DIN 50979 Metallic coatings – Electroplated coatings of zinc and zinc alloys on iron or steel with supplementary Cr(VI)-free treatment.
1. Scope and Coating Types
This standard covers electrodeposited coatings applied to iron and steel to provide corrosion protection. It specifically covers three types of galvanic layers:
- Zn: Zinc coating without alloying elements.
- ZnFe: Zinc-Iron alloy containing 0.3% to 1.0% Iron.
- ZnNi: Zinc-Nickel alloy containing 12% to 16% Nickel.
Important Note: This standard is specifically for systems using Cr(VI)-free passivation. Older chromate systems containing hexavalent chromium are not covered here.
2. Designation System (Coding)
The designation on technical drawings must follow the structure below using double slashes (//) as separators.
Structure:
Galvanic Coating DIN 50979 – [Basis Metal] // [Coating Code][Thickness] // [Passivation] // [Sealing]
2.1 Passivation Codes (Cr(VI)-Free)
Passivation creates a conversion layer to protect the zinc. Since these are Cr(VI)-free, the suffix “n” (for “new”) is used.
| Code | Type | Appearance | Description |
| An | Transparent | Colorless to colored iridescent | Often referred to as “Thin-layer passivation”. |
| Cn | Iridescent | Colored iridescent | Often referred to as “Thick-layer passivation”. |
| Fn | Black | Black | Color nuances are permissible. |
2.2 Sealing Codes
Sealing involves applying organic or inorganic agents (typically ≤ 2 μm thick) to increase corrosion resistance.
| Code | Description |
| T0 | Without sealing. |
| T2 | With sealing. |
2.3 Designation Examples
- Zinc-Nickel (8µm), Iridescent Passivation, No Seal:DIN 50979 – Fe//ZnNi8//Cn//T0
- Zinc-Iron (8µm), Black Passivation, Sealed:DIN 50979 – Fe//ZnFe8//Fn//T2
3. Ordering Information
When sending parts to an electroplater, the following information must be provided:
- Tensile Strength: Crucial for hydrogen embrittlement risk assessment.
- Component Details: Material type, manufacturing method, and heat treatment history.
- Significant Surfaces: Areas defined on the drawing (dot-dash line) where coating thickness and corrosion rules apply.
- Designation: The full coating code (from Section 2).
4. Hydrogen Embrittlement (High-Strength Parts)
4.1 Critical Threshold
Steel parts with a tensile strength (Rm) of ≥ 1000 N/mm2are classified as high-strength and are critical regarding hydrogen embrittlement. This includes parts with local hardening (e.g., case-hardened, cold-worked, or welded areas).
4.2 Heat Treatment Requirements
To prevent brittle fracture, heat treatment (baking) is required to promote hydrogen effusion. This should happen after electroplating.
Table 4: Heat Treatment Guidelines
Temperature: Component temperature must reach ( 215 ± 15)° C.
| Tensile Strength (Rm) in N/mm2 | Minimum Holding Time (Hours) |
| 1000 to 1250 | 6 h |
| 1251 to 1450 | 12 h |
| 1451 to 1600 | 20 h |
| 1601 to 2000 | 24 h |
Note: Care must be taken to ensure the metallic coating does not act as a diffusion barrier preventing hydrogen escape.
5. Technical Requirements and Testing
5.1 Coating Thickness
Thickness should be measured on the significant surface.
- Preferred Method: X-ray fluorescence (DIN EN ISO 3497).
- Alternative Methods: Microscopic (ISO 1463), Coulometric (ISO 2177), or Magnetic (ISO 2178).
5.2 Adhesion (Thermal Shock Test)
Parts are heated to ( 220 ± 15)° C for 30 minutes and then immediately quenched in water at 15°Cto 20°C
- Pass Criteria: No blistering or peeling of the coating.
5.3 Corrosion Resistance (NSS Test)
Testing is performed via Neutral Salt Spray (ISO 9227).
Pre-test Condition: All passivated parts (except Transparent An types) must be heat aged at 120°C for 24 hours before the salt spray test to simulate real-world curing.
Table 5: Transparent (An) and Iridescent (Cn) Coatings
(WR = White Rust/Coating Corrosion; RR = Red Rust/Base Metal Corrosion)
| Coating System | Type | Passivation | Sealer | Barrel/Rack | WR (h) | RR (h) @ 8μm | RR (h) @ 12μm |
| Zn | Zinc | An (Transp.) | T0 (No) | Barrel | 8 | 48 | 72 |
| Zn | Zinc | An (Transp.) | T0 (No) | Rack | 16 | 72 | 96 |
| Zn | Zinc | Cn (Iridesc.) | T0 (No) | Barrel | 72 | 144 | 192 |
| Zn | Zinc | Cn (Iridesc.) | T0 (No) | Rack | 120 | 216 | 264 |
| Zn | Zinc | Cn (Iridesc.) | T2 (Yes) | Barrel | 120 | 192 | 264 |
| Zn | Zinc | Cn (Iridesc.) | T2 (Yes) | Rack | 168 | 264 | 360 |
| ZnNi | Zn-Nickel | Cn (Iridesc.) | T0 (No) | Barrel | 120 | 720 | 720 |
| ZnNi | Zn-Nickel | Cn (Iridesc.) | T0 (No) | Rack | 192 | 720 | 720 |
Table 6: Black (Fn) Coatings
| Coating System | Type | Passivation | Sealer | Barrel/Rack | WR (h) | RR (h) @ 8μm | RR (h) @ 12μm |
| ZnFe | Zn-Iron | Fn (Black) | T2 (Yes) | Barrel | 120 | 264 | 360 |
| ZnFe | Zn-Iron | Fn (Black) | T2 (Yes) | Rack | 168 | 360 | 480 |
| ZnNi | Zn-Nickel | Fn (Black) | T2 (Yes) | Barrel | 168 | 720 | 720 |
| ZnNi | Zn-Nickel | Fn (Black) | T2 (Yes) | Rack | 240 | 720 | 720 |
Note: Corrosion requirements are capped at 720 hours to limit testing costs.
6. Test Report Requirements
The electroplater must provide a report containing:
- Reference to DIN 50979.
- Confirmation of conformity to the standard.
- Name of the electroplater.
- Process details (Rack vs. Barrel).
- For High Strength Parts ( ≥ 1000 N/mm2): Explicit certification that measures were taken to minimize hydrogen embrittlement (e.g., heat treatment records).