Salt Spray Chambers
A salt spray test chamber is an enclosed cabinet that is used in manufacturing environments to test the corrosion resistance of a product or sample of material. An item is placed inside for a period of time while it is subjected to a highly corrosive environment created by a salt solution sprayed from a high-pressure nozzle. The process is known as a salt spray test or a salt fog test. Likewise, the chamber itself may be referred to as a fog chamber.
This corrosion test is meant to provide information about the durability of a surface or coating on a product. The product is attacked for a predetermined amount of time and then assessed for corrosion afterward. The longer the material will last in the chamber without the appearance of corrosion, the more durable the coating or surface is considered to be.
The purpose of a salt chamber is to provide a controlled environment that is highly corrosive in order to obtain corrosion resistance information about metal or metal-coating specimens. Salt spray testing is essential for industries that are geared toward marine products, but other industries utilize this method as well. Industrial settings where these tests are found include metal finishing, aircraft and aerospace, automotive, and military.
Benefits of the Salt Spray Test
The test chamber does not simulate natural real-use environments, so its results are not used to predict the longevity of a material in the real world. It does, however, provide valuable information during manufacturing when used in quality auditing. Since engineers can compare products and materials to one another in a controlled environment before being used in commercial or industrial settings, they are able to choose the most effective materials.
Other Environmental Test Chambers
A limitation of spray testing is that its results can not be used to predict how the tested material will perform in its actual environment. One corrosion test that does just that is the cyclic corrosion test, or CCT. The CCT can cycle the sample through various conditions. It can subject the sample through wet, salty, and dry periods. It may even get more sophisticated with thermal test chambers, pressure test chambers, and altitude test chambers.
History of Salt Spray Chambers
Testing with a saline spray is one of the oldest forms of corrosion testing. It was invented in the early 1900s and its use is documented in 1919 by the American Institute of Mining and Metallurgical Careers. In 1939, the American section of the International Association for Testing Materials (ASTM International) released ASTM B117 which first standardized the practice of the salt corrosion test. Today, it is still one of the most widespread types of environmental testing.
How It Works
When water is applied to metal and other surfaces, there is an exchange of electrons, and the water eventually precipitates out iron hydroxide. This is corrosion. It is going through a process that causes a loss of electrons which chemically changes the material rust.
Corrosion testing equipment works by employing a chemical mechanism called oxygen-concentration cell corrosion which accelerates the rate of corrosion on samples. The salt spray enhances the corrosion by keeping the metal ions in solutions that can act as conductors. The speed of the reactions is increased in the chamber, as well, by keeping the temperature elevated during the testing period.
Items must be suspended or placed in such a way that all parts become exposed to the fog. Typically, the sample is placed at a 15-30 degree angle from the vertical. The item can be observed periodically for evidence of corrosion, but generally, the chamber is closed for the test duration.
Although salt spray testing and salt fog testing are terms that are used interchangeably, there is a slight difference in the procedure. With fog testing, the saline solution is atomized at warm temperatures (about 95F or 35C) so that it produces a mist or fog. In a spray test, on the other hand, the saline solution is sprayed directly onto the item, which is then allowed to dry at room temperature. The process is repeated several times.
For these corrosion testing methods, the saline solution that is applied to the test item should have a specified concentration of sodium chloride. Generally, the concentration will be 5% NaCl. In addition, the pH of the solution is prepared to be neutral 6.5 to 7.2 and is known as NSS or Neutral Salt Spray.
Using pressurized air and a spray nozzle, the salt water is continuously applied to the item for the duration of the corrosion test period. The time for the test is not set by ASTM standards, but the number of hours should be recorded in results.
Generally, test periods endure for 24 hours at a time and can go on for up to 5000 hours. At set intervals, the test item is photographed to allow observation of the amount of corrosion that is accumulating as the test progresses.
The Equipment of the Chamber
Salt spray test chambers must consist of components made of materials that will not have a corrosive reaction from the testing procedures. Examples include plastic, glass, stone, or metal lined with protective plastics, rubber, or epoxies. The shape and size of the chamber must not exceed the limits of the atomized saline solution.
The chamber canopy should be easy to open. Some models have button-operated pneumatic cylinders. Automatic purge features can reduce the amount of fog and humidity that is released when the canopy is opened. Walk-in chambers would have mechanism unique to that opening.
Compressed Air Inlet
Compressed air must enter the chamber and meet with salt water solution in order for a spray to exist. The compressed air enters the take through the compressed air inlet. At pressures between 20 to 87 P.S.I., the air should be delivered from a clean, dry, oil-free supply.
