Assessment of Conductive Sites on Carbon Fiber Reinforced Polymer Plastics' Surfaces Using Electrochemical Methods

Abstract

The components of automobile bodies have been manufactured primarily from steel since cars were first made. With the goal of energy savings, light-weighting is being adopted by substituting steel with a variety of light metals and composite materials. A DOE sponsored project in collaboration with PPG and Ford studied the use of automotive closure panels comprising an inner carbon fiber reinforced polymer (CFRP) sheet and an outer aluminum alloy (AA) sheet. The ends of closure panels are sealed with hem flange joints. A major concern is the formation of a galvanic cell within the joint wherein CFRP is the cathode that will accelerate the corrosion of the AA sheet. Various combinations of AA6xxx alloys with CFRP having two different orientations of carbon fiber, i.e., random and twill, are studied in this work. CFRP can only exert a galvanic driving force on a coupled AA panel by electrical connection at cut edges where sheared carbon fibers are exposed, or if carbon fibers are exposed on the CFRP panel surface by lack of coverage of the insulating epoxy matrix. It is believed that processing and curing variables lead to varied epoxy cover on the surfaces of CFRP. Electrochemical measurements such as the measurement of oxygen reduction limiting current density during cathodic polarization show that as-fabricated surfaces of CFRP panels exhibit some extent of electrochemical activity, indicating some lack of coverage of C fibers. It is of interest to characterize the epoxy cover on the CFRP surfaces in terms of the nature of the active sites (e.g., whether C fibers are totally uncovered or covered by a very thin layer of epoxy) and the density and distribution of the sites. A technique was developed to identify the exact locations of active sites on both random and twill CFRP by electrodeposition of small amounts of Cu. If the amount of deposited copper is small, the deposits indicate the location and number of the active sites on the CFRP surface. These deposits were also analyzed by serial sectioning using Focused Ion Beam/Scanning Electron Microscope. High resolution images of the deposit/epoxy interfaces provide a description of the nature of the defective/conductive regions on CFRP surfaces. Rotating disk electrode experiments were also performed to determine the oxygen reduction reaction (ORR) limiting current densities of CFRP at different electrode rotation rates i.e., different diffusion boundary layer thicknesses. These aspects are related to the overall mass transport in the electrolyte, which provides a deeper understanding of the kinetics on CFRP and the nature of active sites behaving as microelectrodes. The observations from the copper electrodeposition study and rotating disk electrode experiments will be correlated and the role of CFRP as a cathode contributing to the corrosion of AA in the galvanic couple will be presented.This work was supported by the U.S. Department of Energy through award DE-EE0007760, in collaboration with PPG Industries and Ford Motor Company.