Under the action of liquid flow (test solution) at a certain speed, the partial coating of the sample is dissolved. The end point of the complete dissolution of the coating can be determined by directly observing the change of the characteristic color of the metal with the naked eye, or by means of a specific end point indicating device (showing the change of the instantaneous potential or current of the complete dissolution of the coating). The thickness of the metal coating can be calculated according to the time consumed when the partial coating on the sample is completely dissolved.
This method is suitable for measuring the thickness of protective 5 decorative coatings and multilayer coatings on metal products. The measured area should not be less than 0.3cm2. The general measurement error is ±10%, but the actual measurement error is often higher. This kind of therapy has simple equipment, convenient operation, and many types of measurable coatings, so it is widely used in CHINA. It can also be used for coating thickness measurement in the production process to control the quality of electroplating.
1. Measuring device
(1) Measuring device for direct observation of the end point
(see Figure 7-2-2), the lower end of the separatory funnel with a piston is connected to the capillary with a rubber tube, the capillary is 120±5mm, the inner diameter is 1.5-2mm, and the outer diameter of the lower end should not exceed 2.0mm. To achieve the specified flow rate, the capillary port should be filed flat and then smoothed with sandpaper. When calibrating the caliber of the capillary, the piston should be fully opened, so that the capillary can flow out 10±0.1ml of distilled water within 30 seconds when the temperature is 18-20°C under normal pressure.
The hydraulic pressure is maintained by means of a glass tube which is inserted into the funnel through a rubber stopper and which makes it possible for air to enter the funnel through a small hole. The glass tube should be located in the solution and maintain a certain position, that is, 250±5mm from the lower end of the capillary. When the solution flows out of the funnel, the internal pressure decreases, thus forcing air into the funnel from the small hole, thereby ensuring a constant pressure during the measurement.
(2) Instruments with endpoint indicating devices
(See Figure 7-2-3) When it is difficult to observe the end point of dissolution with naked eyes during the dissolution process of some coatings, an end point indicating device can be added on the basis of the instrument in Figure 7-2-3. That is, a platinum wire is sealed at one end of the glass tube (10), and then the glass tube (10) is inserted into the separating funnel through a rubber stopper, and its lower end is level with the glass tube (4). Connect the platinum wire and the sample according to the circuit shown in Figure 7-2-3. The circuit includes: a single-stage amplifier and a power supply, allowing the use of amplification devices with any line structure.
2. Test solution
The test solution is shown in Table 7-2-3. All chemical reagents used should be chemically pure, and distilled water was used to prepare solutions.
3. Measurement method
(1) Preparation of samples
Degrease the sample with an organic solvent (such as gasoline, acetone). If the measurement is carried out immediately after plating, degreasing is not necessary.
(2) Preparation of the instrument
Pour the test solution to 3/4 of the volume of the separatory funnel before measurement, open the stopcock to fill the capillary with the solution, plug the neck of the separatory funnel tightly with a rubber stopper, reopen the stopper, and let the solution flow out of the separatory funnel until the air The bubbles are evenly sucked into the separatory funnel through the glass tube, which indicates that the pressure in the separatory funnel has stabilized.
If there are air bubbles in the rubber tube or capillary when the solution fills the capillary, press the rubber connecting tube firmly when the piston is open to expel the air.
Fix the prepared instrument on the bracket, and make the distance between the capillary tip and the tested surface of the sample be 4-5mm, and the angle between the tested surface of the sample and the horizontal plane be 45±5°.
(3) Determination
Open the piston and count the time with a stopwatch, and close the piston when the end point appears on the surface of the sample. Note the time taken to dissolve the coating and record the solution temperature.
When measuring multi-layer coatings, the time consumed for the dissolution of each coating layer should be recorded separately.
Repeat the measurement twice near the measurement point, take the arithmetic mean of the times, and then determine the thickness of the coating by calculation.
When it is difficult to observe the dissolution end point of some coatings, the device in Figure 7-2-3 can be used for measurement. When the coating is completely dissolved and the middle layer metal or base metal is exposed, the pointer of the ammeter deflects. At this moment, stop the stopwatch and record the time consumed for dissolving the coating.
4. Calculation of the local thickness of the coating
The local thickness of the coating is calculated by the following formula
h=h t t
In the formula:
h——local thickness of coating (μm);
ht———At a certain temperature, the coating thickness (μm/s) dissolved by the test solution per second, its value is found out in Table 7-2-4;
t——the time consumed to dissolve the coating (s)
5. Precautions
(1) This method is suitable for thickness measurement of coatings with a thickness ≥ 2 μm, otherwise the error is greater than 10%.
(2) When the outermost layer of the surface is a sample of nickel coating, in order to remove the passive film on the nickel surface, wipe off the passive film on the surface of the coating with cotton dipped in 1:1 hydrochloric acid solution before testing, and then Wash, dry and test.
(3) When the outermost layer of the surface is a chromium-plated sample, when measuring the thickness of the next layer of coating, the chromium coating should be removed with concentrated hydrochloric acid containing 1%~2% antimony trioxide.
(3) For zinc and cadmium coatings, thickness measurement should generally be carried out before passivation and phosphating. For the passivated sample, in order to remove the passivation film on the coating, wipe the surface of the tested sample with a cotton ball dipped in the hydrochloric acid solution with a concentration of #23. After the film layer is removed, wash and dry quickly, and then measure the thickness.
(5) During the measurement, it is necessary to check the speed of the test solution flowing out of the capillary with distilled water frequently.
