Testing standards and methods of Dacromet coating

According to the national standard of the People's Republic of China on "Technical Conditions of Zinc Chrome Coating" issued by the General Administration of Quality Supervision, Inspection and Quarantine in 2002, standard No. GB/T18684-2002, there are five main tests for Dacromet coating as follows.

1、Appearance

To observe the appearance with the naked eye under natural refractive light. The basic color of zinc-chromium coating should be silvery gray, and it can also be obtained by modification with common other colors, such as black. Zinc-chromium coating should be continuous, without defects such as leakage, bubbles, flaking, cracks, pockmarks, inclusions, etc. The coating should be basically uniform, without obvious local over-thickness. The coating should not be discolored, but small yellow spots are allowed to exist.

2、Detection of coating volume and coating thickness.

The amount of coating or thickness of coating for different grades is divided into four grades. Two methods can be used for detection:

（1）Dissolution weighing method: for samples weight greater than 50g specimen, using the accuracy of 1mg balance to weigh the original mass W1 (mg). Put the samples into 20% NaOH aqueous solution at 70℃~80℃ and soak for 10min to dissolve all the zinc-chromium coating. Take out the sample, wash it fully with water and dry it immediately, and weigh the mass of the sample w2(mg) after the coating is dissolved. Measure and calculate the surface S (dm2) of the workpiece, and calculate the coating amount W (mg/dm2) according to the following formula: W=(WI-W2)/S.

（2）Metallographic microscope method: According to the requirements of GB/T6462, the thickness of the coating is detected by metallographic microscope method.

3、 Adhesion strength test.

The tape test method is used to test the adhesion strength of zinc-chromium coating and the substrate, and the tape test is conducted according to GB/T5270-1985, section 1.4. After the test, the coating should not be peeled off or exposed from the substrate, but the tape is allowed to discolor and adhere to the zinc and aluminum powder particles.

4、 Water resistance performance test.

The sample was immersed in deionized water at 40 ° C ±1 ° C for 240h. After the sample was taken out and dried at room temperature, then carry out the adhesion strength test, the test result should meet the requirements of the adhesion strength test. The adhesion strength test shall be carried out within 2h after the sample is removed from the deionized water. After the water resistance test, the coating shall not be peeled off from the substrate or exposed to the bottom.

5、Salt spray resistance test.

Salt spray resistance test is conducted according to GB/T10125-1997 section 3.2.1 requirements. After the salt spray test, the coating is divided into four grades according to the time of red rust on the coating from 120 hours to 1000 hours.

6、Humidity and heat test:

The damp heat test is conducted in a damp heat tester, which should be able to adjust and control the temperature and humidity. The temperature of the damp heat test chamber is set to 40℃±2℃, the relative humidity is 95%士3%, the samples are hung vertically in the damp heat test chamber, and the samples should not touch each other. When the damp heat tester reaches the set temperature and humidity, start calculating the test time. To check the sample appears red rust or not after  48h continuous test. After two checks, the sample should be checked every 72h, and the sample should be changed position after each check. 240h is checked for the last time. It is stipulated in the standard that only the damp heat resistance test is required on the level 3 and 4 coatings, the coating is required not to show red rust within 240 hours. At the same time, the standard also specifies the method of sample extraction:

In the same batch of products, three samples were randomly selected for each test. If any one of the samples fails to pass the test, then three samples should be randomly selected again for the same test, if any one of them fails again, this batch of products is unqualified.

For assemblies or parts or components with a mass of more than 150 grams, a part of the workpiece is cut as a sample for testing. In order to avoid the exposed steel substrate at the incision to affect the test results, paint, wax or tape should be used to protect the incision.  

The same method can be used to prepare specimens for parts with complex shapes that are difficult to calculate the surface area. In addition to the five tests of coatings mentioned in the above standards, in actual production, ammonia tests, electrical conductivity tests, coating hardness tests, porosity tests, etc. are usually carried out.