MIL-DTL-13924D
4.1.1 Classification. All the tests required for the testing are classified as
quality conformance tests, for which necessary sampling techniques and the methods
of testing are specified in this section.
4.2
Sampling.
4.2.1 Lot. A lot shall consist of coated parts of the same class, same basis
metal, and approximately the same size and shape and coated under similar
conditions. A maximum of 8 hours continuous production shall constitute a lot.
4.2.2 Selection. The number of random samples selected for nondestructive tests
(visual inspection, coverage, color, smut, and workmanship), and destructive tests
(oxalic acid spot test, salt spray test, and embrittlement relief test) from each
lot of coated parts shall be defined by the procuring activity (see 6.2).
4.3 Acceptance and rejection. The selected coated parts shall meet the
requirements in this specification for the represented lot to be acceptable.
Failure of any test samples to meet the requirements in this specification shall
constitute rejection of the entire lot which they represent. Unless otherwise
specified (see 6.2), the tests shall be conducted and the test results accepted
prior to shipment of the lot of coated parts represented by the test samples.
4.3.1 Surface attack. Parts shall be examined for pitted surfaces or
intergranular attack by viewing them at magnifications up to 10x.
4.4
Test procedures.
4.4.1 Smut test. The test shall be made prior to application of corrosion
preventive compound or after vapor degreasing. Each black oxide coated piece shall be
inspected visually under strong light to assure a satisfactory appearance. Each
sample shall also be wiped with a clean white cloth for indications of smut (see 3.7).
A slight amount of smut which is inherent in the process is acceptable for all classes
of coatings and shall not be cause for rejection.
4.4.2 Oxalic acid spot test (class 1, 2, and 3). The black oxide coated pieces of
classes 1, 2, and 3 only, prior to the application of a preservative, shall be handled
with clean cotton gloves or the equivalent. Each sample shall have deposited, on one
flat spot of the black oxide coated surface, three drops (0.00676 oz. (0.2 ml)) of a
five percent solution of oxalic acid. The reaction shall be observed after 30 seconds
and up to eight minutes. After eight minutes the panel shall be rinsed and compared
to figures 1, 2, and 3. A light gray center with a lighter border color (figure 1)
indicates a poor quality coating. A gray-black center with a light border (figure 2)
indicates a borderline quality coating. The coating shall conform to the good quality
coating of figure 3 to be acceptable. A black or dark brown center with a light
border (figure 3) indicates a good quality coating. A good quality coating may show a
light border, indicating exposure of metal around the drops. Parts shall therefore be
judged only on the color and exposure of the metal under the drops.
4.4.3 Resistance to salt spray (class 4). The coated pieces of class 4
(AISI type 300 series corrosion resistant steel) shall be subjected to a 5 percent
salt spray (fog) test in accordance with ASTM B117. Exposure time for the black
coatings, prior to the application of a preservative, or after vapor degreasing,
shall comply with the requirements of 3.9.
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