Mary E. McKnight

Optical Reflectance of Metallic Coatings: Effect of Aluminum Flake Orientation

A set of aluminum-flake pigmented coatings having different flake orientations was prepared using various spraying conditions. The flake surface topography and the orientations of individual flakes were determined from images obtained by laser scanning confocal microscopy. Reflectance measurements were carried out to quantify the optical properties of the coatings. Both a Gaussian distribution (used to represent the measured flake orientation distribution) and a topographic map (including local surface roughness and orientation) of the flakes were then used as input to a ray scattering model to calculate the optical reflectance of each coatings Flake orientation distributions and examples of measured optical reflectance as a function of scattering angle are shown, and the latter are compared to calculated reflectance values.

Measurements and predictions of light scattering by clear coatings

Comparisons are made between calculated and measured angle-resolved light-scattering distributions from clear dielectric isotropic epoxy coatings over a range of rms roughness conditions, resulting in strongly specular scattering to diffuse scattering characteristics. Calculated distributions are derived from topography measurements performed with interferometric microscopes. Two methods of calculation are used. One determines the intensity of scattered light waves with a phase integral in the Kirchhoff approximation. The other is based on the reflection of light rays by locally flat surfaces. The angle-resolved scattering distributions for the coatings are measured with the spectral trifunction automated reference reflectometer (STARR) developed by the National Institute of Standards and Technology. Comparisons between measured and calculated results are shown for three surfaces with rms roughness values of approximately 3, 150, and 800 nm for an angle of incidence of 20 degrees .

Advanced methods and models for describing coating appearance

The National Institute of Standards and Technology has recently initiated a study to advance appearance metrology. A systems approach which applies technological advances in optical metrology, mathematical modeling, and computer rendering to the development of new methods of appearance characterization and to more accurate methods of modeling the appearance of coatings and coated objects will be used. Corresponding experimental and modeling research activities will be conducted in four areas: coating composition, microstructure of coating films, reflectance properties, and appearance descriptions. The parameterized mathematical models and computer rendering, when coupled with advanced measurements, will support the capability of researchers and engineers to assess the contribution of coating constituents to appearance and help design coatings with appropriate initial appearance and durability properties. This paper presents an overview of the planned research.

Relation between AC impedance data and degradation of coated steel: 1. Effects of surface roughness and contamination on the corrosion behavior of epoxy-coated steel

Abstract The effects of surface roughness and inorganic chloride contamination on the corrosion behavior of steel coated with thin, amine-cured epoxy films have been investigated using AC impedance spectroscopy. The results were analyzed using the equivalent circuit models to identify the different steps in the corrosion of coated steel. Two types of corrosion were observed depending on the surface treatment: point-attack corrosion for uncontaminated and blistered corrosion for contaminated surfaces. Both types of corrosion can be discerned by AC impedance spectroscopy and modeled by the equivalent circuits. Coating-deficient areas at the peaks of sandblasted substrate have been calculated accurately from the AC impedance data and verified by scanning electron microscopy.

Thermographic Imaging And Computer Image Processing Of Defects In Building Materials

An image processing system has been coupled to both a thermographic and a video camera to quantify defects from images of building materials. Several applications to building materials are presented including the detection of delaminations in single-ply roofing membrane seams, the characterization of the extent of corrosion under pigmented organic coatings on metallic substrates, the determination of the roughness of a sand-blasted metallic substrate, and the determination of the porosity in hardened cement paste.