Standards of salt spray tester/salt spray corrosion test and its application in automobile industry

With the rapid development of national industry, technology and economy, the application range of industrial products is getting wider and wider. In order to ensure the quality and life of products, many industrial products need to be verified by salt spray tester, especially automobile products. In recent years, the demand for salt spray test is increasing, and many practitioners are confused about how to carry out salt spray test, how to improve test efficiency and how to choose salt spray test standards. Therefore, the author consulted the relevant information at home and abroad and edited the cost. This paper systematically introduces what salt spray tester is. Types of salt spray test; How to carry out salt spray and corrosion test correctly; Choice of international and domestic standards in different industries. Especially, the application of salt spray and corrosion test in automobile industry is introduced.

1. Foreword
With the rapid development of national industry, technology and economy, there are higher requirements for product quality and reliability. Therefore, more and more products need to be tested for temperature and humidity, salt spray, corrosion and so on. The content of this article is as follows:

1) summary of salt spray test,

2) Type of salt spray test,

3) Characteristics of cyclic corrosion salt spray,

4) Industry market demand of cyclic corrosion salt spray test,

5) Application of domestic and international standards for salt spray and corrosion test in various industries,

6) Application of salt spray and corrosion test in automobile industry.

2. Overview of salt spray test:

2.1. Salt spray corrosion is a common and most destructive atmospheric corrosion. Chloride ions in salt spray will penetrate the oxide layer and protective layer on the surface of metal materials, and have electrochemical reaction with internal metals, thus damaging the surface and properties of metal materials, and further affecting and damaging products through pores and cracks on the metal surface.

 

2.2. salt spray test

Salt spray test is an environmental test to test the corrosion resistance of products or metal materials by simulating salt spray environmental conditions with salt spray test equipment. It is a laboratory simulation of the atmospheric salt spray state in the environment, and has the characteristics of standardization and acceleration. Compared with the natural environment, the salt concentration of chloride in salt spray test equipment is usually several times of the salt spray content in natural environment; Even dozens of times. Therefore, the corrosion rate is greatly improved. After exposure to salt spray corrosion in natural environment for one year, it can be completed in artificial simulated salt spray test environment for 24 hours, and similar results are obtained.

3. Types of salt spray test
3.1 Neutral Salt Spray Test (NSS)

An accelerated corrosion test method which is the earliest and most widely used at present appears. Test conditions are continuous salt spray at 35˚ C； The humidity is above 95%; 5% sodium chloride saline solution; The pH value of the solution is set in the neutral range (6.5 ~ 7.2). The mist reduction amount is 1 ~ 2 ml/(h 80 cm2). Salt mist is deposited on the tested test piece, and its surface corrosion state is observed after a certain time.

3.2 acetic acid salt spray test (ASS)

Developed on the basis of neutral salt spray test. On the basis of NSS, some glacial acetic acid was added into 5% sodium chloride solution, and the pH value of the solution was reduced to about 3, forming acid salt mist. The corrosion rate is about 3 times that of NSS test.

3.3 copper accelerated acetic acid salt spray test (CASS)

In order to adapt to the rapid development of economy and industry, in recent years, more products began to adopt this accelerated acetate mist corrosion test. The test condition is that the temperature is 50℃ and a small amount of copper salt-copper chloride is added into the salt solution, which strongly induces corrosion. The pH value of salt spray collection solution is 3.1-3.3, and the corrosion rate can reach 8 times of NSS test.

3.4 Alternating salt spray test (cyclic corrosion salt spray test)

Alternating salt spray test is a comprehensive salt spray test, which combines salt spray, dryness, damp heat (soaking, moisture and condensation, etc.) to simulate the actual application environment of products. Through the infiltration of tidal environment, salt spray corrosion occurs not only on the surface of the product, but also inside the product. In this test, the products are changed alternately under salt fog and damp heat, and finally, the electrical and mechanical properties of the whole product are tested. This test is mainly used for the test of cavity-type whole machine products.

 

Salt spray test types and test requirements

Test method project	Neutral salt spray (NSS)	Acetate mist (AASS)	Copper accelerated acetate fog (CASS)
Temperature	35 oC +/-2℃	35 oC +/-2℃	50 oC +/-2℃
Average settlement (horizontal collection area of 80 square	15 mL/h +/-0.5 mL	15 mL/h +/-0.5 mL	15 mL/h +/-0.5 mL
Sodium chloride concentration (collecting solution)	50 g/L +/-5 g/L	50 g/L +/-5 g/L	50 g/L +/-5 g/L
PH value (collecting solution)	6,5 to 7,2	3.1 to 3.3	3.1 to 3.3

4. Characteristics of cyclic corrosion salt spray test:

4.1 Acceleration

1) In the salt spray stage, salt spray is sprayed on the tested part, and the salt spray solution acts as corrosion medium, causing electrochemical corrosion, rust, blistering or other changes to the tested part. Through the continuous spray of salt spray, the corrosion medium can be kept updated, and the corrosion of the sample surface can be continuously developed and carried out;
 

2) In the wet and hot stage (soaking, wetting and condensation, etc.), the corrosive medium accumulated on the surface of the sample gradually diffuses from the surface to the inside of the sample, thus causing corrosion in the depth direction of the sample;
 

3) In the drying stage, with the decrease of humidity, the concentration of salt solution on the sample surface increases, which leads to the acceleration of surface corrosion rate. Through the combination of salt spray, wet and dry three stages, the simple state of traditional salt spray is changed, so that corrosion not only occurs on the surface of the tested part, but also penetrates into the tested part quickly, which accelerates the corrosion degree of the tested part, shortens the test time and makes the test stricter and faster.
 

4.2 Complex cyclic salt spray test can simulate the real dry-wet alternation state in atmospheric environment. Due to the different cycle combinations, it can present a variety of test environments, which are closer to the real use environment of the tested parts, and can better check the corrosion state of the tested parts in the real environment, and the test state is more complex. Test operations due to ring corrosion need to be switched between multiple cycle combinations. Initially, it can only be done in different test chambers by manual operation. With the development of technology, at present, automatic cyclic salt spray test has been completed through a single test chamber. Due to the complexity of test and structure, the domestic salt spray test box can not be fully supported at present. Only some imported equipment can fully automatically complete the cyclic corrosion salt spray test process.