Salt spray testing is an environmental test which mainly use the artificial simulated salt spray environment conditions created by salt spray chamber to evaluate the corrosion resistance of products or metal materials.
Salt spray testing divided into two categories: one is the natural environmental exposure test, and the other is the artificial accelerated simulation of salt spray environmental test. Artificial simulation of salt fog environment test is to use a certain volume space of the test equipment – salt fog chamber, in its volume space with the artificial method, create salt fog environment to test the quality of product corrosion resistance to salt fog performance.
Compared with the natural environment, the salt concentration of chloride in the salt fog environment can be several times or dozens of times higher than that in the general natural environment, which greatly improves the corrosion speed and shortens the time for the product to be tested by salt fog.
|●||Copper + nickel + chromium or nickel + chromium plating on steel surface||Application of Salt Spray Testing|
|●||Nickel + chromium plating on copper or copper alloy surface|
|●||300 series or 400 series stainless steel surface plated with nickel + chromium|
|●||Copper + nickel + chromium plating on aluminum or aluminum alloy surface|
|●||Zinc alloy surface copper + nickel + chromium plating|
|●||Plastic electroplated parts|
The Artificial Simulated Salt Spray Testing Includes:
A. The neutral salt spray test (NSS) is the earliest and most widely used accelerated corrosion test method. It use 5% sodium chloride brine solution, the PH of the solution is adjusted in the neutral range (6 ~ 7) as the spray solution. Salt spray test temperature is 35 ℃, requirements of sedimentation rate in 1～2ml/80cm2.h
B. The acetate fog test (ASS test) is developed on the basis of the neutral salt fog test. It is to add some glacial acetic acid in 5% sodium chloride solution, so that the PH value of the solution is reduced to about 3, the solution becomes acidic, and finally the formation of salt fog from neutral salt fog into acidic. It corrodes about three times faster than the NSS test.
C. Copper salt accelerated acetic acid salt spray test (CASS) is a newly developed a kind of foreign fast salt-fog corrosion test, test temperature is 50 ℃, salt solution to add a small amount of copper salt, copper chloride, strongly induced corrosion. It corrodes about eight times faster than the NSS test.
Salt Spray Testing Standard and Test Result Determination:
A. Salt spray test standard is the salt fog test conditions, such as temperature, humidity, sodium chloride solution concentration and PH value to do a clear and specific provisions, in addition to the salt spray chamber performance put forward technical requirements. The salt spray test standard for the same product shall be selected according to the characteristics of salt spray test, the corrosion speed of the metal and the sensitivity to salt spray.
B. The purpose of salt spray test is to evaluate the corrosion resistance quality of products or metal materials, and the judgement of salt spray test result is just the judgement of product quality. Whether the judgement result is correct and reasonable is the key to correctly measure the corrosion resistance quality of products or metal.
C. The judging methods of salt spray test results are: rating judging method, weighing judging method, corrosion occurrence judging method, corrosion data statistical analysis method.
|●||The rating judging method is to divide the percentage of corrosion area to total area into several levels according to a certain method.|
|●||Weighing judging method is to weigh the weight of samples before and after the corrosion test, calculate the weight of corrosion loss to evaluate the corrosion resistance quality of samples, it is especially suitable for a certain metal corrosion resistance quality assessment.|
|●||The corrosion occurrence judging method is a qualitative determination method. It determines whether the product will produce corrosion phenomena after the salt spray corrosion test to determine the sample. Generally, this method is adopted in most product standards.|
|●||The method of statistical analysis of corrosion data provides a method for designing corrosion tests, analyzing corrosion data and determining the confidence of corrosion data. It is mainly used for analyzing and statistical corrosion conditions, rather than for the quality determination of a specific product.|
Introduction of Salt Spray Corrosion: Salt spray corrosion is the destruction or deterioration of a material or its properties under the action of the environment. Most corrosion occurs in the atmosphere, which contains oxygen, humidity, temperature changes, pollutants and other corrosion components and corrosion factors.
Salt spray corrosion is one of the most common and destructive atmospheric corrosion. Here the salt fog refers to the atmosphere of chloride, its main corrosion component is the Marine chloride salt – sodium chloride, it mainly comes from the ocean and the inland saline-alkali region.
The corrosion of metal surface by salt spray is caused by the electrochemical reaction of chloride ions which penetrate the oxide layer and protective layer on the metal surface and the inner metal.
At the same time, chloride ions contain a certain hydration energy, which is easy to be adsorbed in the pores and cracks of the metal surface to squeeze out and replace the oxygen in the chlorinated layer, turn the insoluble oxides into soluble chlorides, and turn the passivated surface into an active surface. Cause extremely bad adverse reaction to the product.