PVD coating is often used in corrosive environments, sometimes it is in contact with corrosive materials and media, and sometimes it is used in corrosive atmospheres. For example, many plastic molds exposed to acidic plastics that are highly corrosive (such as PVC, that is, polyvinyl chloride). Besides, the coated automotive parts that used in the coastal or tropical areas suffer severe corrosion. The ubiquitous possibility of corrosion requires that the research, development, and application of coatings must take into account the working environment. Corrosion tests must be performed if there is a possibility of corrosion.
The salt spray test is the most commonly used corrosion resistance test. The test uses universal salt spray test chamber to test the reliability of the tested sample by means of salt spray corrosion. Salt spray is a dispersion system consisting of tiny droplets of salt in the atmosphere. It is one of the three environmental protection series of artificial environments. There are many companies need to simulate the destructive effect of the ocean's surrounding climate on the product, so the salt spray test chamber emerges. The salt spray test chamber is divided into neutral salt spray and acidic salt spray based on their difference of standards and test methods. The salt spray test is also known as the NSS and CASS tests. And the test should be carried out under the relevant national standards (GB/T2423.17-2008).
The corrosion test of PVD coating is not a mandatory national standard. It is only for the purpose of qualitatively understanding the corrosion resistance of the coating. Therefore, the salt test is not necessarily performed in many cases, and the test may be performed according to local conditions and the existing corrosion conditions. For example, in order to study the corrosion resistance of the composite coating (TiN+(Ti,Cr)N+Ti), a certain document test it by electrochemical method. The abrasion and corrosion experiments were carried out in a 0.lmol/L sodium chloride solution, Ag/AgCl is used as reference electrode and platinum wire as an auxiliary electrode. And then connect the EG&GPAR 273A potentiometer and the 1250 Hz inverter analyzer for data acquisition. and use a PMMA box with solution and transfer shafts, a conveyor shafts to connect with friction plates and motors. In addition, a movable rod supports the sample and the entire system exerts the load on the friction plate. The position of the sample is adjustable for optimal test conditions. The sample to be tested shall be packed in polyester resin with only the tested part exposed and the edges sealed to avoid crack corrosion problems. The conductive wire is also placed in the resin, and brazed on the back of the sample. The wear-corrosion test is in the form of a disk-block with a friction plate rotation speed of 40 rpm, a load of 0.08-0.15 kgf, and a friction disk of 43 mm alumina disk or soda lime glass disk. The experimental period is 1h, and the accumulated wear path length is about 325m. The integrity of the coating was observed by monitoring the corrosion voltage; the polarization resistance was obtained by impedance spectroscopy. After the test, an analytical balance was used to obtain the weight loss value. The samples were observed with an optical instrument and a scanning electron microscope (SEM) to study wear morphology after electrochemical testing under wear conditions. The figure below shows the measured wear erosion curve.