There are certain processes, methods and concepts that – once you learn more about them – aren't at all like they sound. A salt spray corrosion test isn't one of them; it's pretty much as straightforward as it sounds. However, the effects that it uncovers on test subjects is nothing short of extraordinary.
Known colloquially as a salt spray fog test, a salt spray corrosion test is designed to assess the extent to which a given item – usually metallic in composition, but not always – will oxidize when placed under stress. The stressor, in these cases, is sodium chloride, a natural enemy to metal and several alloys. It's dispensed over a given time period in an enclosed unit, typically a chamber. This walled-off environment amplifies the oxidative composition of the air to expedite the corrosion process, which under natural circumstances would take a lot longer to become observable.
The purpose of such a spray test is to determine how corrosion-resistant a given tool, part, product or unit actually is. Armed with this information, manufacturers can decide what coatings or paints to apply to ward off corrosion for as long as possible or if the solutions currently in place are sufficient.
"Salt spray corrosion testing can provide insight into how quickly rusting will occur when exposed to harsh elements."
Corrosion is a lot like death and taxes: It's a guarantee. It can be put off, however, and there are a variety of treatments that help to make this possible. Salt spray corrosion testing can provide insight into how quickly rusting will occur when exposed to harsh elements.
How is salt spray testing carried out?
Corrosion isn't something that simply happens. It stems from a combination of factors that are naturally occurring, such as moisture, temperature, humidity and acidity. Because of this, for a salt spray corrosion test to be truly accurate, it requires a high level of precision in terms of introducing the variety of elements and aspects that contribute to oxidation.
A classic example is the chamber itself. The unit must be built to ASTM B117 specifications, which is a standard established by the American Society for Testing and Materials. A chamber that meets the ASTM B117 salt spray test standard means it has the dimensions necessary to reflect the corrosion resistance of a given item.
The temperature setting is another variable that must be very specific. ASTM B117 requires a warmth level of 95 degrees Fahrenheit and that this temperature be maintained for the duration of the salt fog test. This is an additional reason why the chamber must be compliant with ASTM specifications; it ensures that the temperature will remain the same for the duration of the evaluation.
A salt spray apparatus, powered by pressurized air is used to apply the mist evenly, which must have a sodium chloride concentration level of 5% and a pH range between 6.5 and 7.2. This range is designed to reflect the acidity of water that is found in outdoor environments, such as the ocean. Salt spray tests are fully customizable so the pH can be lowered. For example, regular rainfall has a pH value of between 5.0 and 5.5, according to the Environmental Protection Agency.
The one variable that is entirely up for the experimenter to decide is the length of the test itself. Generally speaking, the test can be days, weeks or years, whatever duration the test subject will be exposed to the elements. The longer it goes, the more extensive the corrosion tends to be.
What industries might use a salt spray corrosion test?
Any business whose products are vulnerable to corrosion stands to benefit from salt spray test parameters. This includes the military (e.g. tanks, humvees, etc.), aircraft (e.g. commercial jetliners, helicopters, etc.), automotive (e.g. light vehicles, trucks), motor carriers (e.g. tractor trailers, flatbeds, etc.) and marine (e.g. speed boats, yachts). Aside from being unsightly, corrosion is degenerative, meaning that it gets worse the longer it goes. It can ultimately be so severe as to render a product useless or dangerous. A salt spray test can lengthen the life of a product once the necessary coatings or paints are applied.
"Over 48 billion pounds of road salt is used in the U.S. annually."
It isn't just manufacturers that stand to benefit from salt mist corrosion test data; it can help the planet as well. The primary ingredient of road salt is sodium chloride, which gets used liberally by road crews during the winter when snow leads to slippery conditions for motorists. In fact, over 48 billion pounds of road salt is used in the U.S. annually, according to USA Today.
While road salt does help contribute to safe driving by increasing traction, it nonetheless can adversely affect trees when it leaks into soils from snow melt. It can also get kicked up into the air by tires and inhibit a tree's ability to replace water.
The harm that sodium chloride causes to the environment is part of the reason why more municipalities are either reducing the amount of road salt they use per year or transitioning to a safer, less harmful alternative.
Salt spray corrosion test information can help governments make smarter decisions with regard to how they spend their money and the strategies they use to preserve and protect the environment.
What is salt fog?
As previously mentioned, salt fog and salt spray are often used interchangeably. But there is a slight difference between the two when it comes to the solution that's applied and to what test subject. For example, salt fog is much finer in composition and is usually used to measure the corrosion resistance of alloys, meaning metals that are combined (e.g. steel). Salt spray, on the other hand, is more concentrated and tends to coat or adhere to certain test subjects more effectively (e.g. chromium plating).
"Salt fog is much finer in composition compared to salt spray."
From a sheer testing perspective – meaning the methodologies used – salt fog tests and salt spray tests are one and the same. At Auto Technology, we use salt fog, humidity fogging, solution spray and dwell cycle in our cyclic corrosion test salt spray chambers so you can further customize the process.
What is cyclic corrosion testing?
Outdoor environments are always changing, on a minute to minute basis. It can be pouring rain, and before you know it, nothing but blue skies and comfortably dry air.
Cyclic corrosion testing accounts for these variabilities by exposing materials to several different environments within a single test. At one point, the item may be doused in water, and in another, it may be sprayed in a fine mist or subjected to warm, humid air, a process that is appropriately called "dry off."
The complement to cyclic is non-cyclic. Here, instead of being exposed to alternating environments, it remains the same, whether that be alternating intervals of salt spray coatings, a fixed temperature or humidity level.
The goal is to simulate the environments that the test subject will be exposed to over time and to what degree. Auto Technology is capable of replicating numerous cycles in cyclic corrosion testing so you can truly assess the corrosion resistance of your supplies and what you need to do to improve their defenses. These include:
- Controlled Humidity
- High Humidity
- Salt Fog
- Humidity Fogging
- Wet Bottom Relative Humidity
- Temperature variation (-20 C to 90 C)
Each of these cycles is slightly different from the other and is designed to produce different results depending on the type. For instance, in the dry cycle, the test subject is exposed to forced air via a blower motor, which channels the air through the energized chamber heaters. The temperature of the chamber is determined by the controller as is the humidity. Dry cycle typically pumps in with lesser amounts of moisture.
High humidity is the converse of dry by pumping in air to the chamber with a 100% humidity level. Additionally, instead of a salt spray solution, the test subject is coated in deionized water from an atomizer nozzle.
Can salt spray testing give misleading results?
The one potential problem that can derive from salt spray testing is flawed results. Because it's a controlled environment, it's possible to introduce "ingredients" that cause corrosion that may not be truly representative of what will occur in real world situations. Additionally, the test chamber in which the experiment takes place may not be ASTM compliant.
This is why it's important to go to a professional for all salt spray corrosion tests. They have the training, resources and experience to assure that all the correct salt spray test parameters are properly established and handled.
Why you should choose Auto Technology?
When it comes to corrosion testing and accuracy of results, no one tops Auto Technology. Household name companies from around the world have come to us for environmental testing services that are the very definition of all-inclusive. We have more than 1,200 cubic feet of available testing space and state-of-the-art technology that deliver results. We also build corrosion test chambers so you can assess the strength and resilience of your products whenever you want.
Contact us today to learn more.