Introduction to salt spray corrosion test

A. Salt spray corrosion

Corrosion is the destruction or deterioration of materials or their properties under the action of the environment. Most corrosion occurs in the atmosphere, the atmosphere contains oxygen, humidity, temperature changes and pollutants and other corrosion components and corrosion factors. Salt spray corrosion is one of the most common and destructive atmospheric corrosion. The salt mist here refers to the atmosphere of chloride, whose main corrosive component is the chloride salt in the ocean -- sodium chloride, which mainly comes from the ocean and inland saline areas. The corrosion of salt spray on the surface of metal material is caused by the electrochemical reaction of chloride ions with the inner metal through the oxidation layer and protective layer on the metal surface. At the same time, chloride ion contains 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 chlorination layer, turning the insoluble oxide into soluble chloride, and making the passivation surface become active. Cause extremely bad adverse reaction to the product.

Salt spray test and its connection with practice

Salt spray test is an environmental test which mainly USES the artificial simulated salt spray environment conditions created by salt spray test equipment to evaluate the corrosion resistance of products or metal materials. It is divided into two categories, one is natural environmental exposure test, the other is artificial accelerated simulated salt spray environmental test. The artificial simulated salt spray environment test is a kind of test equipment with a certain volume space -- salt spray corrosion test chamber. Compared with the natural environment, the chloride concentration in the salt spray environment can be several times or dozens of times of the salt spray content in the general natural environment, so that the corrosion rate is greatly increased, the salt spray test on the product is carried out, and the time to obtain the result is also greatly shortened. If a product sample is tested in a natural exposure environment, it may take 1 year to be corroded, while in a simulated salt spray environment, similar results can be obtained within 24 hours. The artificial simulated salt spray test includes neutral salt spray test, acetate spray test, copper salt accelerated salt spray test and alternating salt spray test.

(1) neutral salt spray test (NSS test) is one of the earliest and most widely used accelerated corrosion test methods. It USES 5% sodium chloride brine solution with PH in the neutral range (6 ~ 7) as spray solution. The test temperature was 35℃, and the sedimentation rate of salt spray was required to be between 1 ~ 2ml/80cm ^ 2. H.

(2) acetate spray test (ASS test) is developed on the basis of neutral salt spray test. It is to add some glacial acetic acid in 5% sodium chloride solution, so that the PH value of the solution drops to about 3, the solution becomes acidic, and finally the formation of salt fog from neutral salt fog to acid. Its corrosion rate is about three times faster than that of the NSS test.

(3) copper salt accelerated acetate spray test (CASS test) is a kind of rapid salt spray corrosion test recently developed in foreign countries. The test temperature is 50℃. A small amount of copper salt -- copper chloride is added into the salt solution to strongly induce corrosion. It corroded about eight times faster than the NSS test.

(4) alternating salt spray test is a comprehensive salt spray test, which is actually a neutral salt spray test plus a constant humidity and heat test. It is mainly used in the whole machine of cavity type, through the infiltration of the tidal environment, the salt spray corrosion is generated not only on the surface of the product, but also inside the product. It is the product in salt spray and wet and hot two environmental conditions alternating conversion, finally check the electrical and mechanical properties of the machine products have no change.

Salt spray test standards and test results

Standards are uniform rules for repetitive things and Outlines. Salt spray test standards are specific requirements for salt spray test conditions, such as temperature, humidity, concentration of sodium chloride solution and PH value, and also put forward technical requirements for the performance of salt spray test chamber. The salt spray test standard for the same product shall be selected according to the salt spray test characteristics and the corrosion rate of the metal and the sensitivity to salt spray. Here are a few salt spray test standards, such as GB/T2423.17-1993 "the basic electrical and electronic products environmental test procedure test Ka: salt spray test method of GB/T2423.18-2000 the electrical and electronic products environmental test part 2: test experiment Kb: salt fog, alternating solution (sodium chloride), GB5938-86" light industry products, metal plating and chemical treatment layer corrosion test method ", GB/T1771-91 "the paint and varnish neutral salt fog resistance measurement.

The purpose of salt spray test is to evaluate the quality of products or metal materials resistant to salt spray corrosion. The determination methods of salt spray test results include rating determination method, weighing determination method, corrosion occurrence determination method and corrosion data statistical analysis method. The rating method divides the percentage of the corrosion area and the total area into several levels according to certain methods. Weighing method is to weigh the weight of the samples before and after the corrosion test to calculate the weight of corrosion loss to evaluate the corrosion resistance quality of the samples. The appearance determination method of corrosions is a qualitative determination method. It USES the salt spray corrosion test to determine whether the product is corroded or not. Corrosion data statistical analysis method provides a method to design corrosion test, analyze corrosion data and determine the confidence of corrosion data.

There are three kinds of salt spray test: neutral salt spray test (N SS), AA SS and copper accelerated salt spray test (CA SS, also known as copper chloride acetate spray), among which the most widely used is neutral salt spray test. The basic content of salt spray test is to spray 5% sodium chloride solution in the test chamber at 35 ° c to simulate accelerated corrosion in seawater environment. The corrosion resistance is determined by the duration of resistance. The salt fog box volume is not less than 012 m 3, had better not be less than 014 m3, the temperature in the cabinet to 35 ℃ + 2 ℃, 70 ~ 170 kpa pressure spray, salt mist collector at least two of the 80 cm, collecting area of each 2. Participants plane and vertical direction into 15 ~ 30 ° to salt fog free settling in the surface to be tested, can't direct injection, test stand made of glass, plastic, specimen can't touch each other, each other drops; The temperature in the box was 35℃±2℃, and each collector collected solution (1 ~ 2)mL /h.

The performance parameters of salt spray test are as follows

Along CASS NSS

Compressed air pressure (KGF/marks) 1.00±0.01 1.00±0.01 1.00±0.01

The amount of spray (ml/80 marks ^ 2 /H) was 1.0 ~ 2.0 1.0 ~ 2.0 1.0 ~ 2.0

PH value 3.1 ~ 3.3 3.1 ~ 3.2 6.5 ~ 7.2

Experiment temperature (℃) 35±1℃ 50±1℃ 35±1℃

Air saturator temperature (℃) 47±1℃ 63±1℃ 47±1℃

---------

Note :1.ASS= acetate spray test

2.CASS= copper accelerated acetate spray test

3.NSS= neutral salt spray test
For the method of product evaluation results after salt spray test, please refer to gb5944-86.