2.1 Selecting the Salt Fog Method
After examining requirements documents and applying the tailoring process in Part One of this standard to determine where atmospheric corrosion is anticipated in the life cycle of materiel, use the following to confirm the need for this Method and to place it in sequence with other methods.
2.1.1 Effects of Corrosive Environments
Salt is one of the most pervasive chemical compounds in the world. It is found in the oceans and seas, the atmosphere, ground surfaces, and lakes and rivers. It is impossible to avoid exposure to salt. The worst effects occur, in general, in coastal regions. The effects of exposure of materiel to an environment where there is a corrosive atmosphere can be divided into three broad categories: corrosion effects, electrical effects, and physical effects. Consider the following typical problems to help determine if this Method is appropriate for the materiel being tested. This list is not intended to be all-inclusive.
MIL-STD-810H
METHOD 509.7
2.1.1.1 Corrosion Effects
- a. Corrosion due to electrochemical reaction.
- b. Accelerated stress corrosion.
- c. Formation of acidic/alkaline solutions following salt ionization in water.
2.1.1.2 Electrical Effects
- a. Impairment of electrical materiel due to salt deposits.
- b. Production of conductive coatings.
- c. Corrosion of insulating materials and metals.
2.1.1.3 Physical Effects
- a. Clogging or binding of moving parts of mechanical components and assemblies.
- b. Blistering of paint as a result of electrolysis.
2.1.2 Sequence Among Other Methods
- a. General. Use the anticipated life cycle sequence of events as a general sequence guide (see Part One, paragraph 5.5).
- b. Unique to this Method. If the same test item is to be used for additional testing, it is imperative that the test item be thoroughly cleaned following the Salt Fog test. Salt deposits can interfere with the effects of other tests. In most cases recommend the salt fog test be conducted after the other climatic tests. It is generally inappropriate to conduct the salt fog, fungus and humidity tests on the same test sample because the cumulative of effects from the three environments may be unrealistic. However, if it is necessary to do so, perform the salt fog test following the fungus and humidity tests. Although generally inappropriate, if sand and dust testing is required on the same test item, perform it following salt fog testing. Recommend conducting the sequence of testing in accordance with the LCEP.
2.2 Selecting Procedure Variations
This Method has one procedure. Possible variations are described below.
2.2.1 Salt Solution
Unless otherwise identified, use a 5 ± 1 percent salt solution concentration (paragraph 6.1, reference b.). Use water as described in Part One, paragraph 5.16. The intent is to not introduce contaminants or acidic/alkaline conditions that may affect the test results. (See paragraph 4.5.1.1.b.)
2.2.2 Test Item Configuration
Test Item Configuration The configuration and orientation of the test item during the exposure period of the salt fog test is an important factor in determining the effect of the environment on the test item. Unless otherwise specified, configure the test item and orient it as would be expected during its storage, shipment, or use. The listing below offers the most likely configurations that materiel would assume when exposed to a corrosive atmosphere. For test purposes, choose the most severe/critical configuration.
- a. In a shipping/storage container or transit case.
- b. Outside of its shipping/storage container but provided with an effective environmental control system that partly excludes the salt fog environment.
- c. Outside of its shipping/storage container and set up in its normal operating mode.
- d. Modified with kits for special applications or to compensate for mating components that are normally present, but are not used for this specific test.
2.2.3 Duration
Experience has shown that alternating 24-hour periods of salt fog exposure and drying conditions for a minimum of four 24-hour periods (two wet and two dry), provides more realistic exposure and a higher damage potential than does continuous exposure to a salt atmosphere (paragraph 6.1, reference b). If this option is not acceptable (e.g., security issues, LCEP information, or commodity requirements), perform 48 hours of exposure followed by 48 hours of drying. Increase the number of cycles to provide a higher degree of confidence in the ability of the materials involved to withstand a corrosive environment.
MIL-STD-810H
METHOD 509.7
2.2.4 Temperature
Maintain the temperature in the chamber at 35 ±2 °C (95 ±3.6 °F). This temperature has been historically accepted and is not intended to simulate actual exposure situations. Other temperatures may be used if appropriate.
2.2.5 Air Circulation
2013 Air Circulation Ensure the air velocity in test chambers is minimal (essentially zero).
2.2.6 Fallout Rate
The collection of fog shall be such that each solution receptacle collects 1 to 3 ml of solution per hour for each 80 cm2 of horizontal collecting area based on at least a 16 hr period. A minimum of two salt fog collection receptacles shall be located at the perimeter of the test item one at the point nearest to the nozzle and another at the point farthest from the nozzle. Position the receptacles such that they are not shielded by the test item and will not collect drops of solution from the test item or other sources. If using multiple nozzles or towers, the same principles apply.
2.2.7 Dryout Rate
- Dryout Rate. If corrosion levels from test to test are to be compared, and accepting that the rate of corrosion is much higher during the transition from wet to dry, it is critical to closely control the rate of drying.