There are various corrosion tests available to measure the durability of protective coatings. However, in this article, we focus on salt spray testing. This test produces a corrosion attack on a coated sample to assess the suitability of the particular coating used.
Salt spray testing is a standardized test that allows one to compare the relative corrosion resistance of different coatings under conditions that attempt to mimic real corrosive environments. However, this accelerated corrosion is accelerated or completed in a compressed time frame. For example, this test is widely used to test the relative corrosion rates of ruspert coatings and black oxide coatings. In this case, if the test shows that the wood-plastic coating lasts for 800 hours, while the black oxide coating lasts for 24 to 96 hours, it shows that when the two are under the same corrosion resistance, the wood-plastic coating is better than the black oxide coating. more durable environment. However, using only salt spray testing to compare and draw conclusions about which coating is better can be a misconception without valid knowledge of how different coatings behave in real-world environments. We will discuss this later in this article.
Understanding Salt Spray Testing
How to conduct salt spray test? Typically, this testing is performed in accordance with ASTM B117 or ISO 9227 test procedures. In this process, the samples to be compared are placed in a closed chamber, and then they are exposed to dense fog that simulates severe corrosion conditions. This test uses a sodium chloride solution with a pH range between 6.5 and 7.2. This test usually takes 8 to 3,000 hours. Although, as mentioned, there are many other corrosion tests, the salt spray test is pretty standard. However, experts are divided on how close the professional opinion is to real-life situations. For example, some professionals question the appropriateness of the test in situations that don't involve salt chemicals.
There are many compelling reasons why you should perform salt spray testing, but the main reason is to determine how corrosion will affect your samples. Different surfaces and metals may exhibit different levels of corrosion resistance and this test allows you to compare them. In addition, this test enables you to determine the corrosion propensity of the surface when it is scratched and to evaluate the adhesion of the coating.
There are many examples of relative salt spray test results such as:
Black oxide – samples show corrosion after 24 to 96 hours
Ruspert Coating (200 and 300) - No trace of red rust on samples after 1000 hours
Powder Coating – Samples may last 500 hours before rust is observed
Results for Stainless Steel (304 and 316) – Samples vary depending on surface finish. However, most samples in this category show barely noticeable corrosion after 1000 hours
How does salt spray test time relate to actual duration?
It should be noted that, contrary to what many may think or expect, salt spray testing is no more relevant than other corrosion tests. The X hour test cannot show that the coating will resist corrosion for any number of years. This is because in real life it takes years for rust to develop. So it's good practice to use this test only as a predictor of actual results.
To understand the above, salt spray testing should be looked at as it is actually a quality control measure designed to ensure that process controls are on track and accurate. In other words, the test cannot predict actual results. To illustrate, if you know that galvanizing will last 96 hours or powder coating will last 500 hours, your test sample doesn't mean that something might be wrong with our workmanship. Again, you should be more cautious in drawing conclusions from these numbers that powder coating is more resistant than galvanizing. This inference would be misleading because, as stated earlier; the rating of spray salt is not an indicator of how well one coating will perform better than another in the real world.
To elaborate on this, consider hot-dip galvanizing, which can last 75 to 100 years without maintenance. However, it performed poorly on this test. That's because its actual durability in the real world depends on other factors, such as a glassy skin that develops corrosion products that don't form in this particular type of testing.
In short, salt spray testing is widely used to test the durability of protective coatings. But while this is a standardized test, its results have nothing to do with the number of years a coating can last before it shows signs of corrosion.
