Anodizing Overview

What is anodizing?

By definition, anodizing is “a process to coat a metallic surface electrolytically with a protective or decorative oxide.” A coating of aluminum oxide is grown from the aluminum by passing an electrical current through an acid electrolyte bath in which the aluminum is immersed. The coating thickness and surface characteristics are tightly controlled to meet end product specifications. Unlike most other finishes, anodizing preserves the natural luster, texture, and beauty of the metal itself. The anodized coating is hard, durable, will never peel, and, under normal conditions, will never wear through.

What is anodizing aluminum used for?

Because anodizing is such a versatile process there are literally thousands of different applications. These applications include but are not limited to:

  • Recreational Vehicles
  • Architectural products like windows and doors
  • Appliances
  • Automotive
  • Lighting
  • Food preparation equipment
  • Furniture
  • Sporting goods
  • Medical Equipment
  • Marine
  • Construction
  • Military

Does anodizing alter the bolt/nut thread pattern and diameter or overall size of a part?

Yes. Which means the dimension of the anodized part changes. The amount of change anodizing is a process that converts aluminum to its oxide. The oxide is thicker than the aluminum that is consumed will depend on the anodizing process conditions (temperature, current density, etc.) and alloy. Keep in mind that when calculating the shrinkage of a hole, you must double the amounts given because a hole has two sides. Different anodizes use slightly different process parameters. With all these variables, it is a good idea for the design engineer to contact the anodizing plant under consideration and ask for their input.

Can steel and/or stainless steel be anodized?

Steel and stainless steel can’t be anodized; the process baths used to anodize aluminum would attack and dissolve steel parts. Your parts need to be completely stripped of any steel otherwise the steel will not make it through the process.

What is the purpose of anodizing?

The purpose of anodizing is to form a layer of aluminum oxide that will protect the aluminum beneath it. The aluminum oxide layer has much higher corrosion and abrasion resistance than aluminum. There are some types of anodizing that produce a porous oxide layer that can be colored with organic dyes or metallic pigments giving the aluminum a decorative and protective finish. In short, the main purposes for anodizing are corrosion resistance, abrasive / wear resistance and cosmetics.

Given that anodizing is an electrochemical process, can areas be masked or holes plugged to prevent coverage of an area? (E.g. To create letters or symbols on a surface or to prevent buildup in machined bores or threads).

Yes, sections of a part can be masked. Flat areas can sometimes be more difficult to mask; holes and bores can usually be masked without too much difficulty. Lettering may be accomplished more satisfactorily either by casting them into the part or using laser engraving after anodizing.

From a fabricator’s viewpoint, is it best to do as much machining as possible before anodizing because of the increased surface hardness?

Machining before hardcoat anodizing is much easier and saves considerable wear and tear on the tools. A good rule of thumb is that the hardcoated surface has about the same hardness as nitrided steel (about 50 + Rockwell “C”). Is there a simple process for stripping the anodizing from a part? There is a common procedure performed by anodisers to remove anodise, and such parts can be re-anodised, but there are hazards associated with the removal of anodic oxide from a part. Consult a professional anodiser to explore the details of removing anodise. Do not attempt to remove anodize before sending a part in.

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