S. Ray Taylor

Testing and evaluation of nonchromated coating systems for aerospace applications

Abstract The advanced corrosion resistant aircraft coatings program (ACRAC) is part of the Air Force strategy to improve performance and reduce environmental impact of coatings used on Air Force weapon systems. The program addresses the Air Force near and mid-term strategies to eliminate chromate corrosion inhibitors and reduce steps in the outer mold line coating process. Evaluation of the coating process (surface preparation, conversion coating, primer, topcoat) as a system is a key feature of the ACRAC program. Results to date indicate that the current-state-of-the-art nonchromated coating systems are significantly less effective than those with chromate. A chromate conversion coating is required for the nonchromate primer system to meet minimum requirements. Sol–gel-process based conversion coatings can replace chromate conversion coatings provided a chromated primer is used. Several approaches to incorporating inhibitors into sol–gel coatings are discussed. Electrochemical methods for testing coating performance are discussed and a new procedure based on impedance spectroscopy for evaluating active damage repair is presented.

Understanding the Degradation of Organic Coatings Using Local Electrochemical Impedance Methods I. Commonly Observed Features

Local electrochemical impedance mapping (LEIM) was used to investigate local underfilin corrosion occurring on organic-coated AA2024-T3 exposed to chloride solution. Films (5-20 μm thick) of neat epoxy, polyurethane. and vinyl resins were studied. Frequently observed features in the LEI maps included: (i) an unexpected decrease in admittance in early stages of development, (ii) trenching at the periphery of admittance peaks, (iii) both increases and decreases in peak height over time, (iv) changes in peak width over time, (u) different amplitudes in admittance over different regions of multilobed blisters, and (vi) different admittance magnitudes for different types of defect. These LEIM features could provide significant insight into and documentation of the local breakdown processes of coated metals, if they truly represent electrochemical phenomena associated with changes at the interface. These features were observed on each of the coating chemistries studied. Proposed origins and supportive evidence of these LEIM features is presented.

Evaluation of Natural Oxides on Aluminum in Neutral Borate Electrolyte

Air‐formed and anodized barrier‐type aluminum oxides were studied in neutral borate electrolyte as a preliminary investigation into the possibility of using this environment for nondestructive evaluation of aluminum/polymer laminates. Polarization scans on air‐formed oxides determined a potential range where the faradaic current was minimized and samples appeared unaffected after several days of exposure. X‐ray photoelectron spectroscopy determined that borate exposure, under the prescribed testing conditions, does not permanently change the oxide thickness, and that borate ions are not incorporated into the oxide film. Analysis of the electrochemical impedance spectra of air‐formed oxides and different thicknesses of anodized barrier‐type oxide films established that the impedance response for all thicknesses could be described by a single‐circuit model whose components correspond to a parallel‐plate dielectric‐filled capacitor.

Understanding the Degradation of Organic Coatings Through Local Electrochemical Impedance Methods II. Modeling and Experimental Results of Normal Field Variations above Disk Electrodes

Local electrochemical impedance mapping (LEIM) represents a potentially important tool in the characterization of discrete electrochemical phenomenon on heterogeneous surfaces, such as the degradation of a coated aluminumalloy. Since LEIM is an emerging tool, it is essential to resolve between real material and interfacial changes and a possible sampling artifact to the measured LEIM response. To this end, both analytical and numerical models were developed to calculate the field above an equipotential disk to examine possible LEIM measurement artifacts. Several features in LEI maps which could be the result of an artifact in the measured field above a corroding region were addressed. These included changes in the field due to the following: (i) edge effects. (ii) two closely spaced corroding sites affecting spatial resolution, (iii) a change in radius of a corroding site, including the ability of LEI to resolve small sites. Numerical analysis was found to snore realistically model the finite dimensions and discrete change in potential that were used in actual LEIM experiments. The calculated field above the equipotential disk was then compared to experimental LEI maps of gold disk microelectrodes embedded in SiO 2 . Edge effects were found to be an unlikely source for experimental observations of reduced admittance. The spatial resolution of the LEI probe was predicted through theoretical modeling and determined experimentally to be sufficient to resolve two disk separated by 35 μm. The probe was also predicted and shown experimentally to have the capability to measure a disk of radius 17.5 μm. The successful demonstration of this numerical model will allow the exploration of more complicated material interfaces, such as materials coated with a dielectric layer.

Biomechanical properties of tissue-engineered cartilage from human and rabbit chondrocytes

OBJECTIVE : To describe tissue-engineered cartilage from rabbit and human chondrocytes. STUDY DESIGN AND SETTING : Chondrocytes from rabbit and human ears were seeded onto a tem-plate and implanted for 8 or 16 weeks of in vivo incubation. RESULTS : For the 8-week and 16-week groups, the UTS for cartilage was 3.8 MPa and 3.7 MPa, stiffness was 62.4 MPa and 51.8 MPa, and resilience was 181.8 J/m3 and 109.1 J/m3, respectively. Experimental cartilage was significantly different from controls. From 5 human specimens, the UTS was 5.4 MPa, stiffness was 6.6 MPa, and resilience was 2.0 J/m3. The control had UTS of 8.8 MPa, stiffness of 12.2 MPa, and resilience of 2.9 J/m3. Histology showed mature cartilage but with a fibrovascular infiltrate and increased cellularity. CONCLUSIONS : Mechanical properties of tissue-engineered cartilage can be quantified and are less than that of controls.

The Nondestructive Evaluation of Adhesively Bonded Aluminum Using Electrochemical Impedance Spectroscopy I . Theory and Experimental Investigation of Extrinsic Variables

Electrochemical impedance spectroscopy (EIS) is investigated as a potentially nondestructive method to evaluate the dimensions of edge‐exposed rectangular disbondments in adhesively bonded aluminum structures. Calculations based on equivalent circuit modeling and experiments conducted on controlled laminate geometries demonstrate that changes in defect parameters such as the depth, width, and thickness have specific quantitatively predictable effects on the impedance spectrum of a defective sample. While the relationships between these effects can be complex, the results indicate that the impedance response of the defective sample can characterize changes in defect geometry. This study extends the theory developed for single defect impedance characterization to multiple defects and investigates the theoretical and experimental capabilities of EIS in the evaluation of multiple defects.