As an electrochemical process, anodizing changes the surface of metals to make them more durable to wear, corrosion-resistance, and to give them an attractive finish. The process increases the natural oxide layer that coats metal parts, though it is most commonly used with aluminum and its alloys. This aluminum oxide results in a thin protective layer that protects and covers the top metal layer. Though anodization processes exist for other nonferrous metals – such as zinc, magnesium and titanium – and there are processes that use aluminum plate to anodize color into other metals, what follows mainly addresses the color anodization process used for aluminum, with a more in-depth focus upon hard coat anodized color dyeing.
Why Anodize Metals?
While anodizing for color gives the metal surface an appealing look, there are other benefits.
Color anodizing also offers these benefits:
- Better wear resistance.
- Easy to sterilize and resists contamination, so works well for medical instruments.
- Lubricates better, especially when sealed with polytetrafluoroethylene (PTFE).
- Electrical insulation, abrasion wear resistance, and corrosion protection.
- Resistance to abrasion, though this works best when unsealed.
- Withstands corrosion, especially when sealed.
Along with these benefits, color anodizing aluminum is particularly advantageous.
The Color Anodize Process
To dye aluminum during the anodization process, the metal is immersed into acid, which acts as an electrolyte to conduct electricity. Direct current is then applied to it to create a positive charge on the workpiece. The electricity then splits some water molecules in the acid, creating negatively charged oxygen and positively charged hydrogen.
The hydrogen and oxygen react with the aluminum to form aluminum oxide, along with other related compounds. While the acid helps shape the anodic structure, it simultaneously works to dissolve it. The formation and dissolution upon a metallic component’s surface cause a porous film to form miniscule holes that extend down almost to the raw metal. The origins of the anodic oxide come directly from the metallic substrate, which is made up solely of aluminum.
This layer of aluminum oxide fully integrates with the metallic substrate, so that it will not chip or peel like plated or painted metal. This creates an orderly yet porous structure that permits additional processes like sealing or coloring. The anodization process also increases the ability of paint primers to adhere to the surface. This offers the ability to create a number of cosmetic effects on the metal part’s surface, as thicker porous coats can absorb dyes while thinner coats reflect light.
Ways to Color Anodize Metals
To color anodize with a hard coat, the pores absorb the dye, which is then sealed to retain the color inside the metal’s substrate. The thicker the coating produced during anodizing, the more dye the metal can absorb, resulting in bolder colors. For example, pastels result from a minimal coating, while jet-black colors will generally require a much thicker oxide coating.
Types of Acid Used to Anodize Color
Two general processes are used to anodize color into metals, using either chromic or sulfuric acid. Chromic acid’s advantages include its self-healing and corrosion-resistance properties. though manufacturers generally use black dyes as this type of acid does not color anodize as well with other shades. A sulfuric acid color anodize offers more diverse dyeing properties, and the hard sulfuric acid used in the hard coat color anodize process also allows for other colors when dyeing.
Ways to Anodize Color into Metals
Though there are basically three ways in which manufacturers can anodize color into aluminum, certain methods work better depending upon the purpose for which a component will be used.
Type 1 Anodization
The most basic way to anodize color into a workpiece involves using chromic acid to produce a fine yet supple anodized layer on aluminum components. This opaque layer is not conductive and also has some self-healing properties. Sometimes applied prior to painting to augment paint bonding to the component’s surface, it is more difficult to color anodize with it rather than sulfuric acid. This process also does not lower the metal’s fatigue strength or damage the base material.
Type 2 Anodization
This process uses sulfuric rather than chromic acid to deliver a thicker anodized coating that is more suitable for coloring. Bathed in sulfuric acid, this then produces a porous anodic finish, with the pores allowing the surface of the aluminum to absorb more dye, which is injected directly into the empty pores on the component’s surface and then sealed to better preserve the coloration.
Type 3 Anodization
This method for coloring metal surfaces is alternatively known as a hard coat color anodize or hard anodize and adds to the thickness of aluminum’s naturally occurring oxide coating. This process also uses a sulfuric acid mixture as an electrolyte. It improves the metal’s surface hardness while enabling it to retain PTFE coatings and lubricants, along with enhanced dialectical and thermal properties.
Benefits & Uses for Hard Coat Anodizing
A hard coat color anodize provides a thick anodic overlay of aluminum oxide, typically cooling sulfuric acid to near its freezing point of 50˚ F (10˚ C). Hard coat anodizing involves applying a dense coat of aluminum oxide, which is achieved when a component has been properly cleaned and deoxidized with a suitable electrolyte.
A hard coat color anodize generally uses an electrolyte that creates a denser, thicker anodized layer. Aluminum components used in conditions that require resistance to higher levels of abrasion or corrosion must have a coating that is more durable. This thicker coating also enhances electrical insulation.
Some features of components that are hard anodized include:
- Able to be ground
- Allows for black dye and other colors
- Can repair worn aluminum surfaces
- Do not conduct electricity
- Helps improve surfaces of components for slide applications
- Improved resistance to wear
- Offers finish harder than tooled steel
Applications for anodized color components include:
- Blast shields
- Hinged devices
- Insulation plates
- Sliding components
- Swivel joints
Tips When Color Anodizing
Only the aluminum anodizes during the anodization process. Any other metals present within an aluminum alloy will not respond in the same way. The variances in color during anodizing occurs mainly due to the other metals present in it, though color variations also occur for other reasons, including the color anodize tank chemistry, geometry of the component, tempering or the material’s strength.
Here are some tips for anodizing color into aluminum components:
- Contract color anodizes to companies or contractors that use automated processes in order to reduce inconsistencies.
- Do not mix different aluminum alloys or tempers, as this will produce uneven results.
- Ensure most shaping of components occurs prior to finishing, as small, weblike cracks can occur in the finish when bent after production.
- Heat will change the metal, causing localized discoloration, so beware of how welding affects color anodizing.
- Work only with one source of metal per project to ensure the color comes out consistently the same.
While manufacturers can control the chemistry and conditions within an anodizing tank, including concentrations of the solution, temperatures and time spent within the chemical baths, they cannot control how aluminum parts are shaped or tempered. All these variables make getting the exact color right extremely difficult.
To learn more about Silvex’s hard coat anodizing color solutions, please contact our team today.