ASTM B633 Plating Specifications

About ASTM Plating Specifications

The American Society for Testing and Materials (ASTM) develops technical standards in manufacturing that have been adopted by various industries to enhance performance, safety and ensure higher-quality products.

ASTM specifications are among the most common standards used for Zinc and Zinc-Nickel plating. ASTM B633 is a widely used standard for Zinc and ASTM B841 for Zinc-Nickel.

ASTM B633 Classifications

ASTM B633 includes four classifications. Each classification specifies both Type and Service Condition:

  • Type (Type I,II,III,IV,V, or VI) – to determine which supplemental treatment or chromate to apply.  
  • Service Condition (SC1, SC2, SC3 or SC4) – to define the minimum plating thickness required according to the type of environment.

Service Condition 1 is for mild or indoor applications whereas service condition 4 is for very severe or highly corrosive applications.

ASTM B633 Chromate Specifications Type Chart

ASTM TypeDescriptionASTM B633 Compliant Finishes Offered at GattoRoHS Compliant
ASTM Type I Zinc, as plated (no chromate) Zinc - as is Yes
ASTM Type II Zinc, w/ colored chromate coating Yellow Hexavalent No
Olive Drab Hexavalent No
Black Hexavalent No
ASTM Type III Zinc, w/ colorless chromate coating Standard Clear Trivalent  Yes
ASTM Type IV Zinc, w/ phosphate conversion coating (NOT OFFERED AT GATTO) N/A
ASTM Type V Zinc, w/ colorless passivate High Corrosion Clear Trivalent Yes
ASTM Type VI Zinc, w/ colored passivate Yellow Trivalent Yes
Black Trivalent Yes

ASTM B633 Minimum Plating Thickness Requirements Service Condition Chart

ASTM Service ConditionThickness
SC4 - Very Severe 0.0010”min
SC3 - Severe 0.0005”min
SC2 - Moderate 0.0003”min
SC1 - Mild 0.0002”min


Specifying Plating Thickness and Significant Surfaces for ASTM B633


The electrolytic plating process can cause thickness variations of the coating from point to point on a part. For that reason, areas where minimum plating thickness is required should be labeled on the part drawing or noted on a sample piece. These areas or Significant Surfaces are normally visible and subject to wear and corrosion.

Surfaces such as threads, holes, deep recesses, and corners are not typically designated for minimum thickness requirements. Meeting minimum requirements for these areas could require a thicker coating on other areas of the part.


RoHS Compliance for ASTM B633

RoHS compliance of a finish is determined by which chromate is specified and will either fall into the category of Trivalent Passivation or Hexavalent Chromium. The Restriction of Hazardous Substances Directive (RoHS) restricts hexavalent chromium in products sold to the European Union because of their potential effects on the environment.

  • Trivalent finishes are RoHS compliant because they do not contain hexavalent chromium. Trivalent Chromate is also referred to as Trivalent Passivation.
  • Hexavalent finishes are not RoHS compliant because they contain levels of restricted substances higher than the RoHS threshold.

The type of chromate required is usually indicated on a parts’ engineered drawing or specified by the end user. 

What to Expect from ASTM B633 Plating Specifications:

ASTM B633 Surface Condition

The base metal surface condition can have a significant impact on the performance and appearance of the coating.

Electroplating will replicate the surface of a part. Scratches, pits, cracks, roll/die marks and porous surfaces will become more visible after plating.

Chemical Entrapment in Welds, Seams, and Joined Surfaces

The electroplating process introduces chemistry and liquids that can soak into a parts seams or porous surfaces. Standard rinsing procedures will not entirely flush the solution out of these crevices and over time the plating solution can slowly seep out. This is also referred to as plating bleed-out. Parts with incomplete welds, seams, porous surfaces or any other areas where two pieces are conjoined may be at risk.

The effects of chemical entrapment can vary from minor cosmetic inconsistencies such as staining to plating voids or degradation of the finish.

Though the risk of the entrapped plating solution is always present in parts with these features, Gatto uses a series of techniques throughout our plating process to minimize it’s risks and effects.

Parts should be free of defects prior to plating to ensure the highest quality coating.


ASTM B633 Appearance and Performance Expectations

Learn more about how to achieve a quality appearance for ASTM B633.

The primary purpose of Zinc Plating is to protect the base metal from corrosion. ASTM standards for performance can be met with a range in appearance from bright, to semi-bright to dull. 

Rinsing is a standard part of the plating process and can produce superficial staining on parts.  Secondary Processes such as baking or drying can cause discoloration or variations to the appearance. Neither of these will affect how the finish performs and are not grounds for rejection.

It is common to meet specification guidelines with slight color variations between parts, and among different orders.