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Black Oxide Pitting and Intergranular Attack (IGA) Inspection Process?

March 8, 2012

Q. Paragraph 3.3.1 of the MIL-DTL-13924 [affil. link or DLA] specification for black oxide coating specifies that "the process shall not result in any attack of the surface, either pitting or intergranular." Can someone point me to something that specifies how those tests are to be performed (e.g., magnification required, what to look for)?

Mike Palatas
Management - Gardena, California, USA

April 4, 2012

A. I have not found anything specifying the size or frequency of pits on black oxide parts but I will share with you how I inspect it in the shop I work at. I inspect a sample after cleaning and prior to the black oxide process using a 10X loupe. If the part is smooth and uniform going into the process, it should be smooth and uniform coming out. If there are pits after the process one of 2 things could be happening. Your activating solution (nitric, sulfuric or muriatic) could be too strong. Whichever one you use? Or your black oxide solution could be too concentrated. From my experience I have noticed that most pits occur from over etching in the prep stage. If you need specifics on how much is too much, this will depend on any MIL specs that may be called out on the customer contract. If in doubt, I always contact my customer.

William Kohnen
- Titusville, Florida, USA

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