Analysis of Influencing Factors in Salt Spray Aging Test Experiment

The salt spray corrosion testing methods of products can be divided into two categories: natural environment exposure test and artificial accelerated simulated salt spray environment test. The exposure test in the natural environment is to place the sample in a typical climate area and examine the salt spray corrosion resistance of the sample in the storage environment. The period of the natural environment exposure test is generally very long, which takes several years or even more than ten years. At the same time, the testing personnel have to inspect and record for a long time, which requires a lot of manpower, financial resources and material resources. The test results only apply to some areas and may not apply to other areas.

Analysis of influencing factors in salt spray test:
1. The concentration of the salt solution
The influence of the concentration of the salt solution on the corrosion rateThe quality depends on the type of material and coating. When the concentration is less than 5%, the corrosion rate of steel, nickel and brass increases with the increase in concentration; when the concentration exceeds 5%, the corrosion rate of these metals decreases with the increase in concentration. The above phenomenon can be explained by the oxygen content in the brine. The oxygen content in the saline solution is related to the salt concentration. In the low concentration range, the oxygen content increases with the increase in 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. For metals such as zinc, cadmium and copperr, however, the corrosion rate always increases with the increase in the concentration of the salt solution.

2. Sample placement angle
Sample placement angle has a major influence 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, the 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 sheet is at an angle of 45 degrees to the horizontal line, the corrosion loss weight per square meter is 250g, and when the steel sheet plane is parallel to the vertical line, the corrosion loss weight is 140g per square meter. Standard GB/T 2423.17-93 states that \"The method of placement of flat specimens shall ensure that the tested surface forms an angle of 30 degrees with thevertical orientation.

3. Test temperature and humidity
Temperature and relative humidity affect the effect of salt spray corrosion. 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 electrolytic solution with good conductivity. the relative humidity decreases, the concentration of the salt solution will increase until crystallized salt has precipitated, and the corrosion rate will decrease accordingly. The higher the test temperature, the faster the corrosion rate of the salt spray.
IEC 60355:1971 \" AN ASSESSMENT OF THE PROBLEMS OF ACCELERATED ATMOSPHERIC CORROSION TESTING\" the standard reads: \"For every 10°C increase in temperature, the corrosion rate increases by 2 to 3 times, and the electrolyte conductivity increases by 10 to 20%.\" This is due to the temperature rise, the intensificationof molecular motion and the acceleration of chemical reactions. For the neutral salt spray test, most scholars believe that the test temperature of 35°C is more appropriate. If the test temperature is too high, the salt spray corrosion mechanism will be quite different from the actual situation.

4. The pH value of the saline
The pH value of the saline solution is one of the most important factors influencing 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 galvanized parts such as Fe/Zn, Fe/Cd, Fe/Cu/Ni/Cr 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 6.5~7.2 The neutral salt spray test (NSS) is 1.5 to 2.0 times more difficult. Due to the influence of environmental factors, the pH value of the salt solution will change. For this reason, have the salt spray teststandards at home and abroad determined the pH range of the saline solution and proposed methods to stabilize the pH value of the saline solution during the test to improve the reproducibility of the salt spray test results.

Method of evaluation of results in salt spray test:
1. Judging assessment method
It is to divide the percentage of the corrosion area from the total area into different levels according to a certain level. A certain level is used as the basis of a qualified judgment, which is suitable for the assessment of flat samples.
2. Weighing evaluation method
It is a method to weigh the weight of the sample before and after the corrosion test and calculate the weight lost due to corrosion to judge the quality of the corrosion resistance of the sample. particularly suitable for the corrosion resistance of the sample. The quality of the corrosion resistance of a particular metal is consideredorder.
3. Corrosion appearance assessment method
It is a qualitative assessment method, which judges the sample according to whether the product has corrosion appearance after the salt spray corrosion test.
4. Statistical analysis of corrosion data
It provides methods for designing corrosion tests, analyzing corrosion data and determining the confidence level of corrosion data.The quality judgment of a specific product.

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