Introduction: Why Surface Treatment Matters for Fasteners
When engineers specify bolts, screws, or nuts for automotive assemblies, heavy trucks, or renewable energy structures, the base material is only half the story. The surface coating determines how long the fastener resists corrosion, whether it can be reliably torqued without hydrogen embrittlement, and whether it complies with increasingly strict environmental regulations. Among the modern coating systems available today, Geomet has become one of the most widely adopted — approved by virtually every major OEM worldwide.
What Is Geomet Coating?
Geomet is a chromium-free, zinc-aluminum flake coating applied to metal fasteners and components. The coating is formed by overlapping multiple layers of ultra-thin zinc and aluminum metallic flakes, bonded together with a water-based inorganic binder. The result is a dense, lamellar (layered) film that provides outstanding electrochemical corrosion protection.
A key technical advantage is the coating's extremely thin profile: only 6–8 µm (micrometers). This makes Geomet ideal for precision fasteners where tight dimensional tolerances must be maintained — something that thicker coatings like hot-dip galvanizing (50–80 µm) cannot achieve.
How Does Geomet Work?
Geomet protects fasteners through two complementary mechanisms:
- Barrier protection: The tightly overlapping zinc-aluminum flakes create a physical barrier that blocks moisture and corrosive agents from reaching the steel substrate.
- Cathodic (sacrificial) protection: Zinc is electrochemically more active than steel. If the coating is scratched or damaged, the zinc flakes corrode preferentially, protecting the underlying steel from rust — the same principle as hot-dip galvanizing, but in a far thinner and more controllable film.
Because the process uses a water-based, chromium-free formulation, Geomet complies with RoHS, ELV (End-of-Life Vehicle), and REACH environmental directives — a critical requirement for automotive supply chains in Europe and beyond.
Geomet Grades and Variants
Geomet is available in several grades, each optimized for specific performance or application requirements:
- GEOMET 321 — The standard grade; excellent corrosion resistance, widely specified across automotive OEMs.
- GEOMET 321 VL — A variant with enhanced topcoat for improved friction coefficient control, used by Mercedes-Benz (DBL 9440.40) and Volkswagen (TL 245 Class).
- GEOMET 321 L / M / ML / XL — Variants with different lubrication levels to achieve precise torque-tension (K-factor) performance, as defined in ISO/EN 10683.
- GEOMET 500A — Specified by Peugeot-Citroën (B15 3320-ZLA/ZLV MGL/MGO) for enhanced corrosion and chemical resistance.
- GEOMET 720 — A higher-performance grade with superior salt spray resistance, approved by Mazda (MES CG 311F) and Hyundai-Kia (MS 619-08).
- GEOBLACK 500M — A black-pigmented variant for aesthetic applications, specified by Volvo Truck (STD 121-0003).
OEM Approvals and Industry Standards
One of the strongest indicators of Geomet's reliability is the breadth of its OEM approvals. The table below summarizes the major specifications:
| OEM / Organization | Specification | Geomet Grade |
|---|---|---|
| ISO | ISO/EN 10683 | GEOMET 321 L, M, ML, XL |
| Mazda | MES CG 311F | GEOMET 720 |
| Mercedes-Benz | DBL 9440.40 | GEOMET 321 VL |
| Peugeot-Citroën | B15 3320-ZLA/ZLV MGL/MGO | GEOMET 500A |
| Volkswagen | TL 245 Class | GEOMET 321 VL |
| Volvo Truck | STD 121-0003 | GEOBLACK 500M |
| ASTM | ASTM F1136 | GEOMET 321 |
| BMW | GS 90010-ZNS3 | GEOMET 321 |
| Chrysler | PS 5873 | GEOMET |
| Delphi | DX551501 | GEOMETET |
| Ford Motor Company | WSS-M21P39-A1 | GEOMET 321 |
| Hyundai-Kia | MS 619-08 | GEOMET 720 |
| John Deere | JDM F13D | GEOMET 321 |
| ZF Lemförder | LMN 50-6 3.2 | GEOMET 321 |
This level of cross-OEM acceptance is rare in the fastener coating world and reflects Geomet's consistent, verifiable performance across independent testing programs.
Key Benefits of Geomet Coating
- No hydrogen embrittlement risk: Unlike electroplated zinc, Geomet is applied without an electrolytic process, eliminating the risk of hydrogen absorption — critical for high-strength fasteners (Grade 10.9, 12.9).
- Excellent corrosion resistance: Geomet 321 typically achieves 720+ hours in neutral salt spray (NSS) testing per ISO 9227, with higher grades exceeding 1,000 hours.
- Thin, dimensionally precise film: At 6–8 µm, Geomet preserves thread tolerances on metric fasteners without requiring oversized pre-tapping.
- Consistent friction coefficient: Lubricated variants (L, ML, VL) deliver a controlled K-factor, enabling reliable torque-tension assembly — essential for safety-critical joints.
- Environmentally compliant: Chromium-free and RoHS/ELV/REACH compliant. No hexavalent or trivalent chromium compounds.
- Wide application range: Suitable for bolts, nuts, washers, screws, clips, and stamped parts across automotive, heavy truck, wind energy, and construction sectors.
Geomet vs. Other Common Fastener Coatings
| Property | Geomet | Electroplated Zinc | Hot-Dip Galvanizing | Dacromet |
|---|---|---|---|---|
| Coating thickness | 6–8 µm | 5–25 µm | 50–80 µm | 8–10 µm |
| Hydrogen embrittlement risk | None | High | None | None |
| Chromium-free | ✅ Yes | Depends on passivation | ✅ Yes | ❌ No (Cr6+) |
| Salt spray (hrs, typical) | 720–1,000+ | 96–240 | 500+ | 500–720 |
| Thread tolerance impact | Minimal | Minimal | Significant | Minimal |
| OEM automotive approval | Extensive | Limited (high-strength) | Limited | Being phased out |
Typical Applications
Geomet-coated fasteners are found across demanding industries:
- Automotive: Engine bay bolts, chassis fasteners, brake system hardware, suspension components
- Heavy trucks & commercial vehicles: Frame bolts, axle hardware, exhaust system fasteners
- Renewable energy: Wind turbine tower bolts, solar mounting structure fasteners
- Construction & infrastructure: Structural bolts in corrosive environments
- Agricultural machinery: John Deere-specified hardware for outdoor exposure
Ordering Geomet-Coated Fasteners from FULLERKREG
At FULLERKREG Fasteners, we supply a broad range of DIN/ISO standard fasteners with Geomet surface treatment, including socket head cap screws (DIN 912), flat washers (DIN 125A), and hex bolts across multiple grades. Whether you need GEOMET 321 for standard automotive applications or GEOMET 720 for higher-performance requirements, our team can advise on the correct grade and confirm OEM compliance documentation.
Contact us for bulk pricing, technical datasheets, and sample requests.