Utilizing Salt Corrosion Chamber and Salt Fog Test for Corrosion Control and Testing

1. Briefly Interpretation of Cyclic Corrosion Test
The cyclic corrosion test method is a newly developed method which can reflect the influence of natural environment exposure to sample materials, and is used by companies to formulate related standards. This method is preceded by constant salt fog test, spray-dry test and cyclic corrosion test, while salt fog ultraviole cyclic exposure test and other tests can really reflect the impact of ultraviolet light and condensate water. This cyclic corrosion test method provides more accurate control of parameters and more efficient test time, can more realistically reflect the impact of ultraviolet light and condensate water of sample materials in natural environment exposure, and provides reliable reference samples and information for companies to compile relevant salt fog standards based on real situation.

2. Effects of Exposure Conditions on Cyclic Corrosion Test
Cyclic corrosion test is an important test method for evaluating the corrosion resistance of materials, which subjects materials to different exposure conditions. These exposure conditions include room temperature conditions,salt corrosion chamber conditions, salt fog (spray) conditions, wet conditions, dry conditions, immersion corrosion conditions and water immersion conditions. Under room temperature conditions, the temperature is usually 25±5℃, and the relative humidity is 50% or below; laboratory room temperature conditions can slowly change the performance of samples, and if possible, an automatic control system should be adopted. Chamber conditions can be achieved in an automatic test chamber or manually operated in the laboratory, and the accuracy of temperature deviation can reach ±3℃ or more precisely.

The salt fog conditions are realized by the foggy salt solution in the salt fog test box, and if necessary, other chemical liquids can be used to simulate watershed or other industrial corrosion. Wet conditions require relative humidity of 95-100%, and can be achieved by progressive temperature-humidity test chamber or automatic cycle test chamber. Dry conditions can be achieved in open laboratories or in test boxes, pay attention to the sample surface or thorough drying. The immersion corrosion conditions involve specific concentrations, pH values between 4 and 8, and specific temperatures of electrolytes, while the water immersion conditions require the use of distilled water or deionized water, containers made of plastic or inert materials, pH values between 6 and 8, and temperatures at 24±3℃. Effective utilization of these exposure conditions can accurately evaluate the corrosion performance of materials.

The three most common sample preparation methods for color coated board salt fog tests are plat sample, scratch sample and notch sample. These methods better reflect the product’s use and service life.

3. How to Carry Out Salt Corrosion Chamber
In CCT (Cyclic Corrosion Test) exposure experiments, in addition to the general constant salt fog test precautions, such as uniform load, conversion time, uniform sedimentation, etc., there are other potential problems that also affect the repeatability and reproducibility of the test results.

• In order to guarantee the reliability of the salt fog test, the load of the test box must be balanced, and the conversion time should be fully taken into consideration when changing the exposure conditions. Whether it is manual operation or full automation, the required conversion time will affect the test results. For manual operation, the time required to transfer the sample from one environment or exposure condition to another environment or exposure condition must be monitored and recorded according to changes in the environment, operation, instrument type, load, etc.
• The detection method of CCT salt fog sedimentation rate is also different. The uniformity of spray can only be measured by the complete salt fog deposition after the test, while the traditional salt fog test can be detected during the test. In addition, when the test is interrupted, the tested sample should be placed in an environment not corroded as far as possible, and all the conditions of the test interruption and the treatment of the sample should be recorded in time.
• GM 9540P/B technical standards of General Motors (GM) are widely used in the test field of automotive surface, and the conclusion is derived from the research results of SAE ACAP and AISI. Its stipulated exposure conditions include 0.9% NaCl solution, 0.1% CaCl2 solution and 0.25% NaHCO3 solution. The solubility between 6.0 and 8.0, the durability test period is 80 times, manual 24 hours test is required every day, and there are also automatic salt corrosion chamber for exposure test. One sprayer is used for spraying samples per test. If it is human-controlled, samples can only be exposed to laboratory environment on weekends of each test, and cyclic corrosion test will be conducted according to room temperature (25 degrees Celsius) and relative humidity (30%-50%). The cyclic corrosion test steps are: spray for 8 hours, wet for 8 hours (95%-100% RH), dry for 8 hours (60 degrees Celsius, 10%-20% RH). Repeat the above steps;

At present, the main method of metal protection is to use metals or alloys with strong corrosion resistance to completely cover the easily corroded metal surface, so as to prevent the metal surface from being corroded by atmospheric and other environmental factors. Since the potential of gold and nickel plating is more positive than that of the base metal, it is called cathodic coating. This coating can effectively prevent surface corrosion, but if there are holes or cracks in the coating, there will be pitting corrosion, crevice corrosion, etc. The sacrificial anode protection layer method is used to prevent surface corrosion. Typical examples include plate corrosion and face corrosion.

For salt fog tests, we need to research the corresponding relationship between salt fog tests and actual use conditions of different products to find out the acceleration factors for accelerated tests; at the same time, according to different products, different types of salt fog tests should be adopted, which is conducive to improving the accuracy of test items, especially when carrying out cyclic corrosion tests now, can be combined with ultraviolet, acid rain and other models; in addition, the application specifications of various materials also need to be established in the enterprises to prevent the occurrence of corrosion problems from the design links.

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