Part manufacturers consider zinc plating more of a protective surface finishing process for metals like steel and iron. However, the electrochemical process is more than that as it boasts a high aesthetic value due to the different color options available.

Traditionally, zinc plating gives a grayish blue or a silver hue. However, different techniques, post-processing treatments, and additives can alter the color of the zinc-plated part. This article will introduce and talk about zinc plating techniques, the distinct colors, and possible applications. 

Define zinc plating

Zinc plating or galvanization is an electrochemical or chemical deposition of a thin layer of zinc on metals. It is a protective surface treatment option aimed at improving parts’ durability, corrosion resistance, and longevity.

Step-by-Step Guide to Zinc Plating

Zinc plating takes place using the following steps:

  • Cleaning

Cleaning the substrate can be mechanical, electro-cleaning, or chemical. Physical and electrochemical cleaning will remove contaminants like dirt, grease, or rust. Chemical cleaning using acids will remove oxides on the substrate’s surface 

  • Immersion or Electrolysis

For hot dip galvanizing, the substrate is immersed in a bath containing molten zinc. For electrochemical zinc plating, the substrate is immersed in a bath containing zinc salts (zinc sulfate or zinc chloride) and some additives. The passage of direct electric current through the bath leads to the deposition of zinc on the substrate.

  • Post Treatment 

Post-treatment processes include chromate conversions, passivation, etc. They function in improving the zinc-plated part’s properties and aesthetic value 

Explain different types of zinc plating based on color

Zinc plating and its post-treatment process such as chromate conversions lead to different colors. Below is an overview of the different types of zinc plating based on the color

Type of Zinc Plating MechanismColor
Hot Dip GalvanizingIt involves immersing the substrate (mostly steel) in molten zinc at about 450°C (842°F). The zinc then bonds with the substrateTraditional silver hue
Yellow Chromate CoatingIt is a post-treatment process that involves immersing the zinc-plated part into a solution containing hexavalent chromium compounds. This forms a thin chromate conversion coatingYellow
Trivalent Chromium ProcessIt involves depositing a trivalent chromium conversion coating on the zinc-plated surface.Iridescent (changes based on the angle)
Clear Zinc CoatingAn electrochemical process of depositing zinc on the substrate. The substrate is immersed in a zinc solution and a direct electric current is passed. Blue tint
Colored Zinc Alloy CoatingElectrochemical or mechanical deposition of zinc alloys on the substrate  Variable

Hot Dip Galvanizing

Hot dip galvanizing is a mechanical deposition of zinc on a part. The process is applicable in improving steel’s properties and aesthetic value and it involves dipping the steel part into a molten zinc solution. 

The zinc bonds with the steel metal, resulting in a silver hue or a grayish-blue color. Functionally, the bond between both metals protects the steel from corrosion and makes it durable. 

Common Use and Applications

Hot dip galvanizing is the best method for non-tolerance plating due to the difficulty in controlling the thickness of the painting. Common industries and the parts made using the process are:

  • Construction: Beams, columns, and trusses used in bridges, highways, and buildings
  • Utilities: Electrical transmission towers, telecommunications poles, and water distribution systems.
  • Automotive: Chassis, frames, and body panels 
  • Agriculture: Fencing and livestock enclosures 
  • Infrastructure: Railings, guardrails, and highway barriers. 

Yellow Chromate Coating

Yellow chromate coating or yellow zinc chromate coating is a post-treatment process for zinc-plated parts. It involves electroplating a zinc-plated part by immersing it in a solution containing hexavalent chromium compounds. 

Passing direct current through the bath coats the substrate with a bright yellow or gold-colored chromate conversion coating. The chromate coat protects the material from oxidation and rust. Furthermore, the color indicates corrosion resistance. As long as the color persists, the part is corrosion-resistant.  

Common Use and Applications

Yellow chromate coating is the best option for parts that require short-term protection without stringent requirements based on safety. Common industries and the parts made using the process are:

  • Automotive: Fasteners and brackets 
  • Hardware: Bolts, nuts, and screws 
  • Consumers goods: Appliances, furniture, and recreational equipment

Trivalent Chromium Process

Trivalent chromium coating involves electroplating a zinc-plated part by immersing it in a solution containing trivalent chromium compounds. On passing direct current through the solution, it deposits a greenish-yellow layer that protects the material from oxidation and rust. 

The trivalent chromate coat indicates the presence of corrosion resistance of the parts. As long as the color persists, the part is corrosion-resistant. It also serves as an aesthetic function. 

Environmental and Health Benefits

This is an alternative method to traditional hexavalent chromium plating methods (yellow chromate coating). It does not use hexavalent compounds which are carcinogenic and toxic. As a result, it is more environmentally friendly. 

