What are coatings?
Organic coatings are widely used as a barrier to protect metal structures from the environment hence to prevent the quick formation of corrosion. However, the coating only does its job properly if the entire metal surface is covered and if the adhesion to the surface is sufficient. If not, weak spots may occur which leads the coating to prematurely fail with further, possibly serious, consequences.
How to check the quality of coatings?
“Okay, how do I know whether my coating is doing its job properly?”, by regularly checking the quality of representative samples from each production batch.
METALogic’s laboratory offers several tests to assess the quality of your coatings. The most common way is by using the traditional neutral salt spray test: (NSST). Among the alternatives we offer a test of our own design, the accelerated salt spray test.
The salt spray test: pros and cons
Most commonly used for the quality control of coatings is the salt spray test (SST). The test consists of exposing samples for a certain period of time in a test booth with a controlled atmosphere.
Here is a quick overview of the pros and cons of the SST:
+ standardized and simple
+ relatively inexpensive
+ well established
- it takes up to 1000 hours
Even though at first glance it may seem as if the pros outweigh the cons, the long duration is an important factor to keep in mind since sometimes time is of the essence and a quick answer is required.
About the accelerated spray test
For these more urgent cases, METALogic developed an electrochemical test that requires less time, thus is “accelerated”. To be precise, it has a similar degradation pattern as the NSST, but only in 5% of the time.
The principle of the test is the following: it uses static-cathodic and static-anodic polarization techniques. During anodic polarization, the substrate turns into anode and the anodic/oxidation reaction is accelerated.
What basically happens then is that in places where the coating contains microscopic pinholes or weak spots that allow the electrolyte to easily penetrate to the metal surface, accelerated oxidation takes place. Which causes corrosion products to form, such as rust-coloring. With cathodic polarization, the tested item becomes the cathode and the cathodic reactions are accelerated. The products of this cathodic reaction are detrimental to the coating and provoke it to loosen or detach.
This is in a nutshell summarized how the test works but for further details, take a look at our whitepaper where we describe the entire process.
The main target of the accelerated test is to quickly find where the weak spots are in a coating. Besides that, the test can also give an indication of the time needed for the electrolyte to penetrate the coating.
Another bonus is that the test is able to assess objects with a complex geometry and of different sizes.
The salt spray test and the accelerated spray test: a comparison
Answering the question ‘which test takes the cake’ all depends on your case.
If your case requires a standardized test according to a certain norm or if you want to see if a coating can survive in a certain atmospheric environment, the traditional NSST is the go to option. There is indeed a large database of real life cases compared to NSST survival times which gives the NSST its raison d’être.
However, if you want to check whether your coating has weak spots or defects that aren’t visible to the eye and cannot be detected by a wet sponge holiday detector, and you need the result short notice, the accelerated test provides the answer.
Eager to learn more about the technical aspects and the development of the accelerated spray test? We have the whole process written out in a whitepaper which you can find here. If you have further questions or if you’re interested in performing a quality control test on coatings, do not hesitate to reach out to us. Our experts are eager to take a closer look at your case.
