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MIL-STD 202 Salt Atmosphere Testing

During salt spray testing specimens are subjected to a fine mist of salt solution. While MIL-202 salt atmosphere testing has several useful purposes its deficiencies and limitations must be recognized. This test was originally proposed as an accelerated laboratory corrosion test simulating the effects of seacoast atmospheres on metals. However, it has been erroneously considered by many as an all-purpose accelerated corrosion test.

Resistance to salt atmosphere corrosion is seldom related to resistance to corrosion in other media, even “marine” and seawater atmospheres. However, an idea of the service life and behavior of related metals in these atmospheres can be gained from this test.  Generally, salt atmosphere testing is unreliable for comparing the general corrosion resistance and predictive life service of different kinds of metals.

The salt atmosphere test has received its widest acceptance for evaluating the uniformity of metallic and nonmetallic protective coatings. It has served this purpose with varying amounts of success. In this connection, it is useful for evaluating different lots of the same product, after establishing a standard level of performance.

The salt atmosphere test is especially helpful as a screening test for revealing particularly inferior coatings. When checking the porosity of metallic coatings, the test is more dependably applied to cathodic coatings than anodic coatings. This test can also be used to detect the presence of free iron contaminating the surface of another metal. This is done by inspecting the corrosion of products.

Keystone Compliance is a fully equipped salt atmosphere testing lab. Our test engineers understand the requirements of this method of corrosion testing. The following information is extremely technical in nature. It provides a summary of the requirements Method 101E MIL-STD 202G salt atmosphere testing.

What is the Procedure for the Best Salt Atmosphere Testing?

All testing should be done in a clean salt atmosphere laboratory. Maintenance and conditioning of the test chamber is therefore required as part of the test procedure. The chamber is cleaned each time the salt solution in the reservoir has been used up. This assures that all materials that could adversely affect the results of subsequent tests are removed.

However, no test should be interrupted for the purpose of chamber cleaning. After the cleaning cycle, upon restarting the chamber, the reservoir is filled with salt solution. The chamber is stabilized by operating it until the temperature comes to equilibrium. Intermittent operation of the chamber is acceptable, provided the pH and concentration of the salt solution are kept within limits.

Flat specimens and, where practicable, other specimens are supported in positions where the significant surface is 15 degrees from the vertical. And is parallel to the principal direction of horizontal flow of the fog through the chamber. Other specimens are positioned so as to insure the most uniform exposure. Whenever practicable, the specimens should be supported from the bottom or from the side.

When specimens are suspended from the top, they are suspended by glass or plastic hooks or wax string. If plastic hooks are used, they should be fabricated of material that is non-reactive to the salt solution. The use of metal hooks is not permitted.

Specimens are positioned so that they do not contact each other. This way they do not shield each other from the freely settling fog. Also it prevents corrosion products and condensate from one specimen fallin upon another.

How Should the Test Chamber Be Operated?

A salt fog having a temperature of 95°F minimum passes through the chamber for the specified test duration. The exposure zone of the chamber is maintained at a temperature of 95°F plus or minus 5°F. A  receptacle placed in the exposure zone should collect 0.5-3.0 milliliters of solution per hour for each 10 cm diameter of collecting area. This is the way to test if conditions in the exposure zone are suitable.

At least two clean fog-collecting receptacles are used. One is placed at the perimeter of the test specimens nearest to any nozzle. The other is at the perimeter of the test specimens farthest from the nozzle(s). Receptacles are fastened so they are not shielded by specimens and so no drops of solution from specimens are collected.

The solution thus collected will have a sodium chloride content of 4% to 6% when measured at the exposure zone temperature. The specific gravity and quantity of the solution collected is checked following each salt atmosphere test. Suitable atomization has been obtained in boxes having a volume of less than 12 cubic feet with the following conditions:

  • Nozzle pressure of from 12 to 18 pounds psi.
  • Orifices of from 0.02 to 0.03 inch in diameter.
  • Atomization of 3 quarts of the salt solution per 10 cubic feet of box volume for each 24 hour tet period.

These conditions may have to be modified when using large-size boxes with a volume greater than 12 cubic feet.

What is the Appropriate Duration for MIL-STD 202 Salt Atmosphere Testing?

Unless otherwise specified, the test will be run continuously for the time indicated. Or it will run until definite indication of failure is observed. There should be no interruptions except for adjustment of the apparatus and inspection of the specimen. The length of the salt atmosphere test is indicated in one of the following test conditions.

Test ConditionLength of Test (Hours)
A96
B48
C24
D240

Who Provides the Best Salt Atmosphere Compliance Testing?

Keystone Compliance is recognized as one of the best salt atmosphere labs in the country. Our test engineers are able to provide salt atmosphere certifications for commercial, military, and aerospace products. Contact us to learn why so many manufacturers rely on Keystone Compliance’s testing services to meet their compliance testing needs.

Looking for other MIL-STD-202 compliance tests? Click on a link below to learn more about the other test methods.