The salt spray corrosion test can simulate the salt spray corrosion in the atmospheric environment, speed up the entire corrosion process, and obtain the test results in a short time. It plays an important role in the automotive industry and can test the reliability and quality of products. The first type of salt spray test is the neutral salt spray test, and the corrosive environment is close to neutral. On this basis, various tests have appeared, such as ASS, CASS, etc., and the alternating salt spray test has been developed subsequently, which can circulate damp heat. , salt spray and drying stages, to realize the alternate conversion of various environments, which can effectively test product performance. At this stage, there have been a variety of salt spray corrosion test standard methods, which are suitable for testing different auto parts.
1 Salt spray test
1.1 Salt spray corrosion
Salt spray is a common atmospheric corrosion that exists in the natural environment and is relatively destructive. The salt spray contains a large amount of chloride ions, which will directly penetrate the surface of the metal material. Neither the oxidation nor the protective layer can block the chloride ions. After passing through the surface of the material, it will electrochemically react with the metal, causing huge damage to the material. Surface corrosion will also change the original performance. After long-term corrosion, it will be put into the product through voids and cracks to further corrode.
1.2 Salt spray test
Salt spray corrosion is more damaging to metal materials. In order to reduce salt spray corrosion, a salt spray test can be carried out to simulate the state of atmospheric salt spray corrosion through the laboratory. Compared with the natural environment, the laboratory simulated salt spray corrosion is stronger. , the chloride concentration can reach several times that of the atmospheric environment, which can quickly show the corrosion of metal materials in the laboratory. In the laboratory, the corrosion situation of the natural environment can be simulated for one year in 24 hours, which can greatly shorten the experimental time, and the simulation results are more accurate. During the salt spray test, a large number of professional simulation equipment will be equipped in the laboratory. It is a kind of environmental test to realize the monitoring of the environmental conditions of salt spray.
2 Types of salt spray test
2.1 Neutral salt spray test (NSS)
At this stage, the most commonly used test method is the neutral salt spray test, which is also the earliest test method. During the neutral salt spray test, the salt spray should be continuously provided, the temperature should be around 35 ℃, and the humidity should be above 95%. The amount of sedimentation, the salt spray is deposited on the surface of the measured object, and then the test time is determined according to the actual demand to determine the corrosion condition.
2.2 Acetic acid salt spray test (ASS)
On the basis of the neutral salt spray test, the acetic acid salt spray test was developed. The test conditions are basically the same as those of the neutral salt spray test, but a certain amount of glacial acetic acid must be added to adjust the pH value. The pH of the laboratory is adjusted to be acidic (the pH value is about 3), so that the properties of the salt spray are changed, and the tested object is in an acidic salt spray environment, and the corrosion rate is accelerated, which can reach 3 times the speed of the neutral salt spray test.
2.3 Copper accelerated acetate spray test (CASS)
In recent years, my country’s economy has developed rapidly, and it has also driven industrial development. The corrosion rate of salt spray test also needs to be accelerated. Therefore, more products are tested using copper-accelerated acetate salt spray test, and the test temperature is increased to 50 ℃. Adding a small amount of copper chloride to the sodium chloride solution can greatly increase the degree of corrosion. At the same time, the pH value is generally set at 3.1 to 3.3, the corrosion degree exceeds that of the acetic acid salt spray test, and the corrosion rate can reach about 8 times that of the neutral salt spray test.
2.4 Alternating salt spray test
The alternating salt spray test is a comprehensive test. It uses different test conditions comprehensively and combines and circulates in the laboratory, which can make the simulated environment closer to real life. There is a certain amount of corrosion inside. During the alternating salt spray test, two environments will be alternately converted, one is a salt spray environment, and the other is a humid and hot environment. After the simulation test is completed, the electrical properties and mechanical properties can be detected. It is mainly used to test cavity-type complete machine products.
3 Characteristics of cyclic corrosion salt spray test
3.1 Acceleration
The acceleration of the cyclic corrosion salt spray test is mainly reflected in three aspects.
