Corrosion is the damage or deterioration caused by the action of the environment on a material or its properties. Most corrosion occurs in the atmospheric environment, which contains corrosive components and factors such as oxygen, humidity, temperature changes, and pollutants. Salt spray corrosion is a common and most destructive atmospheric corrosion. The corrosion of metal materials by salt spray is caused by the electrochemical reaction between the chloride ions that penetrate the oxide layer and protective layer of the metal surface and the internal metal. At the same time, chloride ions contain a certain amount of hydration energy, which is easily adsorbed on the pores and cracks on the metal surface to push out and replace oxygen in the oxide layer, transforming insoluble oxides into soluble chlorides, and turning passive surfaces into active surfaces.
Salt spray testing is an environmental test that primarily uses artificial simulated salt spray environmental conditions created by salt spray testing equipment to assess the corrosion resistance of products or metal materials. It is divided into two categories, one is natural environment exposure testing, and the other is artificial accelerated simulated salt spray environment testing. Artificial simulated salt spray environment test is the use of a test equipment with a certain volume space - a salt spray test chamber. Within its volume space, artificial methods are used to create a salt spray environment to assess the quality of the product's salt spray corrosion resistance performance. Compared with natural environments, the salt concentration of chloride in its salt spray environment can be several times or dozens of times that of general natural environments, greatly improving the corrosion rate. Conducting salt spray tests on products also greatly shortens the time for obtaining results. If a product sample is tested in a natural exposure environment, it may take one year for corrosion to occur, while under artificially simulated salt spray environment conditions, similar results can be obtained within 24 hours.
classification
The artificial simulated salt spray test includes neutral salt spray test, acetate spray test, copper salt accelerated acetate spray test, and alternating salt spray test.
Neutral salt spray test
Neutral salt spray testing emerged as the earliest and currently the most widely used accelerated corrosion testing method. Generally, it uses 5% sodium chloride saline solution, and the solution PH value is adjusted in the neutral range (6.5~7.2) as the solution for spray. The test temperature is set at 35 ℃, and the required settling rate of salt spray is between 1-3ml/80cm2. h, and the settling amount is generally between 1-2ml/80cm2. h.
Acetate mist test
The acetate spray test was developed based on the neutral salt spray test. It involves adding some glacial acetic acid to a 5% sodium chloride solution, reducing the pH value of the solution to around 3, making the solution acidic. The resulting salt mist also changes from neutral to acidic. Its corrosion rate is about three times faster than the NSS test.
Copper salt accelerated acetate spray test
The copper salt accelerated acetate spray test is a newly developed rapid salt spray corrosion test abroad. The test temperature is 50 ℃, and a small amount of copper salt copper chloride is added to the salt solution to strongly induce corrosion. Its corrosion rate is approximately 8 times that of the NSS test.
Alternating salt spray test
Alternating salt spray is a comprehensive salt spray test, which is actually a neutral salt spray test plus a constant damp heat test. It is mainly used for cavity type complete machine products. Through the infiltration of humid environment, salt spray corrosion not only occurs on the surface of the product, but also inside the product. It involves alternating the product under two environmental conditions: salt spray and humid heat, and finally assessing the electrical and mechanical properties of the entire product for any changes.
standard
The salt spray test standard specifies the conditions for salt spray testing, such as temperature, humidity, concentration of sodium chloride solution, and pH value. In addition, technical requirements are also proposed for the performance of the salt spray test chamber. The standard for salt spray testing for the same product should be selected based on the characteristics of the salt spray test, the corrosion rate of the metal, and the sensitivity to salt spray. Below are several salt spray test standards, such as GB/T2423.17-1993 'Basic Environmental Testing Procedures for Electrical and Electronic Products - Test Ka: Salt Spray Test Method', GB/T2423.18-2000 'Environmental Testing for Electrical and Electronic Products - Part 2: Test Kb: Salt Spray, Alternating (Sodium Chloride Solution)', and GB5938-86 'Corrosion Resistance Test Method for Metal Coatings and Chemical Treatment Layers of Light Industrial Products', GB/T1771-91 'Paints and Varnishes - Determination of Resistance to Neutral Salt Spray'.
Summary of Salt Spray Testing Standards
ISO 7253-1996 (Coatings)
BS 3900-F12-1997 (Coatings)
BS 7479:1991
IEC 60068-2-11:1981
GB/T 10125-1997 (Coatings)
GB 2423.17-2008
DIN 50021-1988
Standard for acid salt spray testing
ASTM B368-09
ISO 9227-2006
DIN 50021-1988
BS 7479:1991
Copper ion accelerated salt spray test standard
ASTM B368-09
ISO 9227-2006
DIN 50021-1988
BS 7479:1991
Standard for cyclic salt spray testing
ASTM D6899-2003
ASTM G85-02e1 Annex A5
ISO 11997-1:2005
ISO 11997-2:2000
SAE J2334:2002
WSK-M2G299
GM4298P
GM4476P
GM9540P
The purpose of salt spray test is to assess the salt spray corrosion resistance quality of products or metal materials, and the judgment of salt spray test results is the judgment of product quality. Whether the judgment results are correct and reasonable is the key to accurately measuring the salt spray corrosion resistance quality of products or metals. The methods for determining the results of salt spray tests include rating judgment, weighing judgment, corrosion occurrence judgment, and corrosion data statistical analysis. The rating judgment method divides the percentage of corrosion area to total area into several levels according to a certain method, and uses a certain level as the basis for qualified judgment. It is suitable for evaluating flat plate samples; The weighing judgment method is a method of evaluating the corrosion resistance quality of a sample by weighing the weight of the sample before and after the corrosion test, and calculating the weight of the corrosion loss. It is particularly suitable for assessing the corrosion resistance quality of a certain metal; The method of determining the appearance of corrosive substances is a qualitative judgment method, which determines whether the product undergoes corrosion phenomenon after salt spray corrosion test. Generally, this method is mostly used in product standards; The statistical analysis method for corrosion data provides a method for designing corrosion tests, analyzing corrosion data, and determining the confidence level of corrosion data. It is mainly used for analyzing and statistical corrosion conditions, rather than specifically for quality judgment of a specific product.