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| Zinc | Mil Spec | Thickness | Comments |
| QQ-Z-325B | |||
| Either a bright or dull finish is acceptable. Bright Zinc plating closely resembles bright chromium. Zinc coated steel will not rust even when exposed by scratches because of the galvanic protection of the zinc. On weathering, zinc turns to a drab gray color. Zinc should be deposited directly on the base metal (Nickel is permissible undercoat if base metal is a corrosion resisting steel). Parts having a hardness greater than Rc-40 must be given a heat treatment prior to plating. Springs having a hardness over Rc-40 must be given an after plating baking at 375°-25°F for 3 hours. | Class 1 Class 2 Class 3
Type I Type II Type III |
.0010” .00050” .00020”
|
The primary use of chromate finishes on zinc is to retard or prevent formation of white corrosion products on zinc surfaces.
The primary purpose of phosphate coating on zinc is to provide a paint base. Without supplementary treatments With supplementary chromate treatment With supplementary phosphate treatment. |
ASTM Designation: B 633
Electrodeposited Coatings of Zinc on Iron and Steel
Classification
Type |
Description |
I |
As-plated without supplementary treatment |
II |
With colored chromate conversion coatings |
III |
With colorless chromate conversion coatings |
IV |
With phosphate conversion coatings |
Thickness Classes for Coatings
Classification* Number and Conversion Coating Suffix |
Service Condition** | Thickness, Min µm |
Fe/Zn 5 |
SC 4 (very severe) | 25 |
Fe/Zn 12 |
SC 3 (severe) | 12 |
Fe/Zn 8 |
SC 2 (moderate) | 8 |
Fe/Zn 5 |
SC 1 (mild) | 5 |
* Iron or steel with zinc electroplate. Numeral indicates thickness in micrometres.
** Where service conditions are valid only for coatings with chromate conversion coating. Type II for SC 4 and SC 3 and Type III for SC 2 and Sc 1.
Corrosion Resistance Requirements
Types |
Test Period, h |
II |
96 |
III |
12 |
Examples of appropriate service conditions and description of service conditions
SC 4 – Very Severe: Exposure to harsh conditions, or subject to frequent exposure to moisture, cleaners, and saline solutions, plus likely damage by denting, scratching, or abrasive wear. Examples are: plumbing fixtures, pole line hardware.
SC 3 – Severe: Exposure to condensation, perspiration, infrequent wetting by rain, and cleaners. Examples are: tubular furniture, insect screens, window fittings, builder’s hardware, military hardware, washing machine parts, bicycle parts.
SC 2- Moderate: Exposure mostly to dry indoor atmospheres but subject to occasional condensation, wear, or abrasion. Examples are: tools, zippers, pull shelves, machine parts.
SC 1 – Mild: Exposure to indoor atmospheres with rare condensation and subject to minimum wear or abrasion. Examples are: button, wire goods, fasteners.
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