Phosphate and oil
Phosphating grows a layer of fine manganese or zinc phosphate crystals on the steel in an acid bath, and the crystals hold oil. The finish is grey to black. Oiled, it protects studs and nuts in storage and transit, and it is the base that PTFE coatings go over. It is applied by outside Houston processors.
Degrease, blast, phosphate, rinse, oil.
- STEP 01
Degrease
After all machining and forming is finished, the parts are cleaned by solvent, vapour, alkaline or emulsion cleaning.
- STEP 02
Blast
Abrasive blasting removes rust and scale and is required before heavy phosphating unless the drawing says otherwise. The parts go straight into the phosphate after it.
- STEP 03
Phosphate
The parts are immersed in a manganese (Type M) or zinc (Type Z) phosphate bath, whose acid and iron content is checked every four hours.
- STEP 04
Rinse and seal
A cold running-water rinse follows before the parts can dry, then a sealer rinse for Classes 1 to 3.
- STEP 05
Relieve if hard
Steel of 39 HRC and above is held at 210 to 225 °F (98 to 107 °C) for at least 8 hours, or rested for 120 hours, so hydrogen can escape.
- STEP 06
Oil
For Class 2, oil or rust preventive is applied and the surplus is spun or drained off.
- Light protection: bare phosphate need only pass 1.5 hours (Type M) or 2 hours (Type Z) of salt spray, and oiled Class 2 48 hours (M) or 72 hours (Z). The oil or coating on top gives most of the protection.
- Heat: manganese phosphate should not be exposed above 250 °F (121 °C), or zinc phosphate above 200 °F (93 °C).
- Zinc phosphate should not be used where it will contact alkaline materials.
- Not for surfaces that must stay highly polished after coating.
Finishes are applied by outside coaters and platers in Houston, to the specification on the order.
A thin crystal layer the fit has to allow for.
Phosphate is not zero thickness. MIL-DTL-16232 expects 5 to 10 µm for manganese and 5 to 15 µm for zinc, and requires the parts to meet their drawing dimensions after coating, with an allowance for the build-up. The specification's requirement is coating weight; the thickness figures are a guide.
The crystals are friable, so a tight fit can be assembled by forcing it or wiping off the surplus.
Phosphating is an acid process. The specification relieves steel of 39 HRC and above; B7 and L7 studs and 2H and Grade 7 nuts, at 35 HRC maximum, are below that line. On friction, oiled manganese phosphate helps parts run in. The chemically converted Class 4 is recommended for reducing sliding friction, and it narrows the spread of torque against tension on threaded fasteners.
Designations and classes.
How to call it out.
- Give the type and class, for example phosphate to MIL-DTL-16232, Type M, Class 2 for manganese phosphate and oil on B7 studs and 2H nuts.
- If the phosphate is the base for a PTFE topcoat, order the two together as one coating system.
- Ask for coating weight or salt spray results if your specification needs them.
The tests behind it.
Coating thickness and salt spray
Coating thickness is measured with a magnetic or eddy-current gauge, or by X-ray fluorescence for thin platings, and checked against the class ordered. Salt-spray exposure checks corrosion protection on a sample from the lot when the specification calls for it.
Will a plated high-strength bolt crack in service?Hydrogen embrittlement relief and testing
High-strength steel can pick up hydrogen during pickling and plating and fail days later under load. Parts above 39 HRC are baked after plating to drive the hydrogen out, and a sustained-load test on fasteners from the lot proves the relief worked.
Phosphate and oil: common questions.
Does phosphate change the thread size?
Slightly. Manganese phosphate is expected to be 5 to 10 µm thick and zinc phosphate 5 to 15 µm. The thread fit has to allow for it, and the friable crystals can be wiped or forced off where a fit is tight.
How long does phosphate and oil last?
It is meant for storage and transit. Oiled manganese phosphate must pass 48 hours of salt spray and oiled zinc phosphate 72 hours (MIL-DTL-16232). A PTFE topcoat over the phosphate adds far more protection.
Manganese or zinc?
Manganese (Type M) stands up better to alkaline conditions. Zinc (Type Z) is used where paint and oil both go on, and to prevent galling in cold extrusion and deep drawing.
Do B7 studs need a bake after phosphating?
No. The relief treatment is for steel of 39 HRC and above, and B7 is 35 HRC maximum.
Compare with.
PTFE / fluoropolymer coating
A thin fluoropolymer film, usually blue or green, applied over a phosphate base. It gives a low, consistent coefficient of friction, so less torque is needed for the same bolt tension and the torque is set for the coated parts; it resists the atmosphere and chemical splash, and helps nuts come off at the next turnaround.
A clean black finish without dimensional changeBlack oxide
A chemical conversion of the steel surface to black iron oxide, sealed with oil or wax. It adds no thickness, so it suits precision parts and socket products, and gives a uniform dark appearance with light corrosion resistance indoors.
Economical sacrificial protection for indoor and mild serviceZinc electroplating
A thin electroplated zinc layer with a clear, yellow or black chromate, or a chromium-free passivate. Zinc corrodes in preference to the steel underneath, so a scratch does not rust. It is the general-purpose finish for hardware that lives indoors or under cover.