This specification covers the basic requirements for four
grades of corrosion protection for fasteners. Grade A consists
of a zinc phosphate coating with no additional sealer (dry),
Grade B consists of a zinc phosphate coating with a dry organic
sealer, Grade C, and Grade D.1.2 This specification is intended
primarily for fasteners such as nuts, clips, washers, and other
ferrous threaded and non-threaded fasteners that require corrosion protection.
This specification is intended primarily for fasteners
such as nuts, clips, washers, and other ferrous threaded and
non-threaded fasteners that require corrosion protection.
These coatings may or may not have a decorative finish.
The zinc phosphate treatment and subsequent protective coatings are classified into four grades according to the requirements shown in Table 1.
Classification and Performance Requirements of Protective Coatings| Grade No. | Zinc Phosphate, g/m2 | Supplemental Coating | Coating Thickness, µm | Salt Spray, h |
|---|---|---|---|---|
| Grade A | 11, min | none | see 5.3 | 72 A |
| Grade B | 11, min | sealer | see 5.3 | 72 |
| Grade C | 11, min | protective oil-type compound | see 5.3 | 24 |
| Grade D | 11, min | protective oil-type compound | see 5.3 | 72 |
| A When testing Grade A finished parts, a protective oil coating shall be added before salt spray testing. | ||||
Appearance—Unless otherwise agreed upon between
the purchaser and the producer, the color of the protective
coating shall be as-coated gray for Grades A, B, C, and D.
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 (see 8.2). 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.
Dry-to-Touch—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.
The corrosion resistance shall be determined in accordance with Practice B117.
The coating thickness may be determined by microscopic examination of the cross-section taken
perpendicular to
the significant surface or by determining the coating mass
(g/m2
) by a weigh-strip-weigh method and then converting to
a coating thickness (see Note 1). Coating thickness, for referee
purposes, shall be measured by microscopic examination.
Note 1 — The weigh-strip-weigh method involves weighing the test
specimen before and after stripping the deposit with a reagent which does
not attack the base metal.
The formula used for measuring the thickness of a deposit by
the weigh-strip-weigh method is:
T= W/AxD x10 3
where:
T = thickness, µm,
W = mass of deposit, g,
A = area of deposit, m2
, and
D = density of coating, kg/m 3.
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.
Nut—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.
Unless otherwise specified, any rejection based on tests specified herein and made by the purchaser shall be reported to the manufacturer as soon as practical after receipt of the product by the purchaser.
coating; corrosion; fasteners; protection; phosphate; resistance; rust
ASTM Standards: 2
B117 Practice for Operating Salt Spray (Fog) Apparatus.
F1470 Practice for Fastener Sampling for Specified Mechanical Properties and Performance Inspection.