The main factors affecting the results of salt spray corrosion test include: test temperature and humidity, concentration of salt solution, sample placement angle, pH value of salt solution, etc.
Temperature and relative humidity affect the corrosive effect of salt spray. The critical relative humidity for metal corrosion is approximately 70%. When the relative humidity reaches or exceeds this critical humidity, the salt will deliquesce to form an electrolyte with good electrical conductivity. When the relative humidity decreases, the concentration of the salt solution will increase until the crystalline salt is precipitated, and the corrosion rate will decrease accordingly.
The higher the test temperature, the faster the corrosion rate of salt spray. The international standard points out: "For every 10 ℃ increase in temperature, the corrosion rate increases by 2 to 3 times, and the conductivity of the electrolyte increases by 10% to 20%." This is the result of increased temperature, increased molecular motion, and faster chemical reactions. For the neutral salt spray corrosion test, most scholars believe that it is more appropriate to select the test temperature at 35 °C. If the test temperature is too high, the salt spray corrosion mechanism is quite different from the actual situation.
The effect of the concentration of the 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 the 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 increase of the salt concentration. However, when the salt concentration increases to At 5%, the oxygen content reaches relative saturation, and if the salt concentration continues to increase, the oxygen content decreases accordingly. When the oxygen content decreases, the depolarization ability of oxygen also decreases, that is, the corrosion effect is weakened. However, for metals such as zinc, cadmium, and copper, the corrosion rate always increases with the increase of the salt solution concentration.
The placement angle of the sample has a significant effect on the results of the salt spray test. The sedimentation direction of the salt spray is close to the vertical direction. When the sample is placed horizontally, its projected area is the largest, and the surface of the sample bears the largest amount of salt spray, so the corrosion is the most serious. The research results show that when the steel plate is at an angle of 45 degrees to the horizontal line, the corrosion weight loss per square meter is 250 g, and when the plane of the steel plate is parallel to the vertical line, the corrosion weight loss is 140 g per square meter. The standard specifies that the flat specimen should be placed so that the test surface is at an angle of 30 degrees to the vertical.
The pH of the salt solution is one of the main factors affecting the results of the salt spray test. The lower the pH value, the higher the hydrogen ion concentration in the solution, the stronger the acidity and the stronger the corrosiveness. The salt spray test of Fe/Zn, Fe/Cd, Fe/Cu/Ni/Cr and other electroplated parts shows that the corrosiveness ratio of the acetic acid salt spray test (ASS) when the pH value of the salt solution is 3.0 is 6.5～ The neutral salt spray test (NSS) of 7.2 is strictly 1.5 to 2.0 times.
The pH of the salt solution changes due to environmental factors. For this reason, the salt spray test standards at home and abroad have stipulated the pH value range of the salt solution, and put forward a method to stabilize the pH value of the salt solution during the test to improve the reproducibility of the salt spray test results.