From paints to sealants or films to laminates, protective coatings are designed to defend the products they cover — such as wood, metals, alloys and the like — from harsh, often unforgiving conditions, the kind you typically find in the great outdoors.
But how do you test the mettle of these coverings? In other words, what can you do to determine if the coatings applied will hold up and delay what lies beneath from corrosion or rusting?
That's where the salt fog test comes into play. Otherwise known as a salt spray test, a salt fog test is designed to examine the corrosion resistance of all types of wares — such as the aforementioned — by simulating the conditions that typically cause it. However, the corrosion test itself is done in a controlled environment so the duration of the test can be adjusted on an as-needed basis (typically, it's a period of 24 hours, but it can be longer).
If you're not familiar with salt fog and salt spray testing, you undoubtedly have many questions, beyond what a salt spray test is used for in addition to corrosion resistance. For example, you may wonder how a salt spray test is actually performed, if there is a certain standard for the salt spray test to be conducted appropriately, and if so, what that is. You may also be curious about the results you can expect from certain types of tests that fall under the salt fog and salt spray umbrella.
By this article's conclusion, all of these questions will be addressed.
How is a salt spray test performed?
In a real-world setting, corrosion typically takes a long time to actually manifest itself, evidenced by its signature burnt orange color and irregular patterns. A salt fog test speeds up the process significantly by placing the product in question inside an environmental test chamber. Test chambers come in all shapes and sizes, but their most important function is to "lock in" the combination of conditions that ultimately lead to accelerated corrosion.
This is done by using a fine mist to coat the test product — such as a surface metal or clapboard treated with an acrylic paint — with a standardized solution. The most common one used in salt fog tests blends water with sodium chloride, but others include acetic acid salt spray paired with copper chloride.
The combination of the spray, temperatures and relative humidity — all of which can be adjusted on an as-needed basis — is what recreates corrosion on test specimens.
What is a neutral salt spray test?
Neutral refers to the pH level of the salt spray that is used to douse the product under evaluation. Otherwise referred to as ASTM B117, neutral salt spray testing is considered the gold standard corrosion test methods because it's easily repeatable, affordable and effective. For a salt spray to be considered neutral, the pH level must be between 6.5 and 7.2. This is made possible by using a 5% sodium chloride solution that is atomized through pressurized air. The atomization is ultimately what helps the liquid disperse so it evenly coats the test specimen(s). How long ASTM B117 tests actually last is fully customizable, but they're generally no shorter than 24 hours. They can be much longer than that, though, up to 1,000 hours (the equivalent of approximately 41 days).
What results can you expect from an ASTM B117 salt fog test?
It's difficult to say with precision what a product will look like once a fog test is completed. Many factors play a role, including the test standard (e.g. ASTM B117 versus ASTM G85), the length of the salt fog test (usually 24 hours to 1,000 hours) and the products inside the test chambers. The test chamber itself can also impact the results; if it's shoddily constructed, air may leak out, thus impacting the speed of corrosion development.
Ultimately, a salt fog test is designed to provide you with some context as to how a product, like a coating metal, holds up under intense conditions. If corrosion occurs faster than anticipated, you may need to make adjustments in terms of how much a product must be coated for it to withstand certain temperatures and exposures. Alternatively, you may need to adjust your customers' expectations so they can get a better idea of how long metal, paneling, steel or equipment will go before corrosion becomes visible.
What kinds of products might be worth evaluation through salt fog tests?
Perhaps the best answer to this question is to understand what causes corrosion in the first place. Generally speaking, corrosion is a manifestation of a combination of factors, those being moisture and air temperatures. So if you have a product that will be installed outside or used there, a salt fog test can give you an idea as to how long you have before pitting, crevice, intergranular or other forms of corrosion becomes apparent. Commonly tested products include paints, organic coatings, pre-treated surfaces, zinc and zinc-alloy plating and electroplated chromium, nickel, copper and tin.
Whether you're looking for a custom-made environmental test chamber or seek a salt fog test done correctly and professionally on your behalf, Auto Technology has you covered. Our services are second to none as we are industry leaders in environmental testing capabilities. Contact us today to learn more.