1. Appearance: Observe with naked eyes under natural refracted light. The basic hue of the zinc-chromium coating should be silver-gray, and other colors, such as black, can also be obtained by modification. The zinc-chromium coating should be continuous and free of defects such as missing coating, bubbles, peeling, cracks, pitting, and inclusions. The coating should be basically uniform without obvious local over-thickness. The coating does not become discolored, but small yellow spots are allowed.
2. Detection of coating amount and coating thickness: The standard divides the coating amount or coating thickness of different grades of coating into four grades, and two methods can be used for detection:
1. Dissolution weighing method: Use a balance with an accuracy of 1 mg to weigh the original mass W1 (mg) for a sample with a weight of more than 50g, and put the sample in a 20% NaOH aqueous solution at 70℃～80℃, soak for 10min to make zinc The chromium coating is completely dissolved. Take out the sample, wash it thoroughly and dry it immediately, and weigh W2 (mg) of the sample after the coating is dissolved. Measure and calculate the surface area S (dm2) of the workpiece, and calculate the coating amount W (mg/dm2) of the coating according to the following formula: W=(W1-W2)/S
2. Metallographic microscope method: According to the requirements of GB/T6462, use the metallographic microscope method to detect the thickness of the coating.
3. Adhesion strength test: Adopt the tape test method to detect the adhesion strength between the zinc-chromium coating and the substrate. The tape test shall be carried out in accordance with the requirements of section 1.4 of GB/T5270-1985. It is required that the coating shall not peel off or expose the bottom of the substrate after the test, but the tape is allowed to change color and adhere to zinc and aluminum particles.
Water resistance test: immerse the sample in deionized water at 40℃±1℃ for 240h continuously, take the sample out and dry it at room temperature, and then carry out the adhesion strength test. The test result should meet the requirements of the adhesion strength test. The adhesion strength test should be carried out within 2h after the sample is taken out of the deionized water. After the water resistance test, the coating shall not peel off or expose the bottom of the substrate.
4. Salt spray resistance performance test: The salt spray resistance performance test shall be carried out in accordance with the requirements of section 3.2.1 of GB/T10125-1997. After the coating is tested by salt spray, it is divided into four grades according to the time of red rust on the coating from 120 hours to 1000 hours.
5. Damp heat test: The damp heat test is carried out in a damp heat test box, which should be able to adjust and control the temperature and humidity.
Set the temperature of the damp and heat test box to 40℃±2℃ and the relative humidity of 95%±3%. Hang the samples vertically in the damp and heat test box. The samples should not touch each other. When the damp heat test chamber reaches the set temperature and humidity, the test time starts to be calculated. Continuous test 48h check once to check whether the sample appears red rust. After two inspections, check once every 72h. After each inspection, the sample should change position. 240h check the last time. The standard stipulates that only the damp and heat resistance test is carried out for the 3rd and 4th coatings, and the coating shall not appear red rust within 240 hours.
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