Main factors affecting the results of salt spray test (1)

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Main factors affecting the results of salt spray test (1)

Factors: test temperature and humidity, concentration of salt solution, sample placement angle, pH value of salt solution, salt spray settlement and spray mode.

Test temperature and humidity

Temperature and relative humidity affect the corrosion of salt spray. The critical relative humidity of metal corrosion is about 70%. When the relative humidity reaches or exceeds this critical humidity, the salt will deliquesce and form an electrolyte with good conductivity. When the relative humidity decreases, the concentration of salt solution will increase until crystal salt is precipitated, and the corrosion rate will decrease accordingly.

The higher the temperature is, the faster the corrosion rate is. According to iec60355:1971 "an April of the solutions of acceptable testing for electrochemical corrosion" standard of International Electrotechnical Commission, "the corrosion rate increases by 2-3 times and the conductivity of electrolyte increases by 10-20% when the temperature increases by 10 ℃. This is due to the increase of temperature, the aggravation of molecular movement and the acceleration of chemical reaction. For the neutral salt spray test, most scholars think that it is more appropriate to choose the test temperature of 35 ℃. If the test temperature is too high, the salt spray corrosion mechanism is quite different from the actual situation.

Concentration of salt solution

The effect of the concentration of salt solution on the corrosion rate is related to the type of material and coating. When the concentration is below 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. The above phenomenon can be explained by the oxygen content in salt solution. The oxygen content in salt solution is related to the concentration of salt. In the low concentration range, the oxygen content increases with the increase of salt concentration. However, when the salt concentration increases to 5%, the oxygen content reaches relative saturation. If the salt concentration continues to increase, the oxygen content correspondingly decreases. When the content of oxygen decreases, the depolarization ability of oxygen also decreases, that is to say, the corrosion is weakened. But for zinc, cadmium, copper and other metals, the corrosion rate always increases with the increase of salt concentration.

Sample placement angle

The angle of sample placement has a significant effect on the results of salt spray test. The deposition direction of salt spray is close to the vertical direction. When the sample is placed horizontally, its projection 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 the corrosion loss weight per square meter is 250 g when the steel plate is at a 45 degree angle with the horizontal line, and 140 g when the steel plate plane is parallel to the vertical line. According to GB / t2423.17-93, "the flat sample shall be placed at an angle of 30 degrees between the test surface and the vertical direction."