The main factors affecting the results of salt spray test include: test temperature and humidity, salt solution concentration, sample placement Angle, pH value of salt solution, salt spray settling amount and spray method.
- Test temperature and humidity
Temperature and relative humidity affect the corrosion 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 electrical conductivity. When the relative humidity decreases, the concentration of salt solution will increase until 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 electrotechnical commission IEC60355:1971 "the AN APPRAISALOFTHE the PROBLEMS OF ACCELERATED TESTINGFORATMOSPHERICCORROSION" standard points out: "For every 10℃ increase in temperature, the corrosion rate is increased by 2 to 3 times, and the conductivity of the electrolyte is increased by 10 to 20%." This is because the temperature increases, the molecular movement increases, and the chemical reaction speeds up as a result. For the neutral salt spray test, most scholars believe that the test temperature is more appropriate at 35℃. If the test temperature is too high, the salt spray corrosion mechanism is different from the actual situation.
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The concentration of the 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 the concentration. The above phenomenon can be explained by the oxygen content in the salt solution, which is related to the concentration of salt. In the low concentration range, the oxygen content increases with the salt concentration, but when the salt concentration increases to 5%, the oxygen content reaches a relative saturation, and if the salt concentration continues to increase, the oxygen content decreases accordingly. With the decrease of oxygen content, the depolarization ability of oxygen also decreases, that is, the corrosion effect is weakened. However, for zinc, cadmium, copper and other metals, the corrosion rate always increases with the increase of the concentration of salt solution.
- Sample placement Angle
The position Angle of the sample has obvious influence on the result of salt spray test. The sedimentation direction of the 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 also the largest, so the corrosion is the most serious. The results show that the corrosion weight loss per square meter is 250g when the steel plate is at a 45 degree Angle to the horizontal line, and 140g per square meter when the steel plate is parallel to the vertical line. The GB/T2423.17-93 standard stipulates that "the method of placing the flat sample shall be such that the test surface shall be at a 30 degree Angle to the vertical direction."
- The pH of the salt solution
The pH value of salt solution is one of the main factors affecting the result of 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 tests of electroplated parts such as Fe/Zn, Fe/Cd, Fe/Cu/Ni/Cr show that the corrosion of the acetate spray test (ASS) with a salt solution pH of 3.0 is 1.5 to 2.0 times harsher than that of the neutral salt spray test (NSS) with a pH value of 6.5 to 7.2. Due to the influence of environmental factors, the pH value of salt solutions will change.
- Salt spray settling amount and spray method
The finer the salt spray particles are, the larger the surface area they form, the more oxygen they adsorb, and the more corrosive they are. More than 90% of the salt spray particles in nature are less than 1 micron in diameter. The research results show that the oxygen adsorbed on the surface of the salt spray particles with a diameter of 1 micron is relatively balanced with the dissolved oxygen in the particles. No matter how small the salt spray particles are, the amount of oxygen adsorbed will no longer increase.
The most obvious disadvantages of the traditional spray methods, including pneumatic injection method and spray tower method, are the poor uniformity of salt spray settling amount and the large diameter of salt spray particles. The ultrasonic atomization method uses the principle of ultrasonic atomization to atomize the salt solution directly into the salt spray and diffuses it into the test area, which solves the problem of poor uniformity of the salt spray settlement, and the salt spray particle diameter is smaller. Different spray methods also have an effect on the pH of the salt solution.
The ultrasonic atomization method without compressed air has little influence on the pH value of salt solution, but the pressure injection method and spray tower method of compressed air spray have obvious changes in the pH value of salt solution.
