A salt spray test is also called corrosion testing and is performed on metal products. This test estimates the resistance of surface coating against the corrosion initiating conditions. In a neutral salt spray test, the specimen is exposed to the saline fog for a specific duration and when the test cycle is completed, the specimen is analysed visually. There are standards defined by relative authorities to perform the test step by step. ASTM B117 is followed that has defined all the conditions and precautions to be taken while performing this test.
· To perform the test, first of all, a standardised salt spray chamber is required.
· To begin the test, place the sample in the chamber by rubbing off all the contaminants like dust or grease from the specimen surface.
· The angle of placement should be 20° and the should be kept away from the chamber walls. Close the canopy of the chamber and set the time period.
· The duration of the test can be in the multiple of 24, like, 24 hours, 48 hours, 72 hours and so on.
· Many machines are equipped with HMI panel or touch screen panel to allow the profile programming.
· Once the test is complete, the specimen is taken out and analysed visually.
· If the specimen does not show any traces of rust it passed the test, else it is failed.
Precautions to be Taken
· The temperature of the chamber should be approx. 37°C.
· The level of fog that is to be maintained is 5ml.
· During the test, the canopy should not be opened. It disturbs the test conditions that are maintained inside the chamber.
· It is very important that the test should run non-stop. Therefore, it is very necessary to have a water reservoir.
· The sample should not be placed very close to the walls, it may result in uneven exposure to fog.
Importance of Corrosion Testing
In metal industries, this test serves different purposes. Some industries try to evaluate the behaviour of their product in real working conditions, while some try to estimate the shelf life of their product. There is one interesting analysis and that is a determination of salt spray life. It can be defined as the number of hours a specimen can withstand without developing the corrosion. In many industries, it is used as the parameter to judge the regression in the production process. Over the time, due to wear and tear of the machine or with the age of the machine the production process starts to develop loopholes which might not get recognised unless tested. The quality of the coating starts to degrade unintentionally. This degradation can be determined by salt spray testing. If a coated product used to have a salt spray life of 96 hours but now it has started to fail at 84 hours, this means the coating process needs reconsideration.
It is an internal way of setting up quality standards. The manufacturer must consistently adhere to these standards to deliver high-quality products. Salty fog testing is considered an important parameter in the metal industry. It not only tests the sustainability of the coated surface but also determines the real-time behaviour of the product.
There is always a misconception among the manufacturers and they try to relate the salt spray life with real life. But this is not the aim of the test. Because in real life conditions, there is a periodic cycle of wet and dry events. However, in simulated tested conditions, there are constant wet conditions. Understanding the right aim of this test is very important so that right testing purpose can be served.