Determination of coating porosity by filter paper method

       The filter paper method is used to measure the porosity. The test solution on the filter paper interacts with the matrix metal through the pores to generate spots with characteristic colors and display them on the filter paper. Finally, check the number of spots on the filter paper to evaluate the porosity of the coating. This method is suitable for testing the porosity of single metal coatings such as copper, nickel, chromium, tin on steel and nickel, chromium, and tin on copper alloy substrates, or nickel-chromium, copper-nickel, copper-nickel-chromium, etc. Porosity of polymetallic coatings.



     The test solution used to wet the paper is composed of etchant and indicator. The corrosive agent requires that it only acts with the base metal, not with the surface coating, and chloride is usually used. The indicator requires a specific reagent that can interact with the metal ions generated by corrosion to form a characteristic color or precipitation. Generally, potassium ferricyanide or potassium ferrocyanide is used.

      During the test, the test solution on the filter paper enters the inside of the coating through the pores, and corrodes the base metal according to the depth of the pores, producing Fe2+ or Cu2+, Ni2+, etc. These ions react with the indicator in the test solution to generate corresponding ions with a certain color. Precipitation, for example: Fe3[Fe(CN)6]2 is a blue precipitate; Ni3[Fe(CN)6]2 is a yellow-brown precipitate; Cu2[Fe(CN)6] is a reddish-brown precipitate, etc.

    The resulting precipitate oozes out of the pores and is displayed on the filter paper. The color and number of spots on the filter paper can be used to assess where the pores are on the coating and the size of the porosity.

    The test piece itself can be used as a sample. The sample needs to be washed with pure solvent such as benzene, light gasoline, ethylene trichloride or carbon tetrachloride, then washed with absolute ethanol, and fully dried.

    All components required for the test solution are prepared with chemically pure reagents and distilled water. In principle, it can be prepared and used immediately during the test, and it can also be kept in a dark and cold place in a brown bottle for several days. The specific components, application range and color rendering characteristics are shown in Table 8-1.

    Sample preparation and sampling methods are carried out in accordance with product inspection standards. After sampling, use organic solvent or magnesium oxide paste to carefully degrease the site to be inspected, wash it with distilled water, and finally dry it with filter paper or dry it in clean air. The samples that are inspected directly after spraying do not need to be degreasing.

    Soak the filter paper in the test, and brush the test liquid on the sample to fully penetrate, and then paste the soaked test paper. Pay attention to the tight fit without leaving any gaps. After a few minutes, peel off the test paper, rinse it with water, stick it on a glass plate to dry, measure the number and size of colored spots, and convert it into the number of pores. The area of one test paper is more than 10cm2