- May 07, 2018 -
The main factors affecting the salt spray test results include: test temperature and humidity, salt solution concentration, basin click clack waste sample placement angle, salt solution pH value, salt fog deposition amount and spray method.
Test temperature and humidity
Temperature and relative humidity affect the salt spray corrosion. The critical relative humidity for basin click clack waste metal corrosion is about 70%. When the relative humidity reaches or exceeds this critical humidity, the salt will deliquesce to form an electrolyte with good conductivity. When the relative humidity decreases, the salt solution concentration will increase until the crystalline salt precipitates, and the corrosion rate will decrease accordingly.
The higher the test temperature, the faster the salt spray corrosion rate. The International Electrotechnical Commission's IEC 60355:1971 "AN APPRAISAL OF THE PROBLEMS OF ACCELERATED TESTING FOR ATMOSPHERIC CORROSION" standard states: "Each temperature rises by 10°C, the corrosion rate increases by 2 to 3 times, and the electrolyte conductivity increases by 10 to 20%." This is because of the increase in temperature, the increase in molecular motion, and the acceleration of chemical reactions. For the neutral salt spray test, most scholars believe that the test temperature is selected at 35 °C more appropriate. If the test temperature is too high, the salt spray corrosion mechanism differs from the actual situation.
Salt solution concentration
The effect of the salt solution concentration on the corrosion rate is related to the type of basin click clack waste material and coating. When the concentration is less 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 the concentration. The above phenomenon can be explained by the oxygen content in the salt solution. The oxygen content in the salt solution is related to the salt concentration. In the low concentration range, the oxygen content increases with the salt concentration, but when the salt concentration increases to At 5%, the oxygen content is relatively saturated. If the salt concentration continues to increase, the oxygen content decreases accordingly. As the oxygen content decreases, the depolarizing ability of oxygen also decreases, which means the corrosion effect decreases. However, for zinc, cadmium, copper and other metals, the corrosion rate always increases with the concentration of salt solution.
Basin click clack waste sample placement angle
The placement angle of the basin click clack waste sample has a significant effect on the salt spray test results. The settlement direction of salt fog is close to the vertical direction. When the sample is placed horizontally, its projected area is the largest, and the amount of salt fog on the surface of the sample is also the most, so the corrosion is the most serious. The results show that when the steel plate and the horizontal line are at an angle of 45 degrees, the corrosion loss per square meter is 250 g. When the plane of the steel plate is parallel to the vertical line, the corrosion loss weight is 140 g per square meter. The GB/T 2423.17-93 standard stipulates that “the placement method of flat samples should be such that the test surface is at an angle of 30 degrees with the vertical direction.”
Salt solution pH value
The pH of the salt solution is one of the main factors affecting the salt spray test results. The lower the pH, the higher the concentration of hydrogen ions in the solution, and the stronger the acidity and the stronger the corrosion. The salt spray test of Fe/Zn, Fe/Cd, Fe/Cu/Ni/Cr, etc. plating parts showed that the salt spray solution pH value 3.0 of the Acrylic Salt Spray Test (ASS) was more corrosive than the pH value of 6.5- The neutral salt spray test (NSS) of 7.2 is 1.5 to 2.0 times harsher.
Due to environmental factors, the pH of the salt solution changes. To this end, the salt spray test standards at home and abroad stipulated the pH range of the salt solution, and proposed a method for stabilizing the pH value of the salt solution during the test to improve the reproducibility of the salt spray test results.
The finer the salt spray particles are, the larger the surface area formed, the more oxygen is adsorbed, and the stronger the corrosion is. More than 90% of the salt spray particles in nature have a diameter of 1 micron or less. Research results show that the amount of oxygen adsorbed on the surface of salt spray particles with a diameter of 1 μm is relatively balanced with the amount of dissolved oxygen in the particles. Salt fog particles are smaller and the amount of oxygen that is adsorbed no longer increases.
Conventional spray methods include gas pressure spraying and spray tower methods. The most obvious drawbacks are the poor uniformity of salt spray settlement and the large diameter of salt spray particles. Ultrasonic atomization method uses the principle of ultrasonic atomization to atomize the salt solution directly into salt fog and enter the test area through diffusion. This solves the problem of poor uniformity of salt fog settlement, and the salt spray particle diameter is smaller. Different spray methods also have an effect on the pH of the salt solution.