What is the salt spray test?
The salt spray test, also known as the salt fog test, is a popular corrosion testing method. The accelerated standardized test measures the corrosion resistance of steel and other materials exposed to a salt solution.
Invented in 1939, the ASTM B-117 was the first internationally recognized standard, followed by the standards ISO9227, JIS Z 2371 and ASTM G85. Salt spray testing is one of the most proven and widespread corrosion test methods worldwide.
The application of the salt spray testing
To perform the corrosion testing, the examined materials are placed in a salt spray chamber. Then the workpiece is exposed to uninterrupted spray or fog of a salt water solution. Therefore, the name is derived. Besides the testing chamber, a prerequisite for a successful testing procedure is an environment that does not change during the exposure and is maintained stable
In order to carry out the testing process according to the ASTM B117 standard, the temperature must be upheld at 35 °C (+ 1.1 °C to 1.7 °C) Further, the pH value of the testing environment have to range between 6.5 to 7.2. The salt atmosphere is crucial, as well. It must consist of 5 % sodium chloride and 95 % ASTM D1193 Type IV water which is imported at a stable, predetermined air pressure. The duration of the corrosion resistance test is highly dependent on the examined materials and the coating of the metal. The testing chamber must be build accordingly to the applied standard.
Standardization of the testing method
Salt fog testing is a highly standardized corrosion testing procedure under national and international standards. The properties of the testing chamber, the testing procedure and the testing parameters, such as the temperature, the air pressure of the salt solution or the pH are predetermined according to the aforementioned standards. In order to show compliance with the standards and guarantee proper testing conditions, frequently checking of the testing parameters is required. As reference standards, ASTM B117 and ISO 9227 are widely accepted.
What is the difference between ASTM B-117 and ISO 9227?
Established in 1939, the ASTM B-117, was the first internationally recognized salt spray testing standard. The testing procedure is widely accepted and was considered as the gold standard of corrosion testing for years. The ASTM B-117 is a non-cyclic test. That means, that the tested materials undergo continually exposure to salt fog. This circumstance is the major fault of the commonly used test, as the weather conditions fluctuate in the real world.
The ISO 9227 standards suffers from the fault as the ASTM B-117. The standard puts the materials under a permanent exposure to salt fog, as well. Consequently, the testing atmosphere is static, hence the standard does not imitate the real world conditions. The main difference between the two corrosion testing methods is that the ISO 9227 is accepted worldwide, while the ASTM B-117 is only a U.S. standard.
The advantages of the salt spray testing
The salt spray test offers many advantages compared to other corrosion testing methods. One of the main benefits of the procedure is that the test is relatively inexpensive. Further, the test duration is rather short compared to the real-world environment and provides quick results. Therefore, the salt spray test is used to make quick comparisons between expected and the actual corrosion resistance. Thus, it plays an important role in quality control and is applied as a check of the effectiveness of a production process.
However, the salt spray test has a few drawbacks. It does not correlate with the real-world performance of the tested materials. For example, it can only provide results for the life expectancy of coatings in salt water. In addition, it is a very destructive test.
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