The main methods of evaluating the quality of chemical conversion membranes by experiments are as follows.
(1) Microscopic method. By means of metallographic microscope or electron microscope, the chemical conversion film was enlarged to 100-1000 times, and its crystal shape, size and arrangement were observed. The columnar crystal is the best crystalline shape, and the smaller the crystallization ruler is, the better it is. Generally, it is controlled under tens of microns. The more uniform the arrangement is, the smaller the porosity is.
(2) Thickness (or weight) determination. The samples were immersed at 75 C and the mass fraction was 10-15 minutes in the chromic acid solution prescribed by the national standard to remove the chemical film, then the weight difference before and after the film was removed and the film weight was calculated.
(3) Corrosion test method. The most commonly used method for determination of corrosion performance is according to the national standard GB6807-86. It can be dipped in 3% sodium chloride solution at 15-25 ~C or dripped with copper sulfate solution. The formula of copper sulfate drip is as follows: CuSO4:5H1O41g/L, NaCl35g/L, HCl13mL/L. It can also be tested by salt spray test or cycle test after coating.
(4) Impact test method. It is often determined together after coating. When the impact test of the coated phosphating plate is carried out with 49Ncm load, the quality of the chemical conversion film can be determined when the reverse impact point of the impact sample does not produce radioactive cracks.
(5) Secondary adhesion test. The adhesion of chemical conversion film after coating is determined as primary adhesion. Under certain conditions, the adhesion determined after the experiment of temperature-resistant water is called secondary adhesion. Generally, the adhesive force is measured by the scratch method on the sample after water resistance test, and the film falling off grade is observed after stripping of the tape. Generally, it is a parallel comparative experiment.
(6) Membrane porosity measurement method. Take 14% NaCl and 3% potassium ferricyanide solution and 0.1% distilled water solution of surfactant. Store in brown bottle for 24 hours and filter with filter paper. When used, the filter paper is cut and grown to 2.5 cm wide. The paper is immersed in the above solution with plastic tweezers. The excess test solution is dripped and covered on the surface of the chemical film to be measured. After a period of time (1 minute), the test paper is removed and the surface of the film is observed. The blue spots indicate the porous part.
(7) alkali resistance of chemical film. The chemical film was impregnated in 0.1 mol/L sodium hydroxide alkali solution at 25 C. The solubility of the chemical film in the alkali solution was obtained by comparing the quality of the film before and after 5 minutes.
(8) The acid resistance of the chemical conversion film. The acid resistance of chemical conversion membrane was evaluated by comparing the solubility of chemical conversion membrane in acid solution with pH 2.
(9) P ratio of chemical conversion film. P ratio was initially defined as P/(P+H), where P is ferrozinc phosphate and H is zinc phosphate. Therefore, the ratio of P indicates the proportion of ferrozinc phosphate in the chemical conversion film. The chemical conversion film with high P ratio has better corrosion resistance than the chemical conversion film with low P ratio.