MIL-DTL-16232 — heavy phosphate coatings with supplementary oil

Scope — and what it does not cover

MIL-DTL-16232 is the US defence detail specification for heavy phosphate coatings on ferrous parts. It separates the Type — the coating chemistry, manganese base or zinc base — from the Class, which states which supplementary treatment (typically a preservative oil) follows the coating, or none. It replaced MIL-P-16232. It is a coating specification: it does NOT define mechanical properties, does not cover electroplated zinc, and, as with every phosphate document, the corrosion performance belongs to the coating plus the supplementary oil, not to the crystal layer alone.

Standards are copyrighted documents. We explain scope, intent and practical application in our own words; obtain the current edition from ISO, ASTM, DIN or your national standards body.

What actually bites in production

• 1Type and Class are two separate decisions and both must be quoted. A Type without a Class leaves the most important variable — whether there is oil on the part — undefined.
• 2Manganese-base coatings are the heavy, dark, wear-oriented family. Zinc-base coatings are used where a lighter crystal and paint or oil compatibility is wanted.
• 3Coating mass is the acceptance measurement, quoted in g/m² or g/ft² depending on the revision. Check which unit your drawing means.
• 4Because these coatings are heavy, the dimensional effect on threads is real. Thread class and gauging after coating should be agreed before production.
• 5Documents in this family are revised, and sometimes superseded or cancelled. Put the revision letter on the order and confirm it is still current.

What to write on the drawing or PO

• Type (manganese or zinc base) and Class (supplementary treatment), with the revision letter.
• The coating mass requirement and the unit it is written in.
• The supplementary oil or preservative by specification, and whether the coater applies it or someone downstream does.
• Thread class and whether gauging is required after coating.
• Significant surfaces and any areas to be masked.

What goes wrong when it is cited loosely

• "MIL-16232 Type M" written alone: no Class, therefore no oil requirement, therefore no reproducible corrosion result.
• Citing the cancelled MIL-P- number from an old drawing and assuming the current document says the same thing.
• Specifying a heavy manganese coating on a fine-pitch thread that will then not gauge.
• Comparing salt-spray hours between an oiled and an unoiled lot.