1) The salt spray is continuously sprayed to ensure the continuous corrosion of the surface. In the process of cyclic salt spray corrosion, a quantitative amount of salt spray will be sprayed out, and a series of changes will occur in the test piece, including rust, air bubbles, etc., and the salt spray is continuously supplied during this process, which can continuously corrode the test piece .
2) The accumulation of salt mist in the wet and hot stage will spread to the inside of the DUT. After the DUT enters the damp-heat stage, although the salt mist is not sprayed on the surface, the originally accumulated salt mist will still adhere to the surface of the DUT, penetrate into the interior under the humid and hot environment, and deeply corrode the DUT.
3) The drying stage accelerates the surface corrosion rate. After the DUT enters the drying stage, the relative humidity will gradually decrease, which increases the concentration of the salt solution, making it more corrosive and accelerating the corrosion of the DUT.
During the cyclic corrosion salt spray test, the DUT is always in a state of cyclic alternation. The traditional salt spray test method is too simple, and serious corrosion occurs on the surface of the DUT only. Although it will penetrate into the interior, the corrosion simulation will deviate from the reality. The cyclic alternating corrosion will alternate the three states of drying, damp heat and salt spray, the simulated environment is closer to the real, speed up the test speed, and make the test results more accurate and real.
3.2 Complexity
The environment simulated by the cyclic salt spray experiment is closer to the atmospheric environment, and there are many cyclic components. It can be reasonably selected according to the actual test requirements, and it is as close to the use environment as possible, and the complexity of the test is higher. Due to the need to switch multiple cycle combinations for cyclic salt spray corrosion, we have developed the compound salt spray chamber (salt spray + temperature + humidity integrated chamber) to allow our customers to complete the comprehensive testing programs with 1 chamber.
4 Standard method for circulating salt spray for automotive materials
The cyclic salt spray test was first used in the automobile industry, and it can effectively evaluate the reliability of products. There are mainly the following three standard methods of cyclic salt spray.
4.1 CCT cycle
This method is mainly based on the neutral salt spray method, adding two links of drying and damp heat. After many subsequent optimizations and improvements, it has been developed to CCT IV, which is also a method recognized by the industry at present. It is mainly used for testing automotive steel plates and zero part. The damp-heat conditions are 60°C and 95% for 75 minutes, the drying temperature and humidity are 60°C and below 30% for 160 minutes, and the damp-heat stage is experienced again.
4.2 GMW 14872
This method was first applied in the USA. It adopts a combination of superimposed environments. The ambient temperature is 25 °C and 45% relative humidity. After 8 hours, it enters the moist heat stage, and then enters the drying stage. This method can add salt spray according to the actual environment. Factors such as mechanical impact, grit and other factors are added, and the number of cycles can be selected in combination with automotive test components when this method is selected.
4.3 SAE J2334
This method is an internationally published standard method. The temperature is 25 °C, the relative humidity is 45%, and it lasts for 8 hours to enter the damp-heat stage. The temperature is raised to 50 °C, the humidity reaches 100%, and it lasts for 6 hours to enter the salt spray stage. drying section. This method has a longer drying time and is generally used for testing automotive coating products.
4.4 PV 1210
This method is mainly used in Germany and has a relatively wide range of applications. It is connected to the salt spray, damp heat and storage environment. First, the neutral salt spray state is maintained. After 4 hours, it enters the ambient temperature. Finally, it is the damp heat stage. This method cancels the drying stage and uses For testing coated car bodies, sheets and structural components.
5 Individual and comprehensive salt spray and corrosion tests in the automotive industry
With the improvement of the level of science and technology, people put forward higher requirements for the quality of automobile products. Whether in the design, research and development stages, it is necessary to strengthen the rational use of inspection methods. In the design stage, attention should be paid to product functions and service life. However, it is impossible to observe product performance in a short time in the daily environment, and requires a long observation time, which requires the use of salt spray corrosion test, especially when automotive products are exposed to the air most of the time, it is inevitable that salt spray corrosion will occur.
