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MIL-STD 883 Salt Atmosphere (Corrosion) Testing

MIL-883 salt atmosphere testing is proposed as an accelerated laboratory corrosion test. It works to simulate the effects of seacoast atmosphere on devices and package elements. All testing in a salt atmosphere testing lab, like Keystone Compliance. Keystone Compliance has one of the best salt atmosphere labs in the country.

The test engineers at Keystone Compliance have an in-depth understanding of the requirements of MIL-STD-883 salt fog and salt atmosphere testing.  The following information is extremely technical, as it provides a summary of the requirements of MIL-STD 883 salt fog testing. Below are some terms and definitions pulled from MIL-883 salt fog.

Corrosion is the deterioration of coating or base metal or both by chemical or electrochemical action. A corrosion site is the site at which the coating or base metal or both is corroded, or the location of corrosion. Corrosion stain is a semitransparent deposit due to corrosion products.

The corrosion product (deposit) is the effect of corrosion. This could refer to rust or iron oxide, nickel oxide, tin oxide, etc. The product of corrosion may be at the corrosion site, or may flow or run  so as to cover non-corroded areas. This is due to the action of a liquid carrier of salt.

A blister is a localized swelling and separation between the coating(s) and base metal. A pinhole is a small hole occurring in the coating as an imperfection which penetrates entirely through the coating. Pitting is the localized corrosion of coating or base metal, confined to a small area, that takes the form of cavities. Flaking is the separation of small pieces of coating that exposes the base metal.

What is the Proper Procedure for the Best Salt Fog Testing?

The first step is to ensure that the test chamber in the salt atmosphere laboratory is properly cleaned and maintained. This ensures that all materials which could adversely affect the results of the subsequent tests are removed from the chamber. The chamber is cleaned by operating it at 95°F ±5°F with deionized or distilled water as long as necessary. This should be done each time the salt solution in the reservoir has been used up.

Depending on the size of the reservoir and the specified test condition, several test runs could be run before cleaning. For long duration conditions, the reservoir may be refilled via auxiliary reservoirs so that the test cycle is not interrupted. After the cleaning cycle, on restarting the chamber, the reservoir is filled with salt solution. The chamber is stabilized by operating it until the temperature comes to equilibrium.

If operation of the chamber is discontinued for more than one week, the remaining salt solution, if any, must be discarded. Cleaning must then be performed prior to restarting the test chamber. Intermittent operation of the chamber is acceptable provided the pH and concentration of the salt solution are kept within limits defined.

The test procedure for salt solutions requires that the salt concentration be 0.5 to 3.0 percent by weight in deionized or distilled water. The salt used is sodium chloride with no more than 0.1% sodium iodide by weight, and no more than 0.3% total impurities by weight. The pH of the salt solution stays between 6.5 and 7.2 when measured at 95°F ±5°F. Only CP grade (dilute solution) hydrochloric acid or sodium hydroxide can be used to adjust the pH.

Unless otherwise specified, test specimens are not to be preconditioned. If required, prior to mounting, the device leads may be subjected to bending stress initial conditioning. If devices being tested have already undergone the required initial conditioning, as part of another test, the lead bend need not be repeated.

How Should Devices Be Mounted in the Test Chambers of the Salt Fog Testing Lab?

The test devices should be mounted on the holding fixtures according to the applicable orientation(s) below. Devices must be positioned so that they do not contact each other. This is so that they do not shield each other from the freely settling fog. And so that corrosion products and condensation from one device does not fall on another.

There are specific mounting requirements for dual-in-line packages with leads attached to, or exiting from, package sides. An example of this is side-brazed packages and ceramic dual-in-line packages. They are mounted lid upward 15° to 45° from vertical. One of the package sides on which the leads are located is oriented upward at an angle greater than or equal to 15° from vertical.

There are also requirements for packages with leads attached to, or exiting from the opposite side of the lid. This may include TO cans, solid sidewall packages, and metal platform packages, mounted with the lid 15° to 45° from vertical. Half of the samples are tested with the lid upward; the other half are tested with the leads upward. For packages with leads attached to, or exiting from the same side as the lid, only one orientation is required.

Packages with leads attached to, or exiting from package sides, parallel to the lid are mounted with the lid 15° to 45° from vertical. One of the package sides on which the leads are located is oriented upward at an angle greater than or equal to 15° from vertical. For packages with a metal case, half of the samples are tested with the lid upward; the rest are tested with the case upward. All other packages shall be tested with the lid upward.

Leadless and leaded chip carriers are mounted with the lid 15° to 45° from vertical. Half of the samples are tested with the lid upward; the rest are tested with the lid downward. Flat specimens are mounted 15° to 45° from vertical. This includes lids only and lead frames only.

What is the Appropriate Length of the Method of Salt Atmosphere Compliance Testing?

The minimum duration of exposure of the salt atmosphere test is specified by indicating a test condition letter from the following table. Unless otherwise specified, for best salt atmosphere testing, test condition A should apply.

Test Condition

Length of Test (Hours) 

A

24

B

48

C

96

D

240

What is the Failure Criteria for Products Seeking Salt Fog Certification?

It should be noted that corrosion stains are not considered part of the defective area. Corrosion products resulting from lead corrosion that deposit onto areas other than the lead are also considered as part of the defective area. Corrosion at the tips of the leads and corrosion products resulting from such corrosion is disregarded. Portions of leads which cannot be further tested in accordance, due to geometry or design are subject to the failure criteria.

All inspections shall be performed at a magnification of 10X to 20X, unless otherwise specified. For finished products, no device is acceptable that exhibits corrosion defects over more than 5 percent of the area of the finish or base metal of any element other than leads. Corrosion defects to be included in this measurement are: pitting, blistering, flaking, and corrosion products. The defective area may be determined by: comparison with charts or photographs of known defective areas, direct measurement, or image analysis.

Finished products with leads missing, broken, or partially separated also constitute a failure. In addition, any lead which exhibits pinholes, pitting, blistering, flaking, corrosion product that completely crosses the lead, or at the glass seal must be further tested. This testing includes bending the lead 90° at the point of degradation so that tensile stress is applied to the defect region.

Any lead which breaks or shows fracture of the base metal through greater than 50 percent of the cross-sectional area is a reject. In the case of multiple defects the bend is made at the site exhibiting the worst case corrosion. On packages exhibiting defects on more than ten leads, bends are made on a maximum of ten leads exhibiting the worst case corrosion. The examination of the fracture is performed with a magnification of 30X to 60X.

Finished products also fail when specified markings  are missing in whole or in part. They may be faded, smeared, blurred, shifted, or dislodged to the extent that they are not legible. This examination is conducted with normal room lighting and with a magnification of 1X to 3X.

This test is sometimes performed on package elements or partially assembled packages during incoming inspection. Or it may any time prior to completion of package assembly. This may be as an optional quality control gate or as a required test.

No part is acceptable that exhibits corrosion defects over more than 1.0 percent of the area of the finish or base metal of lids. Or that exhibits corrosion defects over more than 2.5 percent of the area of the finish or base metal of any other package element than leads. Corrosion on areas of the finish or base metal that will not be exposed to surrounding ambient after device fabrication are disregarded.

What Salt Fog Laboratory Provides the Best Salt Fog Compliance Testing?

Keystone Compliance is recognized as one of the best salt fog 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.

MIL-STD-883 testing contains several test methods. For more information about these test methods, please click on one of the links below.