Main factors influencing the results of salt spray test (2)

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Main factors influencing the results of salt spray test (2)

PH value of salt solution

The pH value of salt solution is one of the main factors affecting the results of salt spray test. The lower the pH value, the higher the concentration of hydrogen ion in the solution, the stronger the acidity and the stronger the corrosiveness. The salt spray test of Fe / Zn, Fe / CD and Fe / Cu / Ni / Cr Electroplating parts showed that the corrosion of ass with pH of 3.0 was 1.5-2.0 times more severe than that of NSS with pH of 6.5-7.2.

Due to the influence of environmental factors, the pH value of salt solution will change. In order to improve the reproducibility of the salt spray test results, the pH range of the salt solution is specified in the salt spray test standards at home and abroad, and the method of stabilizing the pH value of the salt solution in the process of the test is proposed.

Causes and results of pH value change of salt solution

1) the main cause of the change of pH value of salt solution in the salt spray test is the soluble substances in the air. The properties of these substances may be different. Some of them are acid when they are dissolved in water, and some of them are alkaline when they are dissolved in water;

2) in the process of salt spray test, it is a reversible process that the soluble substance in air dissolves into or escapes from the salt solution. Dissolved substances will reduce the pH value of salt solution, while the escaping substances will increase the pH value of salt solution. When the decreasing rate is equal to the increasing rate, the dissolving rate is greater than the escaping rate, which will reduce the pH value of salt solution. On the contrary, the pH value of salt solution increases. If the dissolution rate is equal to the escape rate, the pH value will not change.

3) there are many factors that affect the pH value of salt solution. For example, the nature and content of soluble substances in air, pressure, contact area and contact time of air and salt solution, etc.

A. nature and content of soluble substances in the air

The air contains CO2, SO2, NO2, H2S, etc. when these gases are dissolved in water, they will generate acid substances, which will reduce the pH value of water. There may also be alkaline dust particles in the air, which will increase the pH value of water when dissolved in water.

B. atmospheric pressure

The solubility of gas in water is directly proportional to atmospheric pressure. At 0 ℃, 0.355 g CO2 can be dissolved in 100 ml water at 1 atm, and 0.670 g CO2 can be dissolved in 100 ml water at 2 ATM. When using compressed air spray, because of the increase of atmospheric pressure, the dissolved amount of CO2 and other acidic substances in the air increased, and the pH value of the salt solution decreased. This process is contrary to the process of CO2 escaping from the salt solution after the spray is affected by the temperature drop.

C. contact area and contact time of air and salt solution

The spray turned the salt solution into a salt spray with a diameter of 1~5 micron m particles. With the increase of contact area, the amount of gas dissolved in or escaping from the liquid increases greatly. When the conditions (such as pressure, temperature, etc.) that affect the gas to dissolve in the liquid and the gas to escape from the liquid remain unchanged, the speed of dissolution and escape will eventually reach equilibrium. Before reaching equilibrium, the amount of dissolved (or escaped) will increase with time.

① the pH value of salt solution stored in a closed container does not change with the increase of storage time. The reason is that there is no contact with air.

② with the increase of gas-liquid contact time, the pH value of the salt solution stored in the uncovered culture dish decreased obviously. It is obviously due to the large contact area with air.

③ in the environment containing alkaline substances, the pH value of the salt solution in the uncovered container increases with the increase of storage time.