting RCtype subcircuits representing the carbon fiber/moisture and vinyl ester/moisture interphases. The pore resistance Rpo determined from the model was found to offer a damage monitoring criteria for the composite material.

In an examination of surface damage to composites, laboratory testings were found to simulate accurately long term surface damage from galvanic marine exposure m seawater. Surface examination after long term galvanic coupling in seawater indicated removal of the polymer matrix above carbon fibers in addition to previously found blisters. Therefore two types of damage, blistering and dissolution, occurred due to galvanic interactions seawater exposure. Blistering was found only 720 hours (30 days) at a potential of -0.65VscE, along with regions of polymer surface dissolution. Imposing a potential of -1.2V s CE resulted in exposing carbon fibers after the covering polymer layer was rapidly removed. Possible electrochemical mechanisms for the polymer dissolution process are discussed.

TiN and ZrN cathodic ion plated coatings were applied to a 304 stainless steel, and exposed to 0.5N NaCl solution. The TiN coatings were also ion implanted with Ti and Au to determine the effect of ion implantation on corrosion behavior. It was found that ion implantation did not enhance the corrosion resistance of the TiN on 304 stainless steel. However the ZrN did protect the stainless steel from corrosion. It is suggested that the ZrN is inherently more protective by formation of a passive layer. TiN even with excess Ti ions from implantation did not form a protective passive layer. Corrosion interfaces of coated and/or implanted stainless steel were modelled by a simple parallel RC-type circuit due to strong interfacial bonding between substrate and coatings. Modelled data indicated the charge transfer resistance of the ZrN was higher than TiN and was related to the enhanced protection of ZrN.

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