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A brief talk about salt spray testers ②

1) Salt spray test classification

      Salt spray test is to artificially simulate the corrosion phenomenon in the natural environment in order to assess the corrosion resistance of materials or products. According to the different test conditions, the salt spray test is mainly divided into four types: neutral salt spray test, acidic salt spray test, copper ion accelerated salt spray test and alternating salt spray test.

1.Neutral Salt Spray Test (NSS) is the earliest and most widely used accelerated corrosion test method. The test uses 5% sodium chloride saline solution, PH value is adjusted in the neutral range (6-7), the test temperature is 35 ℃, the requirement of salt spray settlement rate between 1-2ml/80cm2.h.

2.Acid Salt Spray Test (ASS) is developed on the basis of neutral salt spray test. The test adds glacial acetic acid to a 5% sodium chloride solution, which reduces the pH value of the solution to about 3. The solution becomes acidic, and the salt spray formed at the end also becomes acidic from neutral salt spray. Its corrosion rate is about three times that of the NSS test.

3.Copper ion accelerated salt spray test (CASS) is a newly developed foreign rapid salt spray corrosion test. The test temperature is 50℃, and a small amount of copper salt – copper chloride is added to the salt solution, which strongly induces corrosion, and its corrosion rate is about 8 times that of the NSS test.

4.Alternating salt spray test is a comprehensive salt spray test, which is actually an alternation of neutral salt spray test, damp heat test and other tests. It is mainly used for the whole product of cavity type, through the penetration of humid environment, so that the salt spray corrosion is not only produced on the surface of the product, but also inside the product. It is the product in the salt spray, humid heat and other environmental conditions alternating conversion, and finally assess the electrical and mechanical properties of the whole product with or without changes.

      The above is a detailed introduction to the four classifications of salt spray test and its characteristics. In practical application, the appropriate salt spray test method should be selected according to the characteristics of the product and the purpose of the test.

      Table 1 with reference to GB/T10125-2021 “Artificial atmosphere corrosion test Salt spray test” and related materials gives a comparison of the four salt spray test.

Table 1 Comparative list of four salt spray tests

  Test method          NSS                ASS            CASS          Alternating salt spray test     
  Temperature  35°C±2°℃  35°C±2°℃  50°C±2°℃  35°C±2°℃
Average settling rate for a horizontal area of 80                                                                                                                                                          1.5mL/h±0.5mL/h
  Concentration of NaCl solution                                                                                                                                                              50g/L±5g/L
  PH value                                 6.5-7.2                                  3.1-3.3                                 3.1-3.3                                  6.5-7.2
  Scope of application    Metals and alloys, metal coverings, conversion films, anodic oxide films, organic coverings on metal substrates  Copper + Nickel + Chromium or Nickel + Chromium decorative plating, anodic oxide coatings and organic coverings on aluminium  Copper + Nickel + Chromium or Nickel + Chromium decorative plating, anodic oxide coatings and organic coverings on aluminium    Metals and alloys, metal coverings, conversion films, anodic oxide films, organic coverings on metal substrates


2) Salt spray test judgement

      Salt spray test is an important corrosion test method, used to assess the corrosion resistance of materials in the salt spray environment. The results of the determination method includes rating determination method, weighing determination method, corrosive material appearance determination method and corrosion data statistical analysis method.

1. Rating judgement method is by comparing the ratio of corrosion area and total area, the sample is divided into different levels, with a particular level as the basis for qualified judgement. This method is applicable to the evaluation of flat samples, and can visually reflect the degree of corrosion of the sample.

2. Weighing judgement method is through the weight of the sample before and after weighing corrosion test, calculate the weight of the corrosion loss, so as to judge the degree of corrosion resistance of the sample. This method is particularly suitable for a metal corrosion resistance assessment, can quantitatively assess the degree of corrosion of the sample.

3. The corrosive appearance determination method is a qualitative determination method, through the observation of salt spray corrosion test samples whether to produce corrosion phenomenon to determine. This method is simple and intuitive, so it is widely used in product standards.

4. Statistical analysis of corrosion data provides a method for designing corrosion tests, analysing corrosion data and determining the confidence level of corrosion data. It is mainly used to analyse, statistical corrosion, rather than specifically for a specific product quality determination. This method can process and analyse a large amount of corrosion data to draw more accurate and reliable conclusions.

      In summary, the determination methods of salt spray test have their own characteristics and scope of application, and the appropriate method should be selected for determination according to specific needs. These methods provide an important basis and means for assessing the corrosion resistance of materials.

Post time: Mar-01-2024