Corrosion is the destruction or deterioration of materials or their properties caused by the environment.

1. Most corrosion occurs because of the unique properties in the atmospheric environment. The atmosphere is made up of corrosive components and corrosive factors such as oxygen, humidity, temperature changes and pollutants.

The salt spray corrosion testing method is a standardized and widely used practice in a variety of industries.  The salt spray mentioned here refers to the artificially created atmosphere of chloride. Its main corrosive component is chloride salt such as that found in sodium chloride, which comes from the ocean and inland saline-alkali areas.

The corrosion that occurs on the metal’s surface is the result of the electrochemical reaction between the chloride ion contained in the oxide layer on the metal surface and the protective layer and the internal metal. At the same time, chloride ions contain a certain amount of hydration energy, which can be easily absorbed by the pores and cracks on the metal surface to displace and/or replace the oxygen in the chlorinated layer, turning insoluble oxides into soluble chlorides, and converting the passivated surface into an active surface. The goal of this test is to understand how long the product itself can withstand these adverse reactions during general usage.  

2. Salt spray corrosion test and its real-world applications

Salt spray test is an environmental test that primarily uses artificially simulated salt spray environmental conditions to assess the corrosion resistance of products or metal materials.

It is divided into two categories; one is the natural environment exposure test, and the other is the accelerated simulated salt spray environment test. The artificially simulated salt spray environment test uses a salt spray testing chamber to assess the product’s ability to resist corrosion.

Compared with the properties generally found in the natural environment, the chloride concentration of the salt spray can be several to two times higher. This greatly accelerates the corrosion rate. For example, if a product sample is tested in a natural exposure environment, it may take up to a year to corrode, whereas the artificially simulated can produce similar effects in just 24 hours.

The artificial simulated salt spray tests include a neutral salt spray test, an acetic acid salt spray test, copper salt accelerated acetic acid salt spray test, and an alternating salt spray test.

A. The neutral salt spray test (NSS test) is one of the oldest methods for accelerated corrosion testing and enjoys the widest application field. It uses a 5% sodium chloride salt aqueous solution, with a pH value adjusted to the neutral range (6-7). The test temperature is 35℃, and the sedimentation rate of the salt spray must be between 1～2ml/80cm².h.

B. The acetic acid salt spray test (ASS test) is developed based on the neutral salt spray test. It adds some glacial acetic acid to 5% sodium chloride solution to reduce the pH value of the solution to about 3. In doing so the solution becomes acidic, and the salt fog that has formed turns from neutral salt fog to acid. Its corrosion rate is about 3 times faster than the NSS test.

C. The copper salt accelerated acetic acid salt spray test (CASS test) is a rapid salt spray corrosion test recently developed overseas. The test temperature is 50℃. A small amount of copper salt, copper chloride is added to the salt solution to vigorously induce corrosion. Its corrosion rate is approximately 8 times that of the NSS test.

D.  The alternating salt spray test is a comprehensive salt spray assessment. It is made up of a neutral salt spray test in a salt spray corrosion test chamber plus a constant damp heat test. It is mainly used for cavity-type complete products. As a result of the wet environment created throughout the test, the salt spray is able to penetrate through the surface into the deeper layers of the product. The purpose of alternating the two testing environments (salt spray and damp heat) is to improve the accuracy with which one can judge the electrical and mechanical properties of any given product.

Our salt spray test is based on the GJB548B standard, method 1009, and its fundamental properties are: the concentration of the salt solution must be 0.5%~3.0% (percent by weight) of deionized water or distilled water.  The salt used must be sodium chloride. When measuring at (35±3)℃, the pH value of the salt solution must be between 6.5 and 7.2. Only chemically pure hydrochloric acid or sodium hydroxide (dilute solution) can be used to adjust the pH. To simulate the accelerated corrosion method of a seawater environment, the length of its resistance time determines its ability to resist corrosion.

3. Conclusion

With the development of integrated circuit metal packages, the corresponding environmental adaptability assessments have become more comprehensive and thorough. The salt spray corrosion test is the main methodology for evaluating the environmental corrosion resistance of products. Therefore, improving the corrosive resistance of metal packaging has become a crucial aspect of the manufacturing process. Through technical research, our company strives to solve the issue of corrosion through the process of heat treatment, high temperature sealing process, electroplating process and other methods of processing metal materials.  This way we can effectively improve the overall corrosion resistance of the metal package and meet the needs of customers’ uses for these types of products.