What to Consider when Choosing Your Color for Anodizing Aluminum

Posted January 20, 2017
By Matt Campbell

The Factors that Affect Color Match 

End use customers tend to have a notion in their minds that any color can be matched when anodizing aluminum. It may be possible but there are many factors that come into play that prevent certain colors from being achieved.

Color Considerations for Anodizing Aluminum_DCHN.jpgSome of these factors include:

  • Alloy
  • Anodize types/coating thickness
  • Pretreatment: bright/matte
  • Light fastness
  • Scale-up

Let’s explore these considerations in more detail. If you’ve found yourself in situations when you’ve wanted to achieve a “perfect” match, but faced hurdles, some of this discussion might sound familiar. This may help in gaining clarity and setting expectations for future aluminum anodizing projects.

Alloy Plays an Important Role

Aluminum oxide is formed during the anodizing of aluminum. The oxide is porous and dye is absorbed to form different colors of anodized articles. Alloy plays an important role when choosing a color due to alloying elements (e.g., Cu, Zn, Mg, Si, etc.) that affect the aluminum oxide film that is grown on the aluminum substrate. For wrought alloys, the weight percentage of silicon, copper, zinc, magnesium, etc. can change the aluminum oxide layer’s tone very drastically.

These alloying elements that are embedded in the aluminum oxide coating during growth can make color matching tricky. When dealing with cast alloys, the impurity content can be so high that certain colors are unachievable. The higher impurities in alloys, such as 2024 and 7075, produce a dark gray color after anodize due to the high copper content that is in 2024, and the high zinc content in 7075. Trying to match to lighter colors may prove difficult with these alloys. The alloys 5052 and 3003 produce a clearer anodize layer that may provide a better chance for success when color matching.

Anodize Types and Coating Thickness

The type of anodize is another factor when choosing your color, such as Type II and Type III coatings from industry standard MIL-A-8625F specification. This mainly ties into the coating thickness, temperature, concentration, and the current density that is applied during anodizing. Type II coatings have a thin (0.004”-0.001”), clear, and very porous film suitable for cosmetic applications. The porous film absorbs dye very easily; for this reason, color matching is relatively easy with Type II coatings.

When dealing with Type III, the film thickness is generally thicker (0.0016-0.0024”), with a less porous film and darker coating undertone. Type III coating is mainly used for functional applications and specific coloring may be difficult to achieve. At DCHN, we provide Sanford Quantum, which is designed to provide the corrosion and abrasion resistance of Type III, but with a clear anodic film that is similar to Type II. The key is the use of our patented rectifier that applies low-voltage DC with superimposed AC during anodizing. 

The Role of Pretreatment: Brightness and Matteness

Brightness and matteness play a role when trying to meet the customer’s color standard. There are ways to manipulate the pre-finish of the alloy to achieve a bright or matte finish, but certain alloys have their limits. Alloys with certain alloying elements (e.g., 2000 with Cu as major alloying element, and 7000 series with Zn as major element) require limited or no exposure to the harsh caustics (to achieve matteness) and acids (to achieve brightness) during the pre-treatment process to avoid quality issues. In some cases, a mechanical finish (e.g., tumble, bead blast, or polish) is a better option rather than chemical.

Light Fastness Depends on the Part’s Function

Light fastness should also be considered, depending on the particular function of the part. Different dyes will have varying color fastness that will affect how quickly the dye fades when exposed to outdoor environments.

Scale-Up

When dealing with large loads during anodizing, a coating thickness range is expected. The current density during the coating process forms a window effect and is greatest at the edges. This will result in a slight color variation. It is wise to develop a color range (light, target, and dark) up front during capability studies.

In the end, the main purpose is to meet our customers’ expectations and achieve what they have imagined. We’ll work closely with you, keeping all of these considerations in mind, to help meet all your specifications.

At DCHN, we know that many questions arise when considering aluminum anodizinghardcoat, and other metal finishing jobs. Our white paper, “12 Proven Tips to Save Time & Money for Aluminum Anodizing, Hardcoat, and Other Metal Finishing Services," is a guide full of great tips to help you save time and money. Download it now.

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