The porosity should be determined before all tests of the corrosion resistance of the coating. This point is often overlooked, but in fact, the existence of these micropores, which are not only invisible to the naked eye, but also invisible even when the magnification is low, is the reason that affects the accuracy of the test results.
Obviously, the greater the porosity of the paint film, the faster various corrosive substances will penetrate the surface protected by the paint film.
Usually, when conducting corrosion resistance tests, paint materials are brushed, sprayed, or dipped on the metal surface. When dipping, a paint film with a minimum of porosity can be obtained, while the brushing method of the paint material should be considered very unreliable because it is difficult to avoid "bottoming" when finishing by this method.
Generally speaking, when adopting any finishing method, all can't avoid " revealing the bottom " phenomenon, especially all the more so when single-layer coating film. Microscopic examination often finds unpainted areas along the brush strands or around fine dust particles that fall on the surface during finishing. (Related instruments: aging Test Chamber)
The main reason for the many holes on the paint film is that the solvent is not volatilized uniformly. This situation is caused by the improper selection of the solvent used for the film-forming material and the improper drying process (especially artificial drying).
Methods with different degrees of accuracy can be used to measure: paint film separation, and general paint film can also be measured. All these methods can basically be divided into two types: indicating method and electrical measuring method. The following article mainly introduces the determination of porosity by dissolution of copper sulfate and mercury nitrate.
If it is desired to determine the porosity of a coating or film on a metal plate or any product (iron, zinc, aluminum, copper, brass), it is possible to use a method of separating the more precious metal from a solution of its metal salt in a less precious metal. method on a metal surface. For this purpose, various indicator solutions are available. If iron is used, 5% copper sulfate solution can be used; when zinc or aluminum is used, copper sulfate solution with 0.5% ammonia gasification is used; if copper and brass are used, 5% nitric acid solution is used. (Related instrument: PH meter)
The painted sample is placed in a suitable solution; due to the potential difference, metallic copper or copper is deposited in the pores of the coating film. The immersion time of the sample in the indicator solution depends on the pore size of the coating film and the porosity of the coating film.