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Corrosion is damage or deterioration caused by the material or its properties under the action of the environment. Most of the corrosion occurs in the atmosphere, the atmosphere contains oxygen, humidity, temperature changes and pollutants and other corrosive factors. Salt spray corrosion is a common and most damaging atmospheric corrosion which is used for testing the anti-corrosion property for Galvanized Steel Coils & Sheets.
Salt mist corrosion to the surface of the metal material is due to the chloride ions that penetrate the metal surface of the oxide layer and the protective layer and gets the electro-chemical reaction with the internal metal. At the same time, chlorine ions contain some hydration energy, which is easily absorbed in the pores of the metal surface. And it displaces and cracks the oxide layer in the oxygen-substituted, makes the insoluble oxides into soluble chlorides, and makes the active surface become a passivated condition.
Salt spray test is a major use of salt spray test equipment created by the artificial simulation of salt spray environmental conditions to assess the corrosion resistance of metal materials. It is divided into two categories, one for the natural environmental exposure test, the other is the artificial acceleration of simulated salt spray environmental test.
Artificial simulation of salt spray environmental test is the use of a certain volume of space test equipment - salt spray test chamber-in its volumetric space with artificial methods, resulting in salt spray environment to the product of salt spray corrosion performance quality assessment. It is compared with the natural environment, its salt spray environment chloride concentration can be the general natural salt spray content of several times or several hundred times so that the corrosion rate greatly accelerated and the results of salt spray tests are also greatly shortened. If a sample is tested in a natural exposure environment, it may take one year to be corroded and a similar result can be obtained for 24 hours in the simulated salt spray environment.
Laboratory simulation salt spray test can be divided into four categories:
⑴ neutral salt spray test (NSS test) is the earliest application of the most widely used in an accelerated corrosion test method. It uses 5% sodium chloride solution and the pH value of the solution in the neutral range (6.5 ~ 7.2) as a spray solution. The test temperature is 35 ℃, requiring salt fog sedimentation rate of 1 ~ 2ml / 80cm / h.
⑵ acetic acid salt test (ASS test) is based on the neutral salt spray test to be developed. It is in 5% sodium chloride solution by adding some glacial acetic acid, the solution of the PH value is reduced to about 3, the solution becomes acidic, and finally the formation of salt mist from neutral salt mist into acidic. Its corrosion rate is about 3 times faster than the NSS test.
(3) Copper salt accelerated acetic acid salt test (CASS test) is a newly developed rapid salt spray corrosion test, the test temperature of 50 ℃, salt solution by adding a small amount of copper salt - copper chloride, strongly induced corrosion. Its corrosion rate is about 8 times the NSS test.
⑷ alternating salt spray test is a comprehensive salt spray test, it is actually a neutral salt spray test plus constant hot and humid test. It is mainly used for cavity-type machine products, through the tidal environment of the infiltration, so that salt spray corrosion not only on the product surface but also within the product. It is the product in the salt spray and hot and humid two environmental conditions under the alternating conversion, the final assessment of the whole product of electrical properties and mechanical properties have changed.
The test results of the salt spray test are generally given in qualitative rather than quantitative form. There are four specific ways to determine:
① rating method is to determine the percentage of corrosion area and the total area by a certain method is divided into several levels, with a certain level as a basis for qualified, it is suitable for flat panel samples for evaluation.
② Weighing method is to judge the weight of the sample by weighing the weight of the sample before and after the corrosion test. The corrosion resistance of the sample is evaluated by the weight of the corrosion loss. It is especially suitable for the evaluation of the corrosion resistance of a metal.
③ Determination of corrosive method is a qualitative determination method, it is salt spray corrosion test, the product is corrosive to determine the phenomenon of the sample, the general product standards are mostly used in this method.
④ Corrosion data statistical analysis method provides a design corrosion test, analysis of corrosion data, determine the confidence of corrosion data method, it is mainly used for analysis, statistical corrosion, rather than specifically for a specific product quality determination.