In order for the saline mixture to be sprayed with enough force that it saturates the air around the test item, the compressed air that eventually assists in generating the salt spray must pass through the air saturator, also called the humidifier. To allow for the eventual salt spray to deliver a dense blanket of the solution, the compressed air is bubbled through the tower so that its humidity is raised. The temperature of the air saturator is adjustable set high in the saturator to account for a decrease as it goes to the chamber. An added accessory for the air-saturator tower is an automatic level control.
Salt Solution Reservoir
Separate from the chamber is an attachable tank where the salt water mixture, also called brine, is housed. It is placed so that it can be easily seen, filled, and cleaned. The reservoir may be fitted with an agitating device to keep the air and salt in the water well-mixed. After leaving the reservoir, the brine is filtered before being used in a test. An automatic level control for the salt reservoir is not required, but it is appropriate.
Solution Pump and Flow Meter
The brine is moved out of the tank via peristaltic pumps and piping. All piping that contacts the brine or spray must be made of non-corrodible materials like plastic. Without a pump, the salt water would need to be gravity fed into the system. With a pump and flow meter, the speed of the pumping and the rate of the water flow can be controlled by the user. The mist that leaves the atomizer is a highly corrosive and oxygen rich fog. It is sprayed out at a controlled rate and allowed to fall onto the test samples.
Salt Spray Atomizer
Without atomizing nozzles, there would be no salt spray test. An essential part of the chamber, the atomizer acts as a meeting point of the compressed air and brine. The nozzle should be filtered and all parts must be made of an inert material. Without them, the atomizer would fail to spray uniformly, altering the results of the test. Chambers may have a central fog unit which sprays from the middle of the chamber. Others might have adjustable baffles with which to control the flow.
Chamber Heating Mats
The chamber must contain provisions for heating the chamber. Insulated heating mats are controlled by the chamber climate controller. They are placed outside of the inner chamber in a balanced way so that they evenly radiate heat.
The user has full control over over the chamber operations via the control panel. Often with an LCD screen, the control panel displays the various conditions of the chamber and the test, such as climate and current test period duration. The control panel will also house safety controls such as emergency stop commands. At the very least, the control panel must be able to control warmth.
Located throughout the parts of the chamber are sensors which monitor the conditions and displays them to the user at the control panel. The sensors are precise in their measurements of temperature and relative humidity. Precipitation and water level are also monitored.
Racks, shelving, and other supports are used to hold the sample during the test. The samples are typically placed in such a way that they are at an angle of fifteen to thirty degrees from the vertical. Very large test items should be placed on the bottom of the chamber. They should be hung or set so that dripping condensation from one sample does not fall onto another. Also, the edges of product samples with cut edges should be coated with a protective layer.
The sprayed solution from the test that accumulates must be vented through an exhaust. Vent piping that transports the waste must not allow back pressure and must allow all of the liquid to be evacuated. If the vent is piped through an outer wall, it must be protected from wind currents so that it doesn't cause a pressure change within the cabinet.
Beneath the testing chamber, a condensate drain will collect fluid build up from the test. This drain can be pumped with a marine grade pump in order to control waste spillage. Waste water can be released to a floor-level drain or piped to a holding tank.
Chambers vary in a wide range of ways. Of course, each company will include its own features, but chambers can also differ in various sizes, access types, and compliance standards. Even within a single model, the chamber may be compatible with added accessories or fittings that enhance its use.
Choosing the Right Manufacturer
With a machine like an environmental chamber, models can range from operating only at the most basic level to being fully customizable with added accessory parts. Without a comfortable understanding of the chamber or their needs, buyers may have difficulty choosing the best place to buy their chamber. The initial action for potential buyers is to conduct a needs assessment. They must determine compliance standards that must be following, whether any customizations are needed based on laboratory functions, building location, or convenience. Next, a customer should estimate a flexible budget and collect quotes from various manufacturers.
Most chambers are compliant with ASTM B117 since it has been regulating them since the 1930s. Since then, however, it has developed more standards, such as ASTM G85 for modified salt testing. Other organizations have introduced similar relevant standards for spray/fog testing. These include the International Standardization Organization (ISO 9227), Japanese standards (JIS Z 2371), Society of Automotive Engineers (SAE J 2334), and more. Still, the most recent version of ASTM B117 remains the authority on its standards. Clients should ensure that the chamber that they select is compliant with standards governing their projects.
After speaking to many quality merchants, a client should only purchase from a manufacturer who is willing to listen to the requirements and desires of the customer and be able to suit the client’s needs.
Salt Spray Chambers - Hastest Solutions, Inc.
Salt Spray Chambers - Parameter Generation & Control, Inc.