Difference between neutral salt spray test and cyclic corrosion salt spray test

Introduction to Composite Salt Spray Test
1. The development of salt spray testing and the creation of the alternating salt spray test

    How to simulate an environment closer to the actual application to carry out the alternating salt spray test? To make this clear, it is necessary to start with the development of the test.

    The salt spray method was first used to test the corrosion resistance of materials in about 1914, and in 1939 the neutral salt spray test was incorporated into the ASTM B117 standard. As mentioned above, this traditional salt spray standard required samples to be continuously exposed to a 5% concentration of salt spray at 35°C. Although the ASTM B117 test standard has been amended and improved many times over the 80 years of its application, it has been found that for a long time the correlation between the test results of the method and the actual corrosion effect of the samples exposed outdoors has not been significant. Even so, the current ASTM B117 (or other similar standards neutral salt spray test) is still the main standard of salt spray corrosion testing, widely used in a variety of automotive components in the corrosion resistance test.
     As the demand for corrosion protection of components increased, engineers and researchers continued to try to develop test methods that more accurately simulated the effects of outdoor corrosion. In the 1960s and 1970s, Harrison and Timmons in the United Kingdom developed the Prohesion test, because actual outdoor exposure usually involves both wet and dry conditions, and only by simulating these natural, cyclical environments does accelerated laboratory testing make sense.
    Studies have shown that the relative corrosion rate, structure, and morphology of samples after alternating salt spray testing are very similar to outdoor corrosion results. The test is effective in evaluating a number of corrosion mechanisms, such as general corrosion, galvanic corrosion, and crevice corrosion, and is particularly suited for corrosion testing of industrial protective coatings.
In recent years, SAE and AISI have begun to research and develop the alternating salt spray test applicable to the automotive field. The results are now satisfactory and some new test standards have been formed.

2. How does the Alternating Salt Spray Test work?

    The core of the Alternating Salt Mist Test is to simulate the actual environment in which the samples are exposed. With this dominant idea, the samples are exposed to a series of different conditions, such as salt spray, drying, humidity, spraying, and standing, and the cycle is executed continuously to achieve the purpose of assessing the corrosion resistance of the samples.

The conditions are described as follows
👉 Salt Spray Conditions: Salt spray conditions can be interpreted as an ordinary neutral salt spray, which can be sprayed through a nozzle with a 5% mist of sodium chloride solution.
👉 Dry conditions: dry conditions require full circulation of air in the test space, both to control the drying temperature, but also to avoid the interference of additional corrosive gases and liquids.
👉 Humid conditions (constant temperature and humidity): humid conditions need to provide a specific temperature and humidity to the test space, the temperature range is generally between 35 ℃ ~ 70 ℃, the relative humidity range is generally between 95% ~ 100%.
👉 Spray condition: The test solution (e.g. 5% NaCl solution), is sprayed on the sample under test through a pressurized system and nozzle.
👉 Standing Conditions: Placed in a room temperature environment, the sample usually undergoes a slow drying process that is free of corrosive vapors and gases, with little to no gas flow, at a temperature of typically 25±5'C . The relative humidity is 50% or less.

3. What are the current reference standards for the Alternating Salt Spray Test?

 
    Currently, the alternating salt spray test has been defined in several standardization systems, such as GB, ISO, IEC, SAE, JASO, etc. The main test steps and test methods are similar, with the difference lying in the subtle differences between the different standards in terms of cycling items, cycle time, changeover time and environmental conditions. Taking JASO M 610 standard as an example, this test standard is widely used in the corrosion resistance test of metal components and surface treatment components in automobile-related products designed and produced in Japan, and is an important reference for many standards. The whole set of cycling procedure of its alternating salt spray test is shown in the table below:

       In addition to the relevant alternating salt spray test standards of the standards committees mentioned above, many automobile and component manufacturers are currently developing their own alternating salt spray test standards on this basis, such as BOSCH, BMW, Ford, GM, Volvo, VW, Nissan, and so on. Between the test standards of various companies, the overall test process is relatively similar, and the traceability relationship between the standards is complex. The following table lists the mainstream test standards and the different requirements for cycling modes:

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