ASTM B633 Plating Specifications

Overview of ASTM Plating Standards

The American Society for Testing and Materials (ASTM) establishes manufacturing standards that help industries improve product quality, performance, and safety. These standards are widely adopted across sectors that require consistent and reliable plating processes.

For zinc-based coatings, the two most commonly referenced specifications are:

• ASTM B633 – the primary standard for Zinc electroplating

• ASTM B841 – the standard for Zinc-Nickel electroplating

Both standards define coating characteristics that ensure corrosion protection, durability, and proper performance in the intended environment.

ASTM B633 Classification Details

ASTM B633 divides zinc plating requirements into two main elements: Type and Service Condition. Together, these determine the chromate/passivate used and the minimum plating thickness.

1. Type (I, II, III, IV, V, VI) – Chromate or Passivation Requirement

Each Type specifies what kind of chromate or supplemental surface treatment should be applied over the zinc coating. This may include clear, yellow, black, olive drab, trivalent, or phosphate conversion coatings depending on the required appearance and corrosion protection.

2. Service Condition (SC1, SC2, SC3, SC4) – Minimum Zinc Thickness

Service Conditions define how thick the zinc coating must be, based on the severity of the environment:

• SC1 – Mild or indoor environments

  Light exposure; minimal corrosion risk.

• SC2 – Moderate environments

  General indoor use or limited outdoor exposure.

• SC3 – Severe environments

 Frequent outdoor exposure or moderate corrosive conditions.

• SC4 – Very severe or highly corrosive environments

  Industrial atmospheres, marine environments, or long-term outdoor performance.

The plating Type and Service Condition must be paired correctly to ensure the coating meets durability requirements for the intended application.

Electrolytic zinc plating can naturally produce variations in coating thickness across different areas of a component. To ensure critical sections receive the required minimum deposit, these surfaces — known as Significant Surfaces — should be clearly identified on engineering drawings or sample parts.

These areas are typically exposed in normal use and are the most susceptible to wear, abrasion, and corrosion.

Features such as internal threads, blind holes, sharp recesses, and deep corners are usually not classified as significant surfaces. Achieving the minimum deposit inside these areas often requires excessive buildup on the primary surfaces, which can negatively affect part fit and function.

RoHS Considerations for ASTM B633

RoHS compliance under ASTM B633 depends entirely on the chromate conversion coating selected. Chromates fall into two categories:

• Trivalent Passivation (RoHS Compliant)

  Trivalent chromates do not contain restricted hexavalent chromium and therefore satisfy RoHS environmental regulations. These finishes are commonly used for components intended for global or EU markets.

• Hexavalent Chromate (Not RoHS Compliant)

  Hexavalent chromates exceed RoHS limits and are restricted in products sold in the European Union due to their environmental and health impact.

The specific chromate type required for a job is usually defined in the part’s engineering documentation or specified directly by the customer.