Posted September 17, 2020

Anodizing Guidelines for Part Design

Anodizing Guidelines for Your Next Project

Anodizing is an electrochemical process that changes the texture and
thickness of a material’s surface, creating a decorative, durable, and
corrosion-resistant finish.

Though the focus of this white paper is aluminum anodizing, processes
also exist for titanium, zinc, magnesium, niobium, zirconium, hafnium,
and tantalum. Anodized parts can be found in everything from aircraft
components and architectural materials, to consumer electronics and
firearms.

The Aluminum Anodizing Process

  1. The part is dipped in a hot tank containing a cleaning agent to remove all surface dirt.
  2. An acid solution (chromic, sulfuric, nitric, or phosphoric) de-oxidizes the part and removes the thin, non-uniform aluminum oxide surface.
  3. A sodium hydroxide solution removes the natural shine of the aluminum and provides a soft, matte, textured appearance.
  4. The part is suspended in an anodizing tank containing a mixture of diluted acid and water capable of permitting electrical current flow. The type of acid used, percent solution, and temperature all determine the final finish and color.
  5. One or more cathodes introduce electricity into the tank, creating an electrochemical reaction. A geometrically regular pattern of pores forms as excess positive ions escape. The aluminum on the surface combines with the negatively charged O2 ions, creating a barrier layer of aluminum oxide.
  6. If desired, dye is injected up to the surface of the empty pores to add color. Once the dye is applied, the outer surface is sealed to close the pores and prevent the dye from fading, staining, color bleed, and corrosion.
  7. For non-dyed coatings, the anodized aluminum part is placed in boiling deionized water, converting the unstructured pores of the aluminum oxide to a more solid crystalline hydrate form.

anodizing figure

Cosmetic uses

Anodizing offers a wide array of gloss and color finishes that will not chip or rub off, all while preserving the natural luster, texture, and beauty of the extruded aluminum. The anodic film is molecularly bonded to the substrate and absorbs pigment to the depth of the coating, making color finishes remarkably durable. Even parts exposed to outdoor conditions will typically not fade for a minimum of five years.

Anodizing also provides better adhesion for paint primers and glues, creating an excellent base for paint applications like silk screening, dry-lube painting, and powder coating.

The anodizing process by itself can produce a range of colors—including pale yellow, gold, deep bronze, brown, grey, and black—without the need for dyes.

Mechanical uses

Although aluminum doesn’t rust, exposure to acid rain, salt water, and other contaminants can exploit surface weaknesses and cause damage ranging from discoloration to mechanical failure.

When exposed to the atmosphere, a naturally occurring surface layer of aluminum oxide forms that can provide a small amount of protection against corrosion. Anodizing increases the thickness of this aluminum oxide layer, making parts exceptionally resistant to wear and ideal for use in harsh environments.

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Design considerations

Part Geometry

In the typical aluminum anodizing process, the aluminum oxide is grown down into and out from the surface by equal amounts. Because the oxide created occupies more space than the base metal converted, anodizing raises the surface and increases the part dimensions by half the oxide thickness. A coating that is two micrometers thick will increase the part dimensions by one micrometer per surface. So if the part is anodized on all sides, then all linear dimensions will increase by the oxide thickness.

Anodizing produces an aluminum oxide coating ranging from 0.0002” to 0.003” thick. Coating thickness will vary based on geometry, racking position, and electrical field variations. A difference of 0.0001” to 0.0003” in coating thickness on a single part is common.

anodizing diagram

Compensating for Surface Layer Thickness

This thickening of the anodizing layer should be considered when designing parts and choosing the machining dimension—particularly where there are tight tolerances, very fine threaded holes, precision pin holes, and fine sliding fits.

A common practice is to model each part with anodizing build-up in mind, and to specify that “dimensions apply after all surface finishes” in the engineering drawing. This helps ensure that the machine shop takes the anticipated anodization thickness into account, and compensates for buildup when performing the final machining of the part.

To maintain a part’s original geometry, soft plastic plugs, plastic tape, or painted-on liquid plastics can be used to mask holes and flat areas to prevent anodizing. When anodized holes are required (e.g., for parts that need to resist electrical currents) special oversized taps can be used prior to anodizing to help compensate forthe thicker Type III aluminum oxide layer.

Accommodating for Racking

Racking refers to holding the part while transferring between different anodizing process tanks, as well as providing sufficient contact to allow enough electrical current to run through.

Throughout the anodizing process, each part connects to a hanger to keep it from falling to the bottom of the tank. Because holding fixtures will block the anodizing chemicals from reaching that specific area, parts that must be racked on exposed surfaces will almost always have some sort of visible rack mark. To minimize or eliminate these marks, your design should identify preferred rack connection points that won’t be visible on the finished product.

Production Constraints

Delivery Timeframe

Although anodizing can take just a few hours for a single part, most orders run anywhere from a few dozen to even thousands of parts. A reasonable turnaround estimate is to add at least a week to your production time.

It’s important to communicate with your vendor up front so that they can accommodate your exact needs. Different vendors have different capacities. Some are better at anodizing large parts, others better at very small parts.

For larger quantities, consider a vendor that offers Just In Time (JIT) delivery. Rather than having to wait for your entire order to be completed, a JIT anodizing vendor can deliver initial batches of parts as they are completed.

Price

Anodizing can be very cost effective compared to other types of electroplating, painting, or powder coating. It generally adds around 20% to 25% to the cost of a project, depending on requirements including:

  • Anodizing performed at a higher current and voltage, requiring greater energy use (such as with Type III).
  • Hand masking needed to preserve threading or electrical contacts.
  • Racking, particularly for difficult-to-rack parts.
  • The need for extensive rinsing of parts due to features like small holes, small and deep pockets, or other enclosed features.
  • Unclear or missing design specifications.
  • Special handling, packaging, or protective shipping requirements.
  • Desired color, temper, or finish.
  • Required tolerances for camber, squareness, length, and width.

Types of Aluminum Anodizing

The three most common specification types are:

Type I—Hard Anodizing
Type I anodizing uses a chromic acid based chemical bath that produces a thinner protective coating and a high level of corrosion resistance, as well as an effective primer for painting or coating.

Type II—Sulfuric Acid
Type II anodizing uses sulfuric acid to produce a thicker anodized layer, making it more suitable for dying.

Type III—Sulfuric Acid “Hard Anodizing”
Like Type II, Type III also uses sulfuric acid. But by continuing the electrical current to deepen the pores, it produces a thicker anodized layer that offers greater corrosion protection and resistance to wear.

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Topics: materials, Manufacturing, finishing

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