ASTM B633
ASTM B633: Electrodeposited Coatings of Zinc on Iron and Steel
ASTM B633 is the primary industrial standard used to specify zinc plating on iron and steel components. It defines the requirements for materials, coating thickness, finish types, and testing procedures to ensure corrosion protection.
- Scope: Covers electrodeposited zinc coatings on iron and steel.
- Exclusions: Does not cover zinc-coated wire/sheets (see ASTM A591) or mechanically deposited coatings (see ASTM B695). While often used for fasteners, ASTM F1941 is the specific standard for fastener plating, though B633 is still commonly cited.
Classification & Service Conditions
ASTM B633 uses a "Service Condition" (SC) code to define the required thickness of the zinc plating. The service condition should be selected based on the severity of the environment the part will be exposed to.
| Service Condition | Classification | Thickness (min) | Environment Description | Typical Applications |
|---|---|---|---|---|
| SC4 | Fe/Zn 25 | 25 µm (0.0010") | Very Severe | Harsh outdoor elements, saline solutions, frequent moisture. |
| SC3 | Fe/Zn 12 | 12 µm (0.0005") | Severe | Condensation, perspiration, occasional rain. (e.g., Outdoor furniture, window fittings). |
| SC2 | Fe/Zn 8 | 8 µm (0.0003") | Moderate | Dry indoor atmosphere with occasional moisture/wear. (e.g., Standard fasteners, tools). |
| SC1 | Fe/Zn 5 | 5 µm (0.0002") | Mild | Indoor only, minimum wear. (e.g., Wire goods, buttons). |
Finish Types (Passivation)
The "Type" designation specifies the supplementary finish (chromate/passivate) applied over the zinc to boost corrosion resistance and alter appearance.
Note on RoHS: Types V and VI are explicitly designed for Trivalent (Hexavalent-Free) passivates, making them the preferred choice for RoHS/REACH compliance.
| Type | Description | Finish Appearance | Salt Spray (White Rust) |
|---|---|---|---|
| I | As-plated (No supplementary treatment) | Bright / Dull Silver | N/A |
| II | Coloured Chromate (Hexavalent) | Yellow / Gold / Olive | 96 Hours |
| III | Colourless Chromate | Clear / Blue-Bright | 12 Hours |
| IV | Phosphate Conversion | Grey | N/A |
| V | Colourless Passivate (RoHS) | Clear / Blue-Bright | 72 Hours |
| VI | Coloured Passivate (RoHS) | Iridescent / Yellow | 120 Hours |
Hydrogen Embrittlement Relief
High-strength steels are susceptible to hydrogen embrittlement during the pickling and plating process. ASTM B633 mandates specific thermal treatments to mitigate this risk.
Susceptibility Threshold: Parts with a tensile strength of 1000 MPa (approx. 31 HRC) or greater are considered high-risk.
Pre-Plating Stress Relief: High-strength parts that have been cold-formed, machined, or ground must be stress-relieved before cleaning and plating.
Post-Plating Baking:
- Parts >1000 MPa must be baked after plating to diffuse hydrogen.
- Timing: Baking must commence within 4 hours of the plating operation.
- Temperature: Typically 190°C – 220°C (refer to ASTM B850 for specific schedules).
Exclusion: Steels with tensile strengths >1700 MPa (approx. 46 HRC) should generally not be electroplated due to the extreme risk of embrittlement.