Zinc Coating – Mil-Spec: QQ-Z-325B

Class / Type Thickness
Class 1 .0010″
Class 2 .00050″
Class 3 .00020″
Type 1 Without supplementary treatments
Type 2 With supplementary chromate treatment.
Type 3 With supplementary phosphate treatment

ASTM Designation: B 633

Electrodeposited Zinc Coating on Iron and Steel.

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 CoatingSuffix 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

Type Test Period h
I 96
II 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.