Continuous or cyclic salt spray test?
Traditionally materials and coatings are tested by subjecting them to an aggressive environment. Neutral salt spray tests, such as ISO 9227 NSS (ASTM B117), are often chosen to obtain a test certificate. There is however a tendency towards cyclic tests.
Why?
Continuous, neutral salt spray tests have been performed all over the world under standardized conditions for a long time. So there is a lot of historical data available. However, the test conditions (exposure to an environment at 35°C with continuous salt spray) are often not in line with the real conditions that a material or coating is facing. Changes in temperature, wet-dry cycles, etc. are not taken into account.
To trigger similar degradation mechanisms as in reality, there is a tendency towards cyclic tests. A lot of these tests are standardized. Which test is most suitable depends on the environment/application in which the material will be used. Simulating a maritime climate will often give totally different test results/rankings than simulation of for example an industrial climate.
Furthermore, tailor-made cyclic tests offer a solution for those who want to learn more about their material. The focus is not on standardization or obtaining a test certificate, but on the tailor-made design / test procedure in which the actual conditions are approached as closely as possible, while still achieving a certain acceleration factor. Not only salt solutions can be used, but also other solutions such as e.g. (artificial) swimming pool water, specific cleaning agents, ...
No more neutral salt spray tests?
While neutral salt spray tests may be less suitable when it comes to mimicking realistic exposure conditions, they do have their value. When for example the performance of a coating system must be tested against historical data, they certainly offer a solution. Because of their historical database, for a large number of applications there is a comparison of the actual service life with the service life in the salt spray test.
Conclusion
Choosing the most appropriate test for your application is crucial and not always straightforward. It depends strongly on the objective. Always discuss the purpose and expectations of the test results first, and then choose the correct test.
