Share the method of artificial simulated salt spray test and the factors affecting salt spray corrosion

Date：2022-07-26 15:57:19Views：854

Salt spray test is a clear and specific regulation of salt spray test conditions, such as temperature, humidity, sodium chloride solution concentration and pH value. In addition, it also puts forward technical requirements for the performance of salt spray test chamber. 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 its judgment results are correct and reasonable is the key to correctly measure the salt spray corrosion resistance quality of products or metals.

The artificial salt spray test also includes neutral salt spray test, acetate spray test, copper salt accelerated acetate spray test, and alternating salt spray test.

1. Neutral salt spray test(NSS test) is the most widely used accelerated corrosion test method in the field at present. Select 5% sodium chloride solution and adjust the pH value of the solution to the neutral range (6~7) as the spray solution. The test temperature is 35 ℃, and the salt spray sedimentation rate is between 1~2ml/80cm2.h.

2. Acetate mist test(ASS test) shall be developed under the premise of neutral salt spray test. This is to add some acetic acid to the 5% sodium chloride solution to make the solution possible. When the pH value drops to 3, the solution turns sour, and the resulting salt mist changes from neutral salt mist to acid. Its corrosion rate is about 3 times faster than that of NSS test.

3. Copper salt accelerated acetate spray test(CASS test) is a rapid salt spray corrosion test developed recently abroad. The test temperature is 50 ℃, and a small amount of copper salt copper chloride is added to the solution to strongly induce corrosion. Its corrosion rate is about 8 times that of NSS test.

4. Alternating salt spray testIt is a comprehensive salt spray test, which is actually a neutral salt spray test and a stability cold wet test. It is mainly used for internal cavity products of the whole machine. Through the penetration of humid environment, salt spray corrosion occurs not only on the surface of goods, but also in the interior of goods. This is to replace and convert commodities under salt fog and cold and wet environmental conditions, and finally evaluate whether the mechanical strength and mechanical properties of the whole product have changed.

The main factors affecting the salt spray test results include: test temperature and humidity, salt solution concentration, sample placement angle, solution pH value, salt spray sedimentation, spray method, etc.

1. Test temperature and humidity

Temperature and relative humidity affect the corrosion effect of salt spray. The critical relative humidity of metal corrosion is about 70%. When the relative humidity reaches or exceeds the critical humidity, the salt is easy to deliquesce and produce electrolytes with good conductivity. When the relative humidity decreases, the solution concentration will increase until the crystalline salt precipitates, and the corrosion rate will decrease accordingly.

The higher the salt spray corrosion rate, the faster the test temperature. According to the international organization for standardization iec60355:1971 "anappressaloftheproblemsofacceleratedtestingforatmosphericcorrosion" standard, "for every 10 ℃ rise in temperature, the corrosion rate increases by 2-3 times, and the electrolyte conductivity increases by 10-20%. This is because the temperature rises, the molecular motion intensifies, and the chemical change speed accelerates. For neutral salt spray test, most scholars think that the test temperature of 35 ℃ is more appropriate. If the test temperature is too high, the salt spray corrosion mechanism is quite different from the specific situation.

2. Salt solution concentration

The effect of salt solution concentration on corrosion rate is related to the type of material and coating. When the concentration is lower than 5%, the corrosion rate of steel, nickel and brass increases with the increase of concentration; When the concentration is greater than 5%, the corrosion rate of these metals decreases with the increase of concentration. This situation can be explained by the oxygen content in the solution. The oxygen content in the solution is related to the concentration of salt. In the low concentration range, the oxygen content increases with the increase of salt content. However, when the salt concentration increases to 5%, the oxygen content reaches relative saturation. If the salt content continues to increase, the oxygen content will decrease accordingly. With the decrease of oxygen content, the depolarization ability of oxygen decreases, that is, the corrosion effect decreases. But for zinc, cadmium, copper and other metals, the corrosion rate always increases with the increase of solution concentration.

3. Sample placement Perspective

The placing angle of samples has a significant impact on the results of salt spray test. The sedimentation direction of salt fog is close to the vertical direction. When the sample is placed horizontally, its expansion area is the largest, and the amount of salt spray on the sample surface is the largest, so the corrosion is the most serious. The results show that when the steel plate forms 45 angles with the horizontal line, the corrosion loss weight per square meter is 250g, and when the steel plate plane is parallel to the vertical line, the corrosion weight is 140g per square meter. Gb/t2423.17-93 standard stipulates that "the plate sample shall be placed so that the test surface forms an included angle of 30 with the vertical direction."

4. PH value of solution

One of the main factors that affect the salt spray test results is the pH value of the solution. The lower the concentration of hydrogen ion in pH solution, the stronger the acidity and corrosiveness. The salt spray test of electroplated parts such as fe/zn, fe/cd, fe/cu/ni/cr shows that the solution pH value is 3.0, and the corrosivity of acetate spray test (ASS) is 6.5 ~ 7.2 times that of pH value. The neutral salt spray test (NSS) is 1.5 ~ 2.0 times harsh.

The pH of the solution will change due to environmental factors. Therefore, the salt spray test standards at home and abroad have an impact on the solution, and the pH range is specified, and the solution pH is proposed under stable test conditions in order to improve the reproducibility of salt spray test results.

5. Salt spray sedimentation and spray mode

The finer the salt spray particles, the larger the surface area formed, the greater the amount of oxygen adsorbed, and the stronger the corrosion. More than 90% of salt fog particles in nature have a diameter of 1 μ M scientific research achievements are as follows: diameter 1 μ M the oxygen content adsorbed on the surface of salt spray particles is relatively balanced with the dissolved oxygen content in the particles. No matter how small the salt spray particles are, the adsorbed oxygen content will not increase.

Traditional spraying methods include air pressure spraying and tower spraying. The main disadvantages are the poor uniformity of salt fog sedimentation and the large diameter of salt fog particles. Ultrasonic atomization method uses the principle of ultrasonic atomization to directly atomize the solution into salt spray, and then diffuses into the test area, which solves the problem of poor uniformity of salt spray sedimentation, and the diameter of salt spray particles is small. Different spraying methods will also affect the pH value.

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