Main Factors Affecting the Results of Salt Spray Test

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

1. Test temperature and humidity

Temperature and relative humidity affect the corrosive action of salt spray. The critical relative humidity for 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 concentration of the salt solution will increase until crystallized salt is precipitated, and the corrosion rate will decrease accordingly.

The higher the test temperature, the faster the salt spray corrosion rate. The International Electrotechnical Commission IEC60355: 1971 "AN APPRAISAL OF THE PROBLEMS OF ACCELERATED TESTING FOR ATMOSPHERIC CORROSION" standard states: "When the temperature increases by 10°C, the corrosion rate increases by 2 to 3 times, and the conductivity of the electrolyte increases by 10 to 20%". This is due to the increase in temperature, the intensification of molecular motion, and the acceleration of chemical reactions. For the neutral salt spray test, most scholars believe that the test temperature is more appropriate 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 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 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 When the salt concentration is 5%, the oxygen content reaches relative saturation. If the salt concentration continues to increase, the oxygen content will decrease accordingly. As 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 concentration of the salt solution.

The placement angle of the sample has a significant impact on the results of the salt spray test. The sedimentation direction of the salt mist is close to the vertical direction. When the sample is placed horizontally, its projected area is the largest, and the amount of salt mist on the surface of the sample is also the largest, 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 steel plate plane is parallel to the vertical line, the corrosion weight loss is 140 g per square meter. The GB/T2423.17-93 standard stipulates that "the placement method of the flat sample should make the test surface form an angle of 30 degrees with the vertical direction.

The pH value 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 concentration of hydrogen ions in the solution, the stronger the acidity and the stronger the corrosion. The salt spray test of electroplated parts such as Fe/Zn, Fe/Cd, Fe/Cu/Ni/Cr, etc. shows that the corrosion specific pH value of the acetic acid salt spray test (ASS) with the pH value of the salt solution is 3.0 is 6.5~ The neutral salt spray test (NSS) of 7.2 is 1.5 to 2.0 times more severe.

Due to the influence of environmental factors, the pH value of the salt solution will change. For this reason, the salt spray test standards at home and abroad have stipulated the pH value range of the salt solution, and proposed methods to stabilize the pH value of the salt solution during the test, so as to improve the reproducibility of the salt spray test results.

Chuangtuo Jinggong (Jiangsu) Co., LTD is profession manufacturer of various types of Hexagon Nuts, everything is customized in our factory, Should you have any questions please feel free to come back to us.

Contact: Lorena
Email: market@chtofastener.com
Mobile: 0086 153 6641 6606 (Wechat/Whatsapp ID)