Analysis of influence factors of salt spray aging test experiment

The salt spray corrosion resistance testing methods of products can be divided into two categories: natural environment exposure test and artificially accelerated simulated salt spray environment test. The field exposure test involves placing the sample in a typical climatic zone and examining the sample\'s resistance to salt spray corrosion in the storage environment. The cycle of exposure tests in natural environments is generally very long, requiring several years, or even more than ten years. At the same time, testing personnel need to carry out long-term inspections and records, which requires a large amount of manpower, financial and material resources. Test results only apply to certain areas and may not apply to other areas.

Analysis of influencing factors in the salt spray test:
1. Salt solution concentration
The effect of salt solution concentration on corrosion rate is related to the type of material and coating layer. Where the concentration is less than 5%, the corrosion rate of steel, nickel and brass increases with increasing concentration; when the concentration is greater than 5%, the corrosion rate of these metals decreases with increasing concentration. The above phenomenon can be explained by the oxygen content of the saline solution. The oxygen content of the saline solution is related to the salt concentration. In the low concentration range, the oxygen content increases with increasing salt concentration. However, when the salt concentration increases to 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 capacity 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 increasing salt solution concentration.
2． Example of placement angle
The placement angle of the sample has a significant impact on the results of the salt spray test. The sedimentation direction of salt spray is close to the vertical direction. When the sample is placed horizontally, its projected area is the largest and the sample surface supports the greatest amount of salt spray, so corrosion is the most serious. The research results show that when the steel plate forms an angle of 45 degrees to the horizontal line, the corrosion weight loss per square meter is 250g. When the plane of the steel plate is parallel to the vertical lineale, the weight loss by corrosion is 140 g per square meter. GB/T 2423.17-93 states that “the flat sample should be placed so that the test surface is at an angle of 30 degrees to the vertical direction.
3. Test Temperature and Humidity
Temperature and Relative humidity affect the effects of salt spray corrosion. The critical relative humidity for metal corrosion is approximately 70%. When the relative humidity reaches or exceeds this critical humidity, the salt will deliquesce and form an electrolyte with good electrical conductivity. As the relative humidity decreases, the concentration of the salt solution increases until crystallized salt precipitates and the corrosion rate decreases accordingly. The higher the test temperature, the faster the rate of salt spray corrosion.
The Electrotechnical Commission standard IEC 60355: 1971 “AN EVALUATION OF THE PROBLEMMY ACCELERATED ATMOSPHERIC CORROSION TESTS” specifies: “For every 10°C increase in temperature, the corrosion rate increases by 2 to 3 times and the conductivity of the electrolyte increases by 10 to 20%. “Indeed, as temperature increases, molecular movement intensifies and chemical reactions accelerate. For the neutral salt spray test, most researchers estimate the test temperature to be 35°C. If the test temperature is too high, the salt spray corrosion mechanism will be very different from that of AC current situation.
4． The pH value of the saline solution
The pH value of the saline 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 more acidic and corrosive it is. Salt spray testing of electrolytic parts such as Fe/Zn,Fe/Cd, Fe/Cu/Ni/Cr shows that the corrosiveness ratio of the acetic salt spray (ASS) test of the saline solution with a pH value of 3.0 is 6.5. ~ Neutral Salt Spray (NSS) test of 7.2 is 1.5 to 2.0 times more severe. Due to the influence of environmental factors, the pH value of salt solutions will change. For this reason, domestic and foreign salt spray test standards have stipulated the pH value range of salt solution and proposed methods to stabilize the pH value of salt solution during the test to improve the reproducibility of the results of salt spray tests.

Method for evaluating the results in the salt spray test:
1. Scoring judgment ent method
The percentage of the ratio between the corrosion area and the total area is divided into several levels according to a certain method, so as to A certain level is used as the basis for determinationination of qualification, and it is suitable for evaluation of flat samples.
2. Weighing judgment method
It is a method of weighing the sample before and after the corrosion test and calculating the weight lost by corrosion to judge the corrosion resistance quality of the sample . It is particularly suitable for evaluating the corrosion resistance quality of a certain metal.
3. Method for determining the appearance of corrosion
It is a qualitative determination method which can be used to determine whether the product exhibits corrosion phenomena after the salt spray corrosion test.
4. Method for Statistical Analysis of Corrosion Data
Provides methods for designing corrosion tests, analyzing corrosion data, and determining the reliability of corrosion data. It is mainly used to analyze and count corrosion situations, rather than specifically for a certaindetermination of the quality of a specific product.

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