5.1 Single test
At this stage, my country’s auto industry mainly uses a single test, among which the more critical test items include condensation test, salt spray test, etc. Although the condensation experiment is simple to operate, the accuracy of the results cannot be guaranteed, and the actual results are closely related to the test sequence. In the two cases of long-term humidity and condensation water evaporation, completely different experimental results will be obtained, and the salt spray test will change the corrosion type according to the automobile parts, so there is a serious problem in the single test, although a large number of similar samples can be repeated, The purpose of compensation is achieved, but the correlation between the laboratory simulation environment and the atmospheric environment cannot be guaranteed. At the very beginning, a German business association began to study the salt spray test. The main research direction was the correlation between the test environment and the atmospheric environment, and a series of standards were formulated according to the results of the research. For example, when testing the paint process of automobiles, it is necessary to choose the corrosion state that can simulate the running of the car, focusing on the joints, edges and other positions.
Existing studies have shown that the effect of the laboratory salt spray test meets the test requirements, and the progress of the test simulation is accelerated. When testing according to the test standard, the time can be greatly shortened. For example, it takes 4 to 6 weeks to test auto parts in the atmospheric environment, but The salt spray test can be completed in 4 to 6 cycles. Even if the component has a multi-layer structure, the corrosion process of the component can be well simulated, and a single cycle can achieve the effect of 3 weeks of atmospheric environmental testing. If the igniter in the body cavity is tested, this cycle is equivalent to the effect of testing in the atmospheric environment for 2 weeks. Although the single test effect is relatively good, it still cannot meet all the test requirements of the automobile components, so there are comprehensive tests, alternating tests, etc.
5.2 Comprehensive test
The comprehensive test is developed on the basis of a single test, adding many different simulated environments, mainly the salt spray stage, the drying stage and the moist heat stage. Different combination stages can be alternately cycled, which can make the laboratory environment closer to the reality , and can also achieve the purpose of speeding up the test. At this stage, a large number of standard methods of circulating salt spray for automotive materials have emerged in the world, such as CCT cycle, GMW 14872, SAE J2334, PV 1210, etc., which can be selected according to actual needs. After determining the cycle test method, it is necessary to take into account the test requirements of auto parts and determine a scientific test combination in order to achieve the expected test effect.
5.3 Difficulties and Solutions of Test Methods
In the process of salt spray test, some difficult problems are often encountered, mainly including the following two aspects.
5.3.1 Temperature and climate control issues for alternating tests
In order to be closer to the real environment, climatic factors can also be added to the salt spray test, which requires the equipment control system to have corresponding functions. Foreign products basically adopt a modular structure and are equipped with a direct temperature control system, which has superior performance in all aspects. , The auxiliary heat exchange medium can be used to buffer during the cycle, so that the test chamber is always stable, and the temperature and humidity will not change drastically, so as to ensure the stability of the test. In addition, the direct temperature control system has many advantages, and it can also be continuously tested in a state close to humidity saturation.
5.3.2 Constant spray
During the salt spray test, a constant spray is also used that can help the salt spray test reproduce the environmental conditions. However, the existing methods cannot directly solve the problem of constant spray, such as air conditioning, salt water supply source, etc. The main reason is that salt water is the atomization supply of the nozzle and maintains a constant spray volume during operation. The key is to examine the degree of vacuum, liquid level, etc. Therefore, in order to truly simulate the atmospheric environment and ensure the accuracy of salt spray corrosion results, it is necessary to strengthen the research and development of high-tech products in order to provide more product technical support for the salt spray test.
The salt spray corrosion test can test various performances of automobile parts, and truly restore the atmospheric salt spray corrosion environment, which is widely used in automobile testing work. At this stage, the cyclic corrosion salt spray test is mainly used, which can alternately convert different corrosion environments and improve the accuracy of the test. In the process of salt spray corrosion, the test method must be selected in combination with the auto parts to achieve the effect of twice the result with half the effort.
GAAT offers a full range of salt spray (salt fog) testing chambers including standard size items and customized walk-in configurations. Please feel free to send your inquiry to us and we look forward to working with you!