Common Use and Applications

Trivalent chromium plating is applicable in several industries due to its better environmental impact. They include:

  • Automotive: Trim, emblems, and decorative components
  • Consumer Goods: Faucets, fixtures, and other household appliances 
  • Hardware: Fasteners, fittings, and other hardware items

Clear Zinc Coating

Clear zinc coating is an electrochemical deposition of a thin layer of zinc on substrate metals. After cleaning the substrate using mechanical, chemical, and electrochemical means, immerse it in the bath containing only zinc salts. 

Passing a direct electric current through the bath deposits the layer of zinc on the substrate. The coat has a transparent hue, i.e., it allows the natural color and texture of the substrate to show. 

Clear zinc coating uses only zinc ions (there is an absence of other additives such as chromates), reducing the part’s corrosion resistance. As a result, it reduces their use in harsh environments.

Common Use and Applications

Clear zinc coating is often used in indoor or controlled environments where the corrosion resistance requirement is moderate. Clear zinc coating is applicable in the following industries: 

  • Hardware and Fasteners: Bolts, nuts, screws, and other hardware parts
  • Automotive Components: Brackets, hinges, and clips 
  • Electronics: Electronic enclosures, connectors, and other components 
  • Consumer Goods: Furniture, appliances, and fixtures 
  • Architectural Applications: Hardware, signage, and decorative elements 

Colored Zinc Alloy Coatings

Colored zinc alloy coating is similar to clear zinc coating although it adds other elements like aluminum or nickel. After cleaning the substrate, immerse the part in the bath containing zinc and a controlled amount of aluminum and nickel salts. 

Passing a direct electric current through the bath deposits a thin layer of zinc, aluminum, and nickel on the substrate. Depending on the type and concentration of the additives, you can have a wide range of colors. Below is a list of metals and their color

AdditivesColor
AluminumBluish
NickelYellow, Gold, Bronze
IronDarker color
CobaltBluish or Grayish
CopperRed or Bronze
TinWhite

These distinctive colors due to the different metals make it appealing aesthetically. As a result, they are suitable for decorative and functional applications. Furthermore, the parts have better corrosion resistance compared to clear zinc coating and hot dip galvanization.

Common Use and Applications

Application of colored zinc alloy coatings include:

  • Automotive: Colored Trim, emblems, and decorative accents 
  • Consumer Goods: Furniture, fixtures, and household appliances 
  • Hardware and Fasteners: Bolts, nuts, screws, and other hardware items
  • Architectural Applications: Signage, and building facade
  • Electronics: Enclosures and connectors

The Science Behind Zinc Plating Colors

Zinc plating has different colors depending on the technique, ranging from silver to yellow, black, blue, or green. The color also depends on the coating thickness and additives.

The Role of Chromates

The color-changing ability of chromates is because it is a transition metal (chromium). As a transition metal with a variable oxidation state, there are variable colors. Hence, the hexavalent chromium compounds have a different color from the trivalent ones. 

The color of the chromate also depends on the thickness of the coating and the concentration. Thick chromate coatings are more vibrant, with thinner coatings appearing lighter

Influence of Plating Thickness

The thickness of the zinc plating layer will affect the color. Thick zinc coating will appear brighter than thin coatings. Also, the thicker the coating the tendency of uneven coating chromate thickness and uneven color distribution.

Impact of Post-Treatment Processes

Post-treatment processes in zinc plating include chromate conversion coating, passivation, or sealing and they have the following impacts on zinc-plated parts:

  • Corrosion Resistance

The post-treatment process acts as an additional barrier between the already zinc-plated parts and the environment. As a result, it acts as an additional corrosion protection 

  • Color Enhancement

Chromate conversion coatings can improve the color of the zinc-plated part. It allows the part manufacturer to access other colors like yellow, black, and iridescent.

  • Adhesion Promotion

Chromate conversion coatings can improve the adhesion of paints and powder coatings to a zinc-plated surface.

  • Surface Conditioning

Post-treatment processes like passivation can improve the part’s surface finish by making it smoother and more uniform.

Common Colors and Their Characteristics

Common colors associated with zinc plating and other techniques include: 

  • Bright Silver achieved with a hot dip zinc plating
  • Yellow or Gold achieved by chromate conversion coatings containing hexavalent chromium compounds 
  • Black is achieved using post-treatment processes that involve additives such as selenium or sulfur.
  • Blue or Green is achieved using additives like aluminum or nickel. 

Zintilon Support for Zinc Plating 

The best way to ensure quality zinc coating on a part is to consider a reputable surface finishing service provider At Zintilon, aside from our state-of-the-art manufacturing capability which ensures quality parts. We render quality-focused surface finishing options at a competitive price. Contact us and enjoy a fast lead time and competitive market pricing.

Conclusion

Part manufacturers consider zinc plating more of a protective process than an aesthetic one. Aside from the traditional silver hue given by the process, other zinc plating techniques and post-treatment methods can also alter the color of the end product. 

This article introduced zinc plating techniques and their respective colors. Are you looking for a surface finishing treatment option for your zinc part? Contact us at Zintillon.

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