Salt spray test (or salt fog test) is seemed to be the most popular testing methods applied to evaluate corrosion resistance ability of the powder coating layer. This test makes a corrosive attack against coating samples to assess if the powder coating is appropriate for use. In this article, Ninh Binh DHA will help you to have a better understanding of salt spray test. .
What is salt spray test?
Salt spray test is a standard experiment to compare relative resistance ability against corrosion of different coatings under staging conditions resembling the real environment as much as possible. However, this artificial corrosion is accelerated or implemented in a particular frame of time. This test is widely used to test the relative corrosion rate of powder coating or galvanized coating on steel surfaces. In this case, if the test result shows that the powder coating lasts for 1500 hours while the galvanized coating only lasts 24 to 96 hours, it means that the powder coating is more durable than the galvanized coating in the same corrosive-level environment. However, the final assessment may not be accurate with only a salt spray test to make a conclusion that which coating is better, without having a thorough understanding of how different coatings perform in real-life environments. Let’s discuss this more in this article.
How does the salt spray test process?
What should you know about the salt spray test process? Typically, this test is conducted according to the standard procedure of ASTM B117 or ISO 9227. In this process, compared samples are placed in a closed cabinet, then exposed to a dense salt spray mist simulating severe corrosive conditions. The test uses sodium chloride with a pH from 6.5 to 7.2. Usually, this test is set to run from 8 to 3,000 hours.
However, as mentioned, among many other corrosion tests, the salt spray test is still the most standard one. Experts nowadays still have controversial opinions about if the test can resemble real-life conditions. For example, some experts question the appropriateness of the test when it does not involve exposure to salt chemicals.
There are a number of reasons why you should perform a salt spray test, but the main reason is to determine how much your samples will be affected under corrosive conditions. Different kinds of surfaces and metals may show different levels of corrosion resistance and this test allows you to compare this. As mentioned, we use it to test the corrosion resistance level of powder coating on galvanized metal. If the samples can pass the test in 1500 hours, then correspondingly it can be able to withstand conditions like in Vietnam for 5 years until starting to be rusted and peeled.
Moreover, the test allows you to determine the tendency of the material surfaces to corrode in case it is scratched, and to assess the adhesion of the coating.
To illustrate, let’s look at some examples of relative salt spray test results:
- Black Oxide Coating – Samples begin to corrode after 24 to 96 hours
- Ruspert Coating (200 & 300) – Red rust starts after 1000 hours
- Powder coating: 240 – 3000 hours depending on the quality of the powder coating (the type that can withstand 1500 hours of salt spray is quite good)
- Results of stainless steel (304 & 316) – No significant corrosion after 1000 hours
Correlation between salt spray hour and real life
Although the salt spray test is standardized for testing the corrosion resistance of coatings, if the coating performs worse in the salt spray test, this does not mean that it will be less resistant to corrosion in practice.
For example, hot-dip galvanizing usually only withstands for about 100 hours while powder coating can withstand up to 300, 500, 1000, 1500 or more in the salt spray test. This result may lead to a misunderstanding of the durability of these two coatings in practice as a standard hot-dip galvanizing coating can last for 25-50 years without maintenance thanks to the gradual formation of the oxide layer of the corrodents that does not form in this particular test. On the other hand, the powder coating is difficult to last for such a long time without maintenance.
How should you apply salt spray test?
Yes, you should use the salt spray test to assess the correlation between coatings of the same type (same powder coated or same galvanized, same black oxide, etc.) to have a more objective perspective. However, be careful with conclusions based on the results of salt spray tests.
In conclusion, the salt spray test is standardized and widely applied to evaluate and compare corrosive resistance of the same types of materials, especially powder coating. As an expert having many years of experience in this field, we hope you will find our provided information helpful.
Ninh Binh DHA is a leading supplier of professional powder coating service in Vietnam nowadays. Our products are proven to brilliantly pass 1500 hours of the salt spray test and have been exported to many countries over the world such as America, Australia, Italy, and so on.
Feel free to contact us to find more information and quotes by: Hotline: 0869651843 or Email: sơntinhdiendha@gmail.com
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