Clarifying ASTM B633

With over four decades of experience in zinc plating, this post aims to break down the details of the most common zinc electroplating specification, ASTM B633, shedding light on its significance in the metal finishing industry.

 

What is ASTM B633?

ASTM B633 is one of the most widely used standards that outlines specific requirements for electrodeposited zinc coatings on iron and steel. Set by the American Society for Testing and Materials (ASTM), it serves as a detailed guideline from engineer to plater, to ensure consistency and quality during electroplating.

 

What is the difference between ASTM B633 Types and Classes?

The coating type dictates the appropriate chromate or supplementary treatment, while the class establishes the minimum plating thickness based on the intended environment. Types focus on chromate/treatment selection, while classes determine the necessary plating thickness based on environmental considerations.

 

What are ASTM B633 Finish Types?

 

Type I – Zinc:

This basic zinc coating involves the application of zinc alone. There is no chromate applied over the zinc layer making it subject to potential oxidation post plating.

 

This is commonly referred to as “Zinc - No Chromate” and is best suited for applications that will undergo secondary processing.

 

 

Type II – Colored Chromate Conversion Coating on Zinc:

Introduces a colored hexavalent chromate conversion coating over the zinc. This chromate layer offers enhanced corrosion resistance and a distinct appearance.

Examples include Black Zinc, Yellow Zinc, or Olive Drab Zinc Plating.

 

Type III – Colorless Chromate Conversion Coating on Zinc:

This finish applies a colorless chromate conversion coating, providing a limited level of additional corrosion resistance to zinc oxidation. Type III doesn’t inherently designate Trivalent or Hexavalent, but Trivalent (also known as RoHS Compliant) is most common. 

Type III is often referred to as Zinc and Clear or Clear Zinc Plating and results in a bluish silver appearance.

 

Type IV – Phosphate Conversion Coating on Zinc:

This type involves the application of a phosphate conversion coating over the zinc layer which enhances adhesion and corrosion resistance. It is suited for applications where strong coating adhesion of subsequent coatings such as paint or powder coat is essential, also providing an additional layer of protection.

Note: Gatto does not offer this finish

 

Type V – Colorless Passivate on Zinc:

This coating calls for a colorless trivalent passivate over the zinc layer, providing a clear finish that should offer increased corrosion protection over type III. Type V is RoHS compliant.

 This finish may have a slight iridescent appearance.

 

Type VI – Colored Passivate on Zinc:

This coating includes a colored trivalent chromate following the zinc layer. Similar to Type V, it is RoHS compliant and will also offer substantial additional corrosion protection. Type VI will also add color depending on which finish is specified.

Common finishes include Zinc Trivalent Yellow or Zinc Trivalent Black.

 

What are ASTM B633 Thickness Classes for Coatings?

Each coating classification designates the required minimum thickness of the zinc layer which is determined by the conditions the metal coating will face during its intended use. This is also known as Service Condition, with applications ranging from mild or indoor to severe or highly corrosive environments.

 

 

What is the Difference between ASTM B633 Type II and Type III?

There are three main differences between ASTM B633 Type II and type III.

1.  Color of the Coating: Type II coatings designate a colored coating, where the final color is determined by which chromate is chosen. Black, yellow and olive drab are common for type II. In contrast, Type III coatings are specified as colorless. The chromate applied for type III is clear and will not add additional color to the finish, resulting in a blueish-silver appearance after clear chromate is applied.
2.  Corrosion Resistance: The level of protection against white corrosion differs significantly between ASTM B633 Type II and Type III. Type III coatings tend to be less corrosion resistant and are only required to provide 12 hours of protection in a neutral salt spray test, whereas Type II coatings offer a minimum of 96 hours. This distinction makes Type II better suited for applications requiring enhanced corrosion resistance.
3. Chromate Composition: Type II coatings specify that the chromate is hexavalent, while Type III can be fulfilled with either trivalent or hexavalent chromate. Trivalent chromates do not contain hexavalent chromium and will comply with the RoHS standard, making them most common for type III applications.

 

Is ASTM B633 RoHS Compliant?

RoHS compliance is determined by the type of chromate specified, falling into either Trivalent Chromate Passivation or Hexavalent Chromate categories. In the context of electroplating, hexavalent chromium is a concern due to its environmental and health implications. RoHS compliance restricts the presence of certain substances, including hexavalent chromium.

Trivalent finishes, also known as Trivalent Passivation, are RoHS compliant as they should not contain hexavalent chromium. On the other hand, Hexavalent finishes are non-RoHS compliant due to higher levels of restricted substances surpassing the RoHS threshold.

The coating Type (I, II, III, IV, V or VI), listed in the ASTM B633 specification, will specify whether or not the finish is RoHS Compliant.

 

What is the difference between Chromate and Passivate?

In ASTM B633, "chromate" refers to either trivalent or hexavalent conversion coatings. "Passivate" refers specifically to trivalent conversion coatings or conversion coatings that exclude hexavalent chromium, making them RoHS compliant.

 

Gatto Industrial Platers: Your Trusted Partner in Electroplating

In summary, ASTM B633 serves as a guide to standardize the zinc electroplating process. A comprehensive understanding of the types, classes, and their impact on finish performance is crucial to ensure you obtain the appropriate finish for your parts.

At Gatto Industrial Platers, we prioritize not only meeting industry standards but exceeding them. Our experts can help with any questions regarding ASTM B633 and guide you through the finishing process.