There are a variety of corrosion tests out there that are used to measure the durability of protective coatings. However, in this article, we focus on the Salt Spray Test. This test produces a corrosive attack aimed at coated samples so as to evaluate the suitability of the specific coating for use.
The Salt Spray Test is a standardized test which allows one to compare the relative corrosion resistance of different coatings in conditions that try to mimic the true corrosive environment. However, this instigated corrosion is accelerated or done in a compressed time frame. For instance, this test is widely used to test the relative corrosion rates of a ruspert coating and that of a black oxide coating. In such a case if the test shows that the ruspert coating lasts for say 800 hours while that of black oxide lasts for 24 to 96 hours then it implies that the ruspert coating is more durable than the black oxide coating when both are in the same corrosive environment. However, it could be a misconception to solely use Salt Spray Test for comparison and make conclusion of which coating is more superior, without thoroughly understanding on the behaviour of different coatings in real-world environement. We will discuss this in the later part of this article.
Understanding Salt Spray Test
How is Salt Spray testing carried out? Usually, this test is carried out in line with the ASTM B117 or ISO 9227 testing procedure. In this procedure the samples being compared are placed in an enclosed chamber then they are exposed to a thick metal fog which mimics severely corrosive conditions. The test uses a sodium chloride solution with a pH range between 6.5 and 7.2. Typically this test runs from 8 to 3,000 hours. Though as earlier mentioned, there are a variety of other corrosion tests, salt spray test is very standard. However, opinion is divided among experts about how closely it reflects real life conditions. Some experts, for instance, question how suitable the test is when it does not involve exposure to salt chemicals.
There are many compelling reasons why you should carry out salt spray test but the main reason is to determine how corrosion will affect your specimen. Diverse surfaces and metals can exhibit varying amounts of resistance to corrosion, and this test allows you to compare this. In addition, the test enable you determine a surface's propensity to corrode in case it is scratched and assess stickiness coating.
There are many examples of relative salt spray test results such as:
How does the Salt Spray Test hours correlate with real-world duration?
It should be noted that contrary to what many people think or expect, the Salt Spray Test, just like other corrosion tests does not correlate anything. The test for X hours is not an indicator of any number of years that coating will resist corrosion. This is because, in real life situations, rust takes years to occur. So the best thing is to take this test only as a predictor of real-world outcomes.
To appreciate the above, Salt Spray Test should be seen for what it actually is a quality-control measure carried out to ensure that process control is on track and is accurate. In other words, this testing does not predict real-world outcomes. To illustrate this, if you know that zinc plating lasts 96 hours or powder coating goes for 500hours then your sample for the tests doesn’t then it implies there is a possibility that something might have gone wrong with our process. On the same note, you should be more carefully to conclude from these numbers that powder coating is more resistant than Zinc plating. Such kind of inference would be misleading because as earlier said; the spray salt rating is not an indicator that one coating will outperform the other in the real world.
To elaborate on this, think of hot dip galvanising which can last between 75-100 years with no maintenance. However, it performs quite poorly in this test. This is because its actual endurance in the real world depends on other factors such as the gradual formation of a glassy skin of corrosion products which don’t form in this particular kind of test.
In a nutshell, the Salt Spray Test is widely used test the durability of protective coatings. However, though it is a standardized test, its results do not have any correlation with the number of years that the coating can last before showing signs of corrosion.
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