In order to understand the protective performance of the coating on the surface of the workpiece as soon as possible, the accelerated corrosion that is, under a certain test environment or condition, the coating is enhanced more typically than the usual natural corrosion environment. Corrosion test methods, such as salt spray test, include neutral salt spray test, acetic acid salt spray test and copper salt accelerated salt spray test. In addition to the salt spray test, there are salt water immersion method, artificial sweat test method, gas corrosion method, corrosion mud test method, etc. There are also other corrosion-resistant methods under special conditions agreed by both parties , such as high-temperature anti-discoloration performance, etc. However, in the electroplating industry, the commonly used method for evaluating the corrosion resistance of coatings is the neutral salt spray test method. The main reason is that this method was established as an international standard earlier and was cited by many industries, thus becoming a common method for comparing and requiring the protective performance of coatings.
The neutral salt spray test is a corrosion resistance test method carried out with reference to ISO 3768 [GB 6458 "Neutral Salt Spray Test (NSS Test) for Metal Coverings ").
The test requirements for this method are described below.
(1) Test solution The solution used in the neutral salt spray test is a chemically pure sodium chloride solution prepared with distilled water , and its concentration is (50 ± 5 ) g/L. The pH value of the prepared above solution should be 6.5~7.2, which can be adjusted with hydrochloric acid or sodium hydroxide . In the case of acetic acid salt mist, acetic acid is added to the brine. If it is a CASS test, add cuprous chloride to it.
(2) The test equipment is better to use special salt spray test equipment. The volume of the salt spray chamber is not less than 0.2m3, preferably not less than 0.4m3. And to be equipped with spray gas source and salt water storage tank, salt mist collector, etc.
(3) Test conditions During the test, the temperature inside the salt spray chamber is 35°C± 2°C. The temperature when doing the CASS test is 50 °C ± 1 °C. The spray speed of salt mist is 0.7 ~3.0ml/ (80cm2 • h). The air supply pressure of the Air Compressor should be kept at 0. 7 ~ 1. 8kg/ cm2 (0.07 ~ 0.18MPa). It can be sprayed continuously for 24 hours, which is a cycle; it can also be sprayed for 8 hours, and rested for 16 hours, which is a cycle.
(4) Evaluation of the sample The evaluation of the sample is to open the box and observe the sample every cycle . If the test does not change color, no corrosion, and no other abnormalities (such as blistering, cracking, etc.), it must be done again. A cycle. Until different degrees of corrosion appear. Since there are multiple pieces of samples for each test , and there are usually reference samples, it is impossible for each piece to have the same corrosion degree, which requires a record for each observation. The evaluation of the degree of corrosion resistance is based on the number of passing cycles, the longer the passing cycle, the better the corrosion resistance .