ASTM F1137 Class A/B/C/D Phosphate/Oil Coated Fasteners

Stud Bolts, Nuts, Screws, Threaded Rod & Washers

ASTM F1137 specification covers the basic requirements for six grades of corrosion protection for fasteners. Grade A consists of a heavy zinc phosphate coating with no additional sealer (dry), Grade B consists of a heavy zinc phosphate coating with a dry organic sealer, Grade C consists of a heavy zinc phosphate coating with supplemental dry-to-touch oil type compound, Grade D consists of a heavy zinc phosphate coating with supplemental protective oil type compound,
Grade E consists of a grain-refined microcrystalline zinc phosphate with supplemental oil type compound, and Grade F consists of a manganese phosphate coating with supplemental oil type compound. This specification is intended primarily for fasteners such as nuts, clips, washers, and other ferrous threaded and non-threaded fasteners that require corrosion protection and lubrication. These coatings may or may not have a decorative finish. The values stated in either SI units or inch-pound units are to be regarded separately as standard.

ASTM F1137 Coated Fasteners, Bolts, Screws, Nuts & Washers

ASTM F1137 Classifications

Grade Phosphate Treatment Coating Weight Supplemental Coating Coating Thickness, μm
mg/ft2 g/m2
ASTM F1137 Grade A Heavy Zinc Phosphate 1000, min 11, min none 72A
ASTM F1137 Grade B Heavy Zinc Phosphate 1000, min 11, min sealer 72
ASTM F1137 Grade C Heavy Zinc Phosphate 1000, min 11, min protective oil-type compound  (dry-to-touch) 24
ASTM F1137 Grade D Heavy Zinc Phosphate 1000, min 11, min protective oil-type compound 72
ASTM F1137 Grade E Microcrystalline  Zinc PhosphateB 185 min- 650 max 2 min - 7 max protective oil-type compound 8
ASTM F1137 Grade F Manganese Phosphate 370 min - 1765 max 4 min- 19 max protective oil-type compound 24
  • A) When testing Grade A finished parts, a protective oil coating shall be added before salt spray testing.
  • (B) Microcrystalline zinc phosphate crystals shall be refined such that a mean crystal size of 4-8 µm is obtained, with a crystal size of 10 µm.

ASTM F1137 Requirements

Unless otherwise agreed upon between the purchaser and the producer, the color of the protective coating shall be as-coated gray for all grades.
Corrosion Resistance
These coatings shall be capable of withstanding neutral salt spray for the minimum h specified in Table 1 with no base metal corrosion on significant surfaces. Significant surfaces on threaded fasteners are defined as the exposed surfaces when the fasteners are installed in a normal manner (bolt head, nut head, face, etc.). On other surfaces where control of the coating cannot be obtained under normal processing such as holes, recesses, threads, etc., the above requirements do not apply.
Thread Fit
The maximum thickness of coating which may be applied to threads on threaded products is limited by the basic thread size . Threads may be produced undersize/oversize (before coating) to accommodate the coating thickness, providing the finished product (after coating) meets all specified mechanical properties. All undersize/oversize must be within permissible limits as agreed upon between the supplier and the purchaser.
ASTM F1137 Grades B and ASTM F1137 Grade C shall be evenly coated and dry-to-touch such that when held with filter paper or equivalent and applying hand pressure for 5 to 10 s, there shall be no visible staining of the filter paper when viewed without use of supplementary magnification. For referee purposes, this test shall be performed using a force of 10 N as measured with a load-indicating gage.

ASTM F1137 Inspection

Samples shall be taken in accordance with Practice F1470.
Referee Inspection
The following referee thread inspection procedure may be utilized if the specified "go" gage binds on the screw.
Bolt or Screw
Assemble a phosphate-plated test nut with a 2B or 6H class thread down the full length of the thread.
Assemble a phosphate-plated test bolt or screw with a 2A or 6g class thread for a minimum of one diameter through the nut. The test nut or screw/bolt must run freely for the total length without binding.