STP792

    Variation in the Microstructure of Electrodeposited Black Chrome Solar Coatings

    Published: Jan 1983


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    Abstract

    Electrodeposited black chrome solar coatings have been found to degrade thermally when used at temperatures near 300 °C. This degradation is evidenced by a large (∼ 10 percent) decrease in the solar absorptance for these coatings. The solution of the thermal degradation problem has led to a detailed study of the dependence of the coating microstructure and composition on the plating process variables and how these film properties change after thermal aging. The areal density, ρs, of coatings deposited on steel substrates reaches a constant value as the plating time increases. For thermally unstable coatings, ρs ≈ 2.6 × 10−5 g/cm2, while for thermally stable coatings, ρs ≈ 11 × 10−5 g/cm2. Scanning electron microscope studies of both unstable and stable coatings show that the films are composed of roughly spherical chromium/chromic oxide (Cr/Cr2O3) particles with diameters in the range of 90 to 150 nm. In the stable coatings, the particles tend to form clusters. This microstructure does not change upon heating, although the metallic chromium in the film oxidizes to form fine crystalline chromic oxide (Cr3O3). These data, together with previously published results, have been utilized to develop a model of the black chrome coating. This model is used to calculate the reflectance properties as a function of the average Cr2O3 percentage by volume within the film. The calculated reflectance curves are compared with experimental data for coatings aged for various times at 450 °C in air. Both the model and experimental data show that the solar absorptance initially decreases only slowly as the amount of Cr2O3 increases; however, a rapid decrease occurs when the Cr2O3 content exceeds ≈ 70 volume percent. Thus the authors have concluded that the decrease in solar absorptance with heating can be explained by oxidation of metallic chromium in the films to Cr2O3. It is believed that the clustering of particles in the more stable films decreases the effective exposed surface/volume ratio and thereby tends to inhibit the chromium oxidation. As a result of these investigations, it has been determined that careful control of the film morphology, through careful control of the plating process, is necessary to produce thermally stable black chrome coatings.

    Keywords:

    microstructure, solar coatings, optical properties


    Author Information:

    Pettit, RB
    Members of the technical staff, Sandia National Laboratories, Albuquerque, N. Mex.

    Sweet, JN
    Members of the technical staff, Sandia National Laboratories, Albuquerque, N. Mex.

    Sowell, RR
    Members of the technical staff, Sandia National Laboratories, Albuquerque, N. Mex.


    Paper ID: STP31775S

    Committee/Subcommittee: E04.11

    DOI: 10.1520/STP31775S


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