Scope
ASTM B695 specifies requirements for mechanically deposited zinc coatings on iron and steel. The coating process and resulting zinc layer are appropriate for a broad range of fasteners, springs, and small parts, with particular value for high-strength components where hydrogen embrittlement is a concern.
The companion F959 DTI spec references B695 as the coating method for DTIs used with hot-dip galvanized A325 assemblies, because B695's non-thermal deposition preserves the precise DTI bump geometry that HDG would distort.
How mechanical galvanizing works
The process is distinctly different from both HDG and electroplating:
- Clean. Parts are degreased, acid-cleaned, and copper-flashed (a thin copper strike helps zinc adhesion).
- Load into tumbling barrel. Clean parts, glass beads (the media), water, zinc powder, and chemical promoters are loaded together into a rotating barrel.
- Tumble. As the barrel rotates, glass beads impact the parts continuously. The mechanical impacts cold-weld zinc powder onto the steel surface, building up the coating through peening.
- Separate and rinse. Coated parts are separated from media, rinsed, dried, and optionally chromate-passivated.
The coating is mechanically deposited rather than thermally or electrolytically bonded. No hydrogen is introduced to the steel in the process — the water-based chemistry and cold temperature prevent the acid-induced hydrogen absorption that plagues electroplating.
Coating thickness classes
B695 coating classes parallel the thickness levels of other zinc coatings:
| Class | Minimum thickness | Comparable to |
|---|---|---|
| Class 5 | 0.0002" (5 μm) | Fe/Zn 5 (F1941) |
| Class 8 | 0.00031" (8 μm) | Fe/Zn 8 |
| Class 12 | 0.00047" (12 μm) | Fe/Zn 12 |
| Class 25 | 0.00098" (25 μm) | Fe/Zn 25 |
| Class 40 | 0.0016" (40 μm) | Approaching HDG thickness |
| Class 50 | 0.0020" (50 μm) | HDG-equivalent |
| Class 55 | 0.0022" (55 μm) | Heavy HDG-equivalent |
For structural-bolt applications where mechanical galvanizing substitutes for HDG, Class 50 or 55 is the typical specification — matching the thickness level of HDG without the thermal and hydrogen concerns.
Why mechanical galvanizing matters for high-strength fasteners
The fundamental problem: zinc coating is the default corrosion protection for carbon steel, but the most common application methods (HDG, electroplating) introduce issues for high-strength steel:
- HDG — heat exposure at 840°F can temper steels that were hardened at marginal temperatures; coating thickness is too variable for precision fasteners
- Zinc electroplating — acid pickling and plating introduce dissolved hydrogen; high-strength steels at ≥39 HRC can experience delayed brittle fracture
- Mechanical galvanizing — cold process, no acid pickling, no electrolytic plating — effectively eliminates HE risk
This makes mechanical galvanizing the coating of choice for:
- F3125 A490 — HDG is explicitly prohibited by the standard; mechanical galvanizing is one of the approved alternatives
- A354 BD — HDG discouraged due to HE susceptibility
- F959 DTIs — coating precision critical; HDG distorts the DTI geometry
- High-strength springs
- Precision hardened pins and shafts
Supplementary chromate treatment
Like electroplating, mechanical galvanizing is often followed by a chromate conversion treatment:
- Clear (trivalent) — minimal added protection, aesthetic neutral
- Yellow (iridescent) — substantial added corrosion resistance
- Black — cosmetic, with moderate added protection
Modern mechanical galvanizing uses trivalent chromium chemistries to comply with RoHS, REACH, and ELV regulations.
Service life
Mechanical galvanizing thickness dictates service life much like HDG:
- Class 25 and below: similar to thin electroplate — months to a few years outdoor
- Class 50 (HDG-equivalent): 20–50+ years in most atmospheric conditions
- Class 55: similar to heavy HDG
For structural A490 connections outdoor, Class 50 mechanical galvanizing provides HDG-equivalent service life without the HE risk.
Appearance
Mechanical zinc coatings are typically:
- Matte gray, slightly more uniform than HDG
- No spangles (the crystalline pattern from slow-cooled HDG is absent)
- Less reflective than electroplated zinc
- Uniform thickness across complex geometries (HDG tends to pool in internal corners)
Yellow or black chromate alters the color as expected.
Thread accommodation
Mechanical galvanizing on threaded fasteners follows the same principles as HDG — thicker coating classes (40, 50, 55) require nut overtapping for assembly. Thinner classes may assemble without overtap. Specifications and fastener manufacturer recommendations coordinate the specifics.
Related specifications
- F3125 Annex — Specifies mechanical galvanizing as one of the approved coatings for Grade A490 structural bolts
- F1940 — Hydrogen embrittlement test methods (mechanical galvanizing typically passes where HDG and electroplate do not on high-strength fasteners)
- F959 — Direct tension indicators (mechanical galvanizing is the preferred DTI coating)
- B633 — Electroplating alternative
- F1941 — Electroplating on fasteners
- F2329 — HDG on fasteners
Documentation
California Fastener B695 orders ship with mill certificates showing coating class, coating thickness measurements, chromate type, and applicable hydrogen-embrittlement test results on high-strength fasteners. For structural A490 orders, rotational capacity testing per F3125 